gpg2(1)
NAME
gpg - OpenPGP encryption and signing tool
SYNOPSIS
gpg [--homedir dir] [--options file] [options] command [args]
DESCRIPTION
gpg is the OpenPGP part of the GNU Privacy Guard (GnuPG). It is a tool to provide digital encryption and signing services using the OpenPGP standard. gpg features complete key management and all bells and whistles you can expect from a decent OpenPGP implementation. Note that this version of GnuPG features all modern algorithms and should thus be preferred over older GnuPG versions. If you are looking for version 1 of GnuPG, you may find that version installed under the name gpg1.
RETURN VALUE
The program returns 0 if everything was fine, 1 if at least a signature was bad, and other error codes for fatal errors.
WARNINGS
Use a *good* password for your user account and a *good* passphrase to protect your secret key. This passphrase is the weakest part of the whole system. Programs to do dictionary attacks on your secret keyring are very easy to write and so you should protect your "~/.gnupg/" directory very well. Keep in mind that, if this program is used over a network (telnet), it is *very* easy to spy out your passphrase! If you are going to verify detached signatures, make sure that the program knows about it; either give both filenames on the command line or use '-' to specify STDIN.
INTEROPERABILITY
GnuPG tries to be a very flexible implementation of the OpenPGP standard. In particular, GnuPG implements many of the optional parts of the standard, such as the SHA-512 hash, and the ZLIB and BZIP2 compression algorithms. It is important to be aware that not all OpenPGP programs implement these optional algorithms and that by forcing their use via the --cipher-algo, --digest-algo, --cert-digest- algo, or --compress-algo options in GnuPG, it is possible to create a perfectly valid OpenPGP message, but one that cannot be read by the intended recipient. There are dozens of variations of OpenPGP programs available, and each supports a slightly different subset of these optional algorithms. For example, until recently, no (unhacked) version of PGP supported the BLOWFISH cipher algorithm. A message using BLOWFISH simply could not be read by a PGP user. By default, GnuPG uses the standard OpenPGP preferences system that will always do the right thing and create messages that are usable by all recipients, regardless of which OpenPGP program they use. Only override this safe default if you really know what you are doing. If you absolutely must override the safe default, or if the preferences on a given key are invalid for some reason, you are far better off using the --pgp6, --pgp7, or --pgp8 options. These options are safe as they do not force any particular algorithms in violation of OpenPGP, but rather reduce the available algorithms to a "PGP-safe" list.
COMMANDS
Commands are not distinguished from options except for the fact that
only one command is allowed.
gpg may be run with no commands, in which case it will perform a
reasonable action depending on the type of file it is given as input
(an encrypted message is decrypted, a signature is verified, a file
containing keys is listed).
Please remember that option as well as command parsing stops as soon as
a non-option is encountered, you can explicitly stop parsing by using
the special option --.
Commands not specific to the function
--version
Print the program version and licensing information. Note that
you cannot abbreviate this command.
--help
-h Print a usage message summarizing the most useful command line
options. Note that you cannot abbreviate this command.
--warranty
Print warranty information.
--dump-options
Print a list of all available options and commands. Note that
you cannot abbreviate this command.
Commands to select the type of operation
--sign
-s Make a signature. This command may be combined with --encrypt
(for a signed and encrypted message), --symmetric (for a signed
and symmetrically encrypted message), or --encrypt and
--symmetric together (for a signed message that may be decrypted
via a secret key or a passphrase). The key to be used for
signing is chosen by default or can be set with the --local-user
and --default-key options.
--clearsign
Make a clear text signature. The content in a clear text
signature is readable without any special software. OpenPGP
software is only needed to verify the signature. Clear text
signatures may modify end-of-line whitespace for platform
independence and are not intended to be reversible. The key to
be used for signing is chosen by default or can be set with the
--local-user and --default-key options.
--detach-sign
-b Make a detached signature.
--encrypt
-e Encrypt data. This option may be combined with --sign (for a
signed and encrypted message), --symmetric (for a message that
may be decrypted via a secret key or a passphrase), or --sign
and --symmetric together (for a signed message that may be
decrypted via a secret key or a passphrase).
--symmetric
-c Encrypt with a symmetric cipher using a passphrase. The default
symmetric cipher used is AES-128, but may be chosen with the
--cipher-algo option. This option may be combined with --sign
(for a signed and symmetrically encrypted message), --encrypt
(for a message that may be decrypted via a secret key or a
passphrase), or --sign and --encrypt together (for a signed
message that may be decrypted via a secret key or a passphrase).
--store
Store only (make a simple literal data packet).
--decrypt
-d Decrypt the file given on the command line (or STDIN if no file
is specified) and write it to STDOUT (or the file specified with
--output). If the decrypted file is signed, the signature is
also verified. This command differs from the default operation,
as it never writes to the filename which is included in the file
and it rejects files which don't begin with an encrypted
message.
--verify
Assume that the first argument is a signed file and verify it
without generating any output. With no arguments, the signature
packet is read from STDIN. If only a one argument is given, it
is expected to be a complete signature.
With more than 1 argument, the first should be a detached
signature and the remaining files make up the the signed data.
To read the signed data from STDIN, use '-' as the second
filename. For security reasons a detached signature cannot read
the signed material from STDIN without denoting it in the above
way.
Note: If the option --batch is not used, gpg may assume that a
single argument is a file with a detached signature and it will
try to find a matching data file by stripping certain suffixes.
Using this historical feature to verify a detached signature is
strongly discouraged; always specify the data file too.
Note: When verifying a cleartext signature, gpg verifies only
what makes up the cleartext signed data and not any extra data
outside of the cleartext signature or header lines following
directly the dash marker line. The option --output may be used
to write out the actual signed data; but there are other
pitfalls with this format as well. It is suggested to avoid
cleartext signatures in favor of detached signatures.
--multifile
This modifies certain other commands to accept multiple files
for processing on the command line or read from STDIN with each
filename on a separate line. This allows for many files to be
processed at once. --multifile may currently be used along with
--verify, --encrypt, and --decrypt. Note that --multifile
--verify may not be used with detached signatures.
--verify-files
Identical to --multifile --verify.
--encrypt-files
Identical to --multifile --encrypt.
--decrypt-files
Identical to --multifile --decrypt.
--list-keys
-k
--list-public-keys
List all keys from the public keyrings, or just the keys given
on the command line.
Avoid using the output of this command in scripts or other
programs as it is likely to change as GnuPG changes. See
--with-colons for a machine-parseable key listing command that
is appropriate for use in scripts and other programs. Never use
the regular output for scripts - it is only for human
consumption.
--list-secret-keys
-K List all keys from the secret keyrings, or just the ones given
on the command line. A # after the letters sec means that the
secret key is not usable (for example, if it was created via
--export-secret-subkeys). See also --list-keys.
--list-sigs
Same as --list-keys, but the signatures are listed too. This
command has the same effect as using --list-keys with --with-
sig-list.
For each signature listed, there are several flags in between
the "sig" tag and keyid. These flags give additional information
about each signature. From left to right, they are the numbers
1-3 for certificate check level (see --ask-cert-level), "L" for
a local or non-exportable signature (see --lsign-key), "R" for a
nonRevocable signature (see the --edit-key command "nrsign"),
"P" for a signature that contains a policy URL (see --cert-
policy-url), "N" for a signature that contains a notation (see
--cert-notation), "X" for an eXpired signature (see --ask-cert-
expire), and the numbers 1-9 or "T" for 10 and above to indicate
trust signature levels (see the --edit-key command "tsign").
--check-sigs
Same as --list-sigs, but the signatures are verified. Note that
for performance reasons the revocation status of a signing key
is not shown. This command has the same effect as using --list-
keys with --with-sig-check.
The status of the verification is indicated by a flag directly
following the "sig" tag (and thus before the flags described
above for --list-sigs). A "!" indicates that the signature has
been successfully verified, a "-" denotes a bad signature and a
"%" is used if an error occurred while checking the signature
(e.g. a non supported algorithm).
--locate-keys
Locate the keys given as arguments. This command basically uses
the same algorithm as used when locating keys for encryption or
signing and may thus be used to see what keys gpg might use. In
particular external methods as defined by --auto-key-locate may
be used to locate a key. Only public keys are listed.
--fingerprint
List all keys (or the specified ones) along with their
fingerprints. This is the same output as --list-keys but with
the additional output of a line with the fingerprint. May also
be combined with --list-sigs or --check-sigs. If this command
is given twice, the fingerprints of all secondary keys are
listed too. This command also forces pretty printing of
fingerprints if the keyid format has been set to "none".
--list-packets
List only the sequence of packets. This command is only useful
for debugging. When used with option --verbose the actual MPI
values are dumped and not only their lengths. Note that the
output of this command may change with new releases.
--card-edit
Present a menu to work with a smartcard. The subcommand "help"
provides an overview on available commands. For a detailed
description, please see the Card HOWTO at
https://gnupg.org/documentation/howtos.html#GnuPG-cardHOWTO .
--card-status
Show the content of the smart card.
--change-pin
Present a menu to allow changing the PIN of a smartcard. This
functionality is also available as the subcommand "passwd" with
the --card-edit command.
--delete-keys name
--delete-keys name
Remove key from the public keyring. In batch mode either --yes
is required or the key must be specified by fingerprint. This is
a safeguard against accidental deletion of multiple keys.
--delete-secret-keys name
gRemove key from the secret keyring. In batch mode the key must
be specified by fingerprint. The option --yes can be used to
advice gpg-agent not to request a confirmation. This extra pre-
caution is done because gpg can't be sure that the secret key
(as controlled by gpg-agent) is only used for the given OpenPGP
public key.
--delete-secret-and-public-key name
Same as --delete-key, but if a secret key exists, it will be
removed first. In batch mode the key must be specified by
fingerprint. The option --yes can be used to advice gpg-agent
not to request a confirmation.
--export
Either export all keys from all keyrings (default keyrings and
those registered via option --keyring), or if at least one name
is given, those of the given name. The exported keys are written
to STDOUT or to the file given with option --output. Use
together with --armor to mail those keys.
--send-keys key IDs
Similar to --export but sends the keys to a keyserver.
Fingerprints may be used instead of key IDs. Option --keyserver
must be used to give the name of this keyserver. Don't send your
complete keyring to a keyserver --- select only those keys which
are new or changed by you. If no key IDs are given, gpg does
nothing.
--export-secret-keys
--export-secret-subkeys
Same as --export, but exports the secret keys instead. The
exported keys are written to STDOUT or to the file given with
option --output. This command is often used along with the
option --armor to allow easy printing of the key for paper
backup; however the external tool paperkey does a better job for
creating backups on paper. Note that exporting a secret key can
be a security risk if the exported keys are send over an
insecure channel.
The second form of the command has the special property to
render the secret part of the primary key useless; this is a GNU
extension to OpenPGP and other implementations can not be
expected to successfully import such a key. Its intended use is
to generated a full key with an additional signing subkey on a
dedicated machine and then using this command to export the key
without the primary key to the main machine.
GnuPG may ask you to enter the passphrase for the key. This is
required because the internal protection method of the secret
key is different from the one specified by the OpenPGP protocol.
--export-ssh-key
This command is used to export a key in the OpenSSH public key
format. It requires the specification of one key by the usual
means and exports the latest valid subkey which has an
authentication capability to STDOUT or to the file given with
option --output. That output can directly be added to ssh's
‘authorized_key’ file.
By specifying the key to export using a key ID or a fingerprint
suffixed with an exclamation mark (!), a specific subkey or the
primary key can be exported. This does not even require that
the key has the authentication capability flag set.
--import
--fast-import
Import/merge keys. This adds the given keys to the keyring. The
fast version is currently just a synonym.
There are a few other options which control how this command
works. Most notable here is the --import-options merge-only
option which does not insert new keys but does only the merging
of new signatures, user-IDs and subkeys.
--recv-keys key IDs
Import the keys with the given key IDs from a keyserver. Option
--keyserver must be used to give the name of this keyserver.
--refresh-keys
Request updates from a keyserver for keys that already exist on
the local keyring. This is useful for updating a key with the
latest signatures, user IDs, etc. Calling this with no arguments
will refresh the entire keyring. Option --keyserver must be used
to give the name of the keyserver for all keys that do not have
preferred keyservers set (see --keyserver-options honor-
keyserver-url).
--search-keys names
Search the keyserver for the given names. Multiple names given
here will be joined together to create the search string for the
keyserver. Option --keyserver must be used to give the name of
this keyserver. Keyservers that support different search
methods allow using the syntax specified in "How to specify a
user ID" below. Note that different keyserver types support
different search methods. Currently only LDAP supports them all.
--fetch-keys URIs
Retrieve keys located at the specified URIs. Note that different
installations of GnuPG may support different protocols (HTTP,
FTP, LDAP, etc.). When using HTTPS the system provided root
certificates are used by this command.
--update-trustdb
Do trust database maintenance. This command iterates over all
keys and builds the Web of Trust. This is an interactive command
because it may have to ask for the "ownertrust" values for keys.
The user has to give an estimation of how far she trusts the
owner of the displayed key to correctly certify (sign) other
keys. GnuPG only asks for the ownertrust value if it has not yet
been assigned to a key. Using the --edit-key menu, the assigned
value can be changed at any time.
--check-trustdb
Do trust database maintenance without user interaction. From
time to time the trust database must be updated so that expired
keys or signatures and the resulting changes in the Web of Trust
can be tracked. Normally, GnuPG will calculate when this is
required and do it automatically unless --no-auto-check-trustdb
is set. This command can be used to force a trust database check
at any time. The processing is identical to that of --update-
trustdb but it skips keys with a not yet defined "ownertrust".
For use with cron jobs, this command can be used together with
--batch in which case the trust database check is done only if a
check is needed. To force a run even in batch mode add the
option --yes.
--export-ownertrust
Send the ownertrust values to STDOUT. This is useful for backup
purposes as these values are the only ones which can't be re-
created from a corrupted trustdb. Example:
gpg --export-ownertrust > otrust.txt
--import-ownertrust
Update the trustdb with the ownertrust values stored in files
(or STDIN if not given); existing values will be overwritten.
In case of a severely damaged trustdb and if you have a recent
backup of the ownertrust values (e.g. in the file ‘otrust.txt’,
you may re-create the trustdb using these commands:
cd ~/.gnupg
rm trustdb.gpg
gpg --import-ownertrust < otrust.txt
--rebuild-keydb-caches
When updating from version 1.0.6 to 1.0.7 this command should be
used to create signature caches in the keyring. It might be
handy in other situations too.
--print-md algo
--print-mds
Print message digest of algorithm ALGO for all given files or
STDIN. With the second form (or a deprecated "*" as algo)
digests for all available algorithms are printed.
--gen-random 0|1|2 count
Emit count random bytes of the given quality level 0, 1 or 2. If
count is not given or zero, an endless sequence of random bytes
will be emitted. If used with --armor the output will be base64
encoded. PLEASE, don't use this command unless you know what
you are doing; it may remove precious entropy from the system!
--gen-prime mode bits
Use the source, Luke :-). The output format is still subject to
change.
--enarmor
--dearmor
Pack or unpack an arbitrary input into/from an OpenPGP ASCII
armor. This is a GnuPG extension to OpenPGP and in general not
very useful.
--tofu-policy auto|good|unknown|bad|ask key...
Set the TOFU policy for all the bindings associated with the
specified keys. For more information about the meaning of the
policies, see: [trust-model-tofu]. The keys may be specified
either by their fingerprint (preferred) or their keyid.
How to manage your keys
This section explains the main commands for key management
--quick-gen-key user-id [algo [usage [expire]]]
This is a simple command to generate a standard key with one
user id. In contrast to --gen-key the key is generated directly
without the need to answer a bunch of prompts. Unless the
option --yes is given, the key creation will be canceled if the
given user id already exists in the key ring.
If invoked directly on the console without any special options
an answer to a ``Continue?'' style confirmation prompt is
required. In case the user id already exists in the key ring a
second prompt to force the creation of the key will show up.
If any of the optional arguments are given, only the primary key
is created and no prompts are shown. For a description of these
optional arguments see the command --quick-addkey. The usage
accepts also the value ``cert'' which can be used to create a
certification only primary key; the default is to a create
certification and signing key.
If this command is used with --batch, --pinentry-mode has been
set to loopback, and one of the passphrase options
(--passphrase, --passphrase-fd, or passphrase-file) is used, the
supplied passphrase is used for the new key and the agent does
not ask for it. To create a key without any protection
--passphrase '' may be used.
--quick-addkey fpr [algo [usage [expire]]]
Directly add a subkey to the key identified by the fingerprint
fpr. Without the optional arguments an encryption subkey is
added. If any of the arguments are given a more specific subkey
is added.
algo may be any of the supported algorithms or curve names given
in the format as used by key listings. To use the default
algorithm the string ``default'' or ``-'' can be used. Supported
algorithms are ``rsa'', ``dsa'', ``elg'', ``ed25519'',
``cv25519'', and other ECC curves. For example the string
``rsa'' adds an RSA key with the default key length; a string
``rsa4096'' requests that the key length is 4096 bits.
Depending on the given algo the subkey may either be an
encryption subkey or a signing subkey. If an algorithm is
capable of signing and encryption and such a subkey is desired,
a usage string must be given. This string is either ``default''
or ``-'' to keep the default or a comma delimited list of
keywords: ``sign'' for a signing subkey, ``auth'' for an
authentication subkey, and ``encr'' for an encryption subkey
(``encrypt'' can be used as alias for ``encr''). The valid
combinations depend on the algorithm.
The expire argument can be used to specify an expiration date
for the subkey. Several formats are supported; commonly the ISO
YYYY-MM-DD format is used. The values ``never'', ``none'', or
``-'' can be used for no expiration date.
--gen-key
Generate a new key pair using the current default parameters.
This is the standard command to create a new key. In addition
to the key a revocation certificate is created and stored in the
‘openpgp-revocs.d’ directory below the GnuPG home directory.
--full-gen-key
Generate a new key pair with dialogs for all options. This is
an extended version of --gen-key.
There is also a feature which allows you to create keys in batch
mode. See the manual section ``Unattended key generation'' on
how to use this.
--gen-revoke name
Generate a revocation certificate for the complete key. To only
revoke a subkey or a key signature, use the --edit command.
This command merely creates the revocation certificate so that
it can be used to revoke the key if that is ever needed. To
actually revoke a key the created revocation certificate needs
to be merged with the key to revoke. This is done by importing
the revocation certificate using the --import command. Then the
revoked key needs to be published, which is best done by sending
the key to a keyserver (command --send-key) and by exporting
(--export) it to a file which is then send to frequent
communication partners.
--desig-revoke name
Generate a designated revocation certificate for a key. This
allows a user (with the permission of the keyholder) to revoke
someone else's key.
--edit-key
Present a menu which enables you to do most of the key
management related tasks. It expects the specification of a key
on the command line.
uid n Toggle selection of user ID or photographic user ID with
index n. Use * to select all and 0 to deselect all.
key n Toggle selection of subkey with index n or key ID n. Use
* to select all and 0 to deselect all.
sign Make a signature on key of user name If the key is not
yet signed by the default user (or the users given with
-u), the program displays the information of the key
again, together with its fingerprint and asks whether it
should be signed. This question is repeated for all users
specified with -u.
lsign Same as "sign" but the signature is marked as non-
exportable and will therefore never be used by others.
This may be used to make keys valid only in the local
environment.
nrsign Same as "sign" but the signature is marked as non-
revocable and can therefore never be revoked.
tsign Make a trust signature. This is a signature that combines
the notions of certification (like a regular signature),
and trust (like the "trust" command). It is generally
only useful in distinct communities or groups. For more
information please read the sections ``Trust Signature''
and ``Regular Expression'' in RFC-4880.
Note that "l" (for local / non-exportable), "nr" (for non-
revocable, and "t" (for trust) may be freely mixed and prefixed
to "sign" to create a signature of any type desired.
If the option --only-sign-text-ids is specified, then any non-text
based user ids (e.g., photo IDs) will not be selected for signing.
delsig Delete a signature. Note that it is not possible to
retract a signature, once it has been send to the public
(i.e. to a keyserver). In that case you better use
revsig.
revsig Revoke a signature. For every signature which has been
generated by one of the secret keys, GnuPG asks whether a
revocation certificate should be generated.
check Check the signatures on all selected user IDs. With the
extra option selfsig only self-signatures are shown.
adduid Create an additional user ID.
addphoto
Create a photographic user ID. This will prompt for a
JPEG file that will be embedded into the user ID. Note
that a very large JPEG will make for a very large key.
Also note that some programs will display your JPEG
unchanged (GnuPG), and some programs will scale it to fit
in a dialog box (PGP).
showphoto
Display the selected photographic user ID.
deluid Delete a user ID or photographic user ID. Note that it
is not possible to retract a user id, once it has been
send to the public (i.e. to a keyserver). In that case
you better use revuid.
revuid Revoke a user ID or photographic user ID.
primary
Flag the current user id as the primary one, removes the
primary user id flag from all other user ids and sets the
timestamp of all affected self-signatures one second
ahead. Note that setting a photo user ID as primary makes
it primary over other photo user IDs, and setting a
regular user ID as primary makes it primary over other
regular user IDs.
keyserver
Set a preferred keyserver for the specified user ID(s).
This allows other users to know where you prefer they get
your key from. See --keyserver-options honor-keyserver-
url for more on how this works. Setting a value of
"none" removes an existing preferred keyserver.
notation
Set a name=value notation for the specified user ID(s).
See --cert-notation for more on how this works. Setting a
value of "none" removes all notations, setting a notation
prefixed with a minus sign (-) removes that notation, and
setting a notation name (without the =value) prefixed
with a minus sign removes all notations with that name.
pref List preferences from the selected user ID. This shows
the actual preferences, without including any implied
preferences.
showpref
More verbose preferences listing for the selected user
ID. This shows the preferences in effect by including the
implied preferences of 3DES (cipher), SHA-1 (digest), and
Uncompressed (compression) if they are not already
included in the preference list. In addition, the
preferred keyserver and signature notations (if any) are
shown.
setpref string
Set the list of user ID preferences to string for all (or
just the selected) user IDs. Calling setpref with no
arguments sets the preference list to the default (either
built-in or set via --default-preference-list), and
calling setpref with "none" as the argument sets an empty
preference list. Use gpg --version to get a list of
available algorithms. Note that while you can change the
preferences on an attribute user ID (aka "photo ID"),
GnuPG does not select keys via attribute user IDs so
these preferences will not be used by GnuPG.
When setting preferences, you should list the algorithms
in the order which you'd like to see them used by someone
else when encrypting a message to your key. If you don't
include 3DES, it will be automatically added at the end.
Note that there are many factors that go into choosing an
algorithm (for example, your key may not be the only
recipient), and so the remote OpenPGP application being
used to send to you may or may not follow your exact
chosen order for a given message. It will, however, only
choose an algorithm that is present on the preference
list of every recipient key. See also the
INTEROPERABILITY WITH OTHER OPENPGP PROGRAMS section
below.
addkey Add a subkey to this key.
addcardkey
Generate a subkey on a card and add it to this key.
keytocard
Transfer the selected secret subkey (or the primary key
if no subkey has been selected) to a smartcard. The
secret key in the keyring will be replaced by a stub if
the key could be stored successfully on the card and you
use the save command later. Only certain key types may be
transferred to the card. A sub menu allows you to select
on what card to store the key. Note that it is not
possible to get that key back from the card - if the card
gets broken your secret key will be lost unless you have
a backup somewhere.
bkuptocard file
Restore the given file to a card. This command may be
used to restore a backup key (as generated during card
initialization) to a new card. In almost all cases this
will be the encryption key. You should use this command
only with the corresponding public key and make sure that
the file given as argument is indeed the backup to
restore. You should then select 2 to restore as
encryption key. You will first be asked to enter the
passphrase of the backup key and then for the Admin PIN
of the card.
delkey Remove a subkey (secondary key). Note that it is not
possible to retract a subkey, once it has been send to
the public (i.e. to a keyserver). In that case you
better use revkey.
revkey Revoke a subkey.
expire Change the key or subkey expiration time. If a subkey is
selected, the expiration time of this subkey will be
changed. With no selection, the key expiration of the
primary key is changed.
trust Change the owner trust value for the key. This updates
the trust-db immediately and no save is required.
disable
enable Disable or enable an entire key. A disabled key can not
normally be used for encryption.
addrevoker
Add a designated revoker to the key. This takes one
optional argument: "sensitive". If a designated revoker
is marked as sensitive, it will not be exported by
default (see export-options).
passwd Change the passphrase of the secret key.
toggle This is dummy command which exists only for backward
compatibility.
clean Compact (by removing all signatures except the selfsig)
any user ID that is no longer usable (e.g. revoked, or
expired). Then, remove any signatures that are not usable
by the trust calculations. Specifically, this removes
any signature that does not validate, any signature that
is superseded by a later signature, revoked signatures,
and signatures issued by keys that are not present on the
keyring.
minimize
Make the key as small as possible. This removes all
signatures from each user ID except for the most recent
self-signature.
cross-certify
Add cross-certification signatures to signing subkeys
that may not currently have them. Cross-certification
signatures protect against a subtle attack against
signing subkeys. See --require-cross-certification. All
new keys generated have this signature by default, so
this option is only useful to bring older keys up to
date.
save Save all changes to the key rings and quit.
quit Quit the program without updating the key rings.
The listing shows you the key with its secondary keys and all
user ids. The primary user id is indicated by a dot, and
selected keys or user ids are indicated by an asterisk. The
trust value is displayed with the primary key: the first is the
assigned owner trust and the second is the calculated trust
value. Letters are used for the values:
- No ownertrust assigned / not yet calculated.
e Trust calculation has failed; probably due to an expired
key.
q Not enough information for calculation.
n Never trust this key.
m Marginally trusted.
f Fully trusted.
u Ultimately trusted.
--sign-key name
Signs a public key with your secret key. This is a shortcut
version of the subcommand "sign" from --edit.
--lsign-key name
Signs a public key with your secret key but marks it as non-
exportable. This is a shortcut version of the subcommand "lsign"
from --edit-key.
--quick-sign-key fpr [names]
--quick-lsign-key fpr [names]
Directly sign a key from the passphrase without any further user
interaction. The fpr must be the verified primary fingerprint
of a key in the local keyring. If no names are given, all useful
user ids are signed; with given [names] only useful user ids
matching one of theses names are signed. By default, or if a
name is prefixed with a '*', a case insensitive substring match
is used. If a name is prefixed with a '=' a case sensitive
exact match is done.
The command --quick-lsign-key marks the signatures as non-
exportable. If such a non-exportable signature already exists
the --quick-sign-key turns it into a exportable signature.
This command uses reasonable defaults and thus does not provide
the full flexibility of the "sign" subcommand from --edit-key.
Its intended use is to help unattended key signing by utilizing
a list of verified fingerprints.
--quick-adduid user-id new-user-id
This command adds a new user id to an existing key. In contrast
to the interactive sub-command adduid of --edit-key the new-
user-id is added verbatim with only leading and trailing white
space removed, it is expected to be UTF-8 encoded, and no checks
on its form are applied.
--quick-revuid user-id user-id-to-revoke
This command revokes a User ID on an existing key. It cannot be
used to revoke the last User ID on key (some non-revoked User ID
must remain), with revocation reason ``User ID is no longer
valid''. If you want to specify a different revocation reason,
or to supply supplementary revocation text, you should use the
interactive sub-command revuid of --edit-key.
--passwd user_id
Change the passphrase of the secret key belonging to the
certificate specified as user_id. This is a shortcut for the
sub-command passwd of the edit key menu.
OPTIONS
gpg features a bunch of options to control the exact behaviour and to
change the default configuration.
Long options can be put in an options file (default
"~/.gnupg/gpg.conf"). Short option names will not work - for example,
"armor" is a valid option for the options file, while "a" is not. Do
not write the 2 dashes, but simply the name of the option and any
required arguments. Lines with a hash ('#') as the first non-white-
space character are ignored. Commands may be put in this file too, but
that is not generally useful as the command will execute automatically
with every execution of gpg.
Please remember that option parsing stops as soon as a non-option is
encountered, you can explicitly stop parsing by using the special
option --.
How to change the configuration
These options are used to change the configuration and are usually
found in the option file.
--default-key name
Use name as the default key to sign with. If this option is not
used, the default key is the first key found in the secret
keyring. Note that -u or --local-user overrides this option.
This option may be given multiple times. In this case, the last
key for which a secret key is available is used. If there is no
secret key available for any of the specified values, GnuPG will
not emit an error message but continue as if this option wasn't
given.
--default-recipient name
Use name as default recipient if option --recipient is not used
and don't ask if this is a valid one. name must be non-empty.
--default-recipient-self
Use the default key as default recipient if option --recipient
is not used and don't ask if this is a valid one. The default
key is the first one from the secret keyring or the one set with
--default-key.
--no-default-recipient
Reset --default-recipient and --default-recipient-self.
-v, --verbose
Give more information during processing. If used twice, the
input data is listed in detail.
--no-verbose
Reset verbose level to 0.
-q, --quiet
Try to be as quiet as possible.
--batch
--no-batch
Use batch mode. Never ask, do not allow interactive commands.
--no-batch disables this option. Note that even with a filename
given on the command line, gpg might still need to read from
STDIN (in particular if gpg figures that the input is a detached
signature and no data file has been specified). Thus if you do
not want to feed data via STDIN, you should connect STDIN to
‘/dev/null’.
--no-tty
Make sure that the TTY (terminal) is never used for any output.
This option is needed in some cases because GnuPG sometimes
prints warnings to the TTY even if --batch is used.
--yes Assume "yes" on most questions.
--no Assume "no" on most questions.
--list-options parameters
This is a space or comma delimited string that gives options
used when listing keys and signatures (that is, --list-keys,
--list-sigs, --list-public-keys, --list-secret-keys, and the
--edit-key functions). Options can be prepended with a no-
(after the two dashes) to give the opposite meaning. The
options are:
show-photos
Causes --list-keys, --list-sigs, --list-public-keys, and
--list-secret-keys to display any photo IDs attached to
the key. Defaults to no. See also --photo-viewer. Does
not work with --with-colons: see --attribute-fd for the
appropriate way to get photo data for scripts and other
frontends.
show-usage
Show usage information for keys and subkeys in the
standard key listing. This is a list of letters
indicating the allowed usage for a key (E=encryption,
S=signing, C=certification, A=authentication). Defaults
to yes.
show-policy-urls
Show policy URLs in the --list-sigs or --check-sigs
listings. Defaults to no.
show-notations
show-std-notations
show-user-notations
Show all, IETF standard, or user-defined signature
notations in the --list-sigs or --check-sigs listings.
Defaults to no.
show-keyserver-urls
Show any preferred keyserver URL in the --list-sigs or
--check-sigs listings. Defaults to no.
show-uid-validity
Display the calculated validity of user IDs during key
listings. Defaults to yes.
show-unusable-uids
Show revoked and expired user IDs in key listings.
Defaults to no.
show-unusable-subkeys
Show revoked and expired subkeys in key listings.
Defaults to no.
show-keyring
Display the keyring name at the head of key listings to
show which keyring a given key resides on. Defaults to
no.
show-sig-expire
Show signature expiration dates (if any) during --list-
sigs or --check-sigs listings. Defaults to no.
show-sig-subpackets
Include signature subpackets in the key listing. This
option can take an optional argument list of the
subpackets to list. If no argument is passed, list all
subpackets. Defaults to no. This option is only
meaningful when using --with-colons along with --list-
sigs or --check-sigs.
--verify-options parameters
This is a space or comma delimited string that gives options
used when verifying signatures. Options can be prepended with a
`no-' to give the opposite meaning. The options are:
show-photos
Display any photo IDs present on the key that issued the
signature. Defaults to no. See also --photo-viewer.
show-policy-urls
Show policy URLs in the signature being verified.
Defaults to yes.
show-notations
show-std-notations
show-user-notations
Show all, IETF standard, or user-defined signature
notations in the signature being verified. Defaults to
IETF standard.
show-keyserver-urls
Show any preferred keyserver URL in the signature being
verified. Defaults to yes.
show-uid-validity
Display the calculated validity of the user IDs on the
key that issued the signature. Defaults to yes.
show-unusable-uids
Show revoked and expired user IDs during signature
verification. Defaults to no.
show-primary-uid-only
Show only the primary user ID during signature
verification. That is all the AKA lines as well as photo
Ids are not shown with the signature verification status.
pka-lookups
Enable PKA lookups to verify sender addresses. Note that
PKA is based on DNS, and so enabling this option may
disclose information on when and what signatures are
verified or to whom data is encrypted. This is similar to
the "web bug" described for the --auto-key-retrieve
option.
pka-trust-increase
Raise the trust in a signature to full if the signature
passes PKA validation. This option is only meaningful if
pka-lookups is set.
--enable-large-rsa
--disable-large-rsa
With --gen-key and --batch, enable the creation of RSA secret
keys as large as 8192 bit. Note: 8192 bit is more than is
generally recommended. These large keys don't significantly
improve security, but they are more expensive to use, and their
signatures and certifications are larger. This option is only
available if the binary was build with large-secmem support.
--enable-dsa2
--disable-dsa2
Enable hash truncation for all DSA keys even for old DSA Keys up
to 1024 bit. This is also the default with --openpgp. Note
that older versions of GnuPG also required this flag to allow
the generation of DSA larger than 1024 bit.
--photo-viewer string
This is the command line that should be run to view a photo ID.
"%i" will be expanded to a filename containing the photo. "%I"
does the same, except the file will not be deleted once the
viewer exits. Other flags are "%k" for the key ID, "%K" for the
long key ID, "%f" for the key fingerprint, "%t" for the
extension of the image type (e.g. "jpg"), "%T" for the MIME type
of the image (e.g. "image/jpeg"), "%v" for the single-character
calculated validity of the image being viewed (e.g. "f"), "%V"
for the calculated validity as a string (e.g. "full"), "%U" for
a base32 encoded hash of the user ID, and "%%" for an actual
percent sign. If neither %i or %I are present, then the photo
will be supplied to the viewer on standard input.
The default viewer is "xloadimage -fork -quiet -title 'KeyID
0x%k' STDIN". Note that if your image viewer program is not
secure, then executing it from GnuPG does not make it secure.
--exec-path string
Sets a list of directories to search for photo viewers and
keyserver helpers. If not provided, keyserver helpers use the
compiled-in default directory, and photo viewers use the PATH
environment variable. Note, that on W32 system this value is
ignored when searching for keyserver helpers.
--keyring file
Add file to the current list of keyrings. If file begins with a
tilde and a slash, these are replaced by the $HOME directory. If
the filename does not contain a slash, it is assumed to be in
the GnuPG home directory ("~/.gnupg" if --homedir or $GNUPGHOME
is not used).
Note that this adds a keyring to the current list. If the intent
is to use the specified keyring alone, use --keyring along with
--no-default-keyring.
If the the option --no-keyring has been used no keyrings will be
used at all.
--secret-keyring file
This is an obsolete option and ignored. All secret keys are
stored in the ‘private-keys-v1.d’ directory below the GnuPG home
directory.
--primary-keyring file
Designate file as the primary public keyring. This means that
newly imported keys (via --import or keyserver --recv-from) will
go to this keyring.
--trustdb-name file
Use file instead of the default trustdb. If file begins with a
tilde and a slash, these are replaced by the $HOME directory. If
the filename does not contain a slash, it is assumed to be in
the GnuPG home directory (‘~/.gnupg’ if --homedir or $GNUPGHOME
is not used).
--homedir dir
Set the name of the home directory to dir. If this option is not
used, the home directory defaults to ‘~/.gnupg’. It is only
recognized when given on the command line. It also overrides
any home directory stated through the environment variable
‘GNUPGHOME’ or (on Windows systems) by means of the Registry
entry HKCU\Software\GNU\GnuPG:HomeDir.
On Windows systems it is possible to install GnuPG as a portable
application. In this case only this command line option is
considered, all other ways to set a home directory are ignored.
To install GnuPG as a portable application under Windows, create
an empty file name ‘gpgconf.ctl’ in the same directory as the
tool ‘gpgconf.exe’. The root of the installation is than that
directory; or, if ‘gpgconf.exe’ has been installed directly
below a directory named ‘bin’, its parent directory. You also
need to make sure that the following directories exist and are
writable: ‘ROOT/home’ for the GnuPG home and
‘ROOT/var/cache/gnupg’ for internal cache files.
--display-charset name
Set the name of the native character set. This is used to
convert some informational strings like user IDs to the proper
UTF-8 encoding. Note that this has nothing to do with the
character set of data to be encrypted or signed; GnuPG does not
recode user-supplied data. If this option is not used, the
default character set is determined from the current locale. A
verbosity level of 3 shows the chosen set. Valid values for
name are:
iso-8859-1
This is the Latin 1 set.
iso-8859-2
The Latin 2 set.
iso-8859-15
This is currently an alias for the Latin 1 set.
koi8-r The usual Russian set (rfc1489).
utf-8 Bypass all translations and assume that the OS uses
native UTF-8 encoding.
--utf8-strings
--no-utf8-strings
Assume that command line arguments are given as UTF8 strings.
The default (--no-utf8-strings) is to assume that arguments are
encoded in the character set as specified by --display-charset.
These options affect all following arguments. Both options may
be used multiple times.
--options file
Read options from file and do not try to read them from the
default options file in the homedir (see --homedir). This option
is ignored if used in an options file.
--no-options
Shortcut for --options /dev/null. This option is detected before
an attempt to open an option file. Using this option will also
prevent the creation of a ‘~/.gnupg’ homedir.
-z n
--compress-level n
--bzip2-compress-level n
Set compression level to n for the ZIP and ZLIB compression
algorithms. The default is to use the default compression level
of zlib (normally 6). --bzip2-compress-level sets the
compression level for the BZIP2 compression algorithm
(defaulting to 6 as well). This is a different option from
--compress-level since BZIP2 uses a significant amount of memory
for each additional compression level. -z sets both. A value of
0 for n disables compression.
--bzip2-decompress-lowmem
Use a different decompression method for BZIP2 compressed files.
This alternate method uses a bit more than half the memory, but
also runs at half the speed. This is useful under extreme low
memory circumstances when the file was originally compressed at
a high --bzip2-compress-level.
--mangle-dos-filenames
--no-mangle-dos-filenames
Older version of Windows cannot handle filenames with more than
one dot. --mangle-dos-filenames causes GnuPG to replace (rather
than add to) the extension of an output filename to avoid this
problem. This option is off by default and has no effect on non-
Windows platforms.
--ask-cert-level
--no-ask-cert-level
When making a key signature, prompt for a certification level.
If this option is not specified, the certification level used is
set via --default-cert-level. See --default-cert-level for
information on the specific levels and how they are used. --no-
ask-cert-level disables this option. This option defaults to no.
--default-cert-level n
The default to use for the check level when signing a key.
0 means you make no particular claim as to how carefully you
verified the key.
1 means you believe the key is owned by the person who claims to
own it but you could not, or did not verify the key at all. This
is useful for a "persona" verification, where you sign the key
of a pseudonymous user.
2 means you did casual verification of the key. For example,
this could mean that you verified the key fingerprint and
checked the user ID on the key against a photo ID.
3 means you did extensive verification of the key. For example,
this could mean that you verified the key fingerprint with the
owner of the key in person, and that you checked, by means of a
hard to forge document with a photo ID (such as a passport) that
the name of the key owner matches the name in the user ID on the
key, and finally that you verified (by exchange of email) that
the email address on the key belongs to the key owner.
Note that the examples given above for levels 2 and 3 are just
that: examples. In the end, it is up to you to decide just what
"casual" and "extensive" mean to you.
This option defaults to 0 (no particular claim).
--min-cert-level
When building the trust database, treat any signatures with a
certification level below this as invalid. Defaults to 2, which
disregards level 1 signatures. Note that level 0 "no particular
claim" signatures are always accepted.
--trusted-key long key ID
Assume that the specified key (which must be given as a full 8
byte key ID) is as trustworthy as one of your own secret keys.
This option is useful if you don't want to keep your secret keys
(or one of them) online but still want to be able to check the
validity of a given recipient's or signator's key.
--trust-model pgp|classic|tofu|tofu+pgp|direct|always|auto
Set what trust model GnuPG should follow. The models are:
pgp This is the Web of Trust combined with trust signatures
as used in PGP 5.x and later. This is the default trust
model when creating a new trust database.
classic
This is the standard Web of Trust as introduced by PGP 2.
tofu
TOFU stands for Trust On First Use. In this trust model,
the first time a key is seen, it is memorized. If later
another key is seen with a user id with the same email
address, a warning is displayed indicating that there is
a conflict and that the key might be a forgery and an
attempt at a man-in-the-middle attack.
Because a potential attacker is able to control the email
address and thereby circumvent the conflict detection
algorithm by using an email address that is similar in
appearance to a trusted email address, whenever a message
is verified, statistics about the number of messages
signed with the key are shown. In this way, a user can
easily identify attacks using fake keys for regular
correspondents.
When compared with the Web of Trust, TOFU offers
significantly weaker security guarantees. In particular,
TOFU only helps ensure consistency (that is, that the
binding between a key and email address doesn't change).
A major advantage of TOFU is that it requires little
maintenance to use correctly. To use the web of trust
properly, you need to actively sign keys and mark users
as trusted introducers. This is a time-consuming process
and anecdotal evidence suggests that even security-
conscious users rarely take the time to do this
thoroughly and instead rely on an ad-hoc TOFU process.
In the TOFU model, policies are associated with bindings
between keys and email addresses (which are extracted
from user ids and normalized). There are five policies,
which can be set manually using the --tofu-policy option.
The default policy can be set using the --tofu-default-
policy policy.
The TOFU policies are: auto, good, unknown, bad and ask.
The auto policy is used by default (unless overridden by
--tofu-default-policy) and marks a binding as marginally
trusted. The good, unknown and bad policies mark a
binding as fully trusted, as having unknown trust or as
having trust never, respectively. The unknown policy is
useful for just using TOFU to detect conflicts, but to
never assign positive trust to a binding. The final
policy, ask prompts the user to indicate the binding's
trust. If batch mode is enabled (or input is
inappropriate in the context), then the user is not
prompted and the undefined trust level is returned.
tofu+pgp
This trust model combines TOFU with the Web of Trust.
This is done by computing the trust level for each model
and then taking the maximum trust level where the trust
levels are ordered as follows: unknown < undefined <
marginal < fully < ultimate < expired < never.
By setting --tofu-default-policy=unknown, this model can
be used to implement the web of trust with TOFU's
conflict detection algorithm, but without its assignment
of positive trust values, which some security-conscious
users don't like.
direct Key validity is set directly by the user and not
calculated via the Web of Trust.
always Skip key validation and assume that used keys are always
fully valid. You generally won't use this unless you are
using some external validation scheme. This option also
suppresses the "[uncertain]" tag printed with signature
checks when there is no evidence that the user ID is
bound to the key. Note that this trust model still does
not allow the use of expired, revoked, or disabled keys.
auto Select the trust model depending on whatever the internal
trust database says. This is the default model if such a
database already exists.
--auto-key-locate parameters
--no-auto-key-locate
GnuPG can automatically locate and retrieve keys as needed using
this option. This happens when encrypting to an email address
(in the "user@example.com" form), and there are no
user@example.com keys on the local keyring. This option takes
any number of the following mechanisms, in the order they are to
be tried:
cert Locate a key using DNS CERT, as specified in rfc4398.
pka Locate a key using DNS PKA.
dane Locate a key using DANE, as specified in draft-ietf-dane-
openpgpkey-05.txt.
wkd Locate a key using the Web Key Directory protocol. This
is an experimental method and semantics may change.
ldap Using DNS Service Discovery, check the domain in question
for any LDAP keyservers to use. If this fails, attempt
to locate the key using the PGP Universal method of
checking 'ldap://keys.(thedomain)'.
keyserver
Locate a key using whatever keyserver is defined using
the --keyserver option.
keyserver-URL
In addition, a keyserver URL as used in the --keyserver
option may be used here to query that particular
keyserver.
local Locate the key using the local keyrings. This mechanism
allows the user to select the order a local key lookup is
done. Thus using '--auto-key-locate local' is identical
to --no-auto-key-locate.
nodefault
This flag disables the standard local key lookup, done
before any of the mechanisms defined by the --auto-key-
locate are tried. The position of this mechanism in the
list does not matter. It is not required if local is
also used.
clear Clear all defined mechanisms. This is useful to override
mechanisms given in a config file.
--auto-key-retrieve
--no-auto-key-retrieve
This option enables the automatic retrieving of keys from a
keyserver when verifying signatures made by keys that are not on
the local keyring.
If the method "wkd" is included in the list of methods given to
auto-key-locate, the Signer's User ID is part of the signature,
and the option --disable-signer-uid is not used, the "wkd"
method may also be used to retrieve a key.
Note that this option makes a "web bug" like behavior possible.
Keyserver or Web Key Directory operators can see which keys you
request, so by sending you a message signed by a brand new key
(which you naturally will not have on your local keyring), the
operator can tell both your IP address and the time when you
verified the signature.
--keyid-format none|short|0xshort|long|0xlong
Select how to display key IDs. "none" does not show the key ID
at all but shows the fingerprint in a separate line. "short" is
the traditional 8-character key ID. "long" is the more accurate
(but less convenient) 16-character key ID. Add an "0x" to
either to include an "0x" at the beginning of the key ID, as in
0x99242560. Note that this option is ignored if the option
--with-colons is used.
--keyserver name
This option is deprecated - please use the --keyserver in
‘dirmngr.conf’ instead.
Use name as your keyserver. This is the server that --recv-keys,
--send-keys, and --search-keys will communicate with to receive
keys from, send keys to, and search for keys on. The format of
the name is a URI: `scheme:[//]keyservername[:port]' The scheme
is the type of keyserver: "hkp" for the HTTP (or compatible)
keyservers, "ldap" for the LDAP keyservers, or "mailto" for the
Graff email keyserver. Note that your particular installation of
GnuPG may have other keyserver types available as well.
Keyserver schemes are case-insensitive. After the keyserver
name, optional keyserver configuration options may be provided.
These are the same as the global --keyserver-options from below,
but apply only to this particular keyserver.
Most keyservers synchronize with each other, so there is
generally no need to send keys to more than one server. The
keyserver hkp://keys.gnupg.net uses round robin DNS to give a
different keyserver each time you use it.
--keyserver-options name=value
This is a space or comma delimited string that gives options for
the keyserver. Options can be prefixed with a `no-' to give the
opposite meaning. Valid import-options or export-options may be
used here as well to apply to importing (--recv-key) or
exporting (--send-key) a key from a keyserver. While not all
options are available for all keyserver types, some common
options are:
include-revoked
When searching for a key with --search-keys, include keys
that are marked on the keyserver as revoked. Note that
not all keyservers differentiate between revoked and
unrevoked keys, and for such keyservers this option is
meaningless. Note also that most keyservers do not have
cryptographic verification of key revocations, and so
turning this option off may result in skipping keys that
are incorrectly marked as revoked.
include-disabled
When searching for a key with --search-keys, include keys
that are marked on the keyserver as disabled. Note that
this option is not used with HKP keyservers.
auto-key-retrieve
This is the same as the option auto-key-retrieve.
honor-keyserver-url
When using --refresh-keys, if the key in question has a
preferred keyserver URL, then use that preferred
keyserver to refresh the key from. In addition, if auto-
key-retrieve is set, and the signature being verified has
a preferred keyserver URL, then use that preferred
keyserver to fetch the key from. Note that this option
introduces a "web bug": The creator of the key can see
when the keys is refreshed. Thus this option is not
enabled by default.
honor-pka-record
If --auto-key-retrieve is used, and the signature being
verified has a PKA record, then use the PKA information
to fetch the key. Defaults to "yes".
include-subkeys
When receiving a key, include subkeys as potential
targets. Note that this option is not used with HKP
keyservers, as they do not support retrieving keys by
subkey id.
timeout
Tell the keyserver helper program how long (in seconds)
to try and perform a keyserver action before giving up.
Note that performing multiple actions at the same time
uses this timeout value per action. For example, when
retrieving multiple keys via --recv-keys, the timeout
applies separately to each key retrieval, and not to the
--recv-keys command as a whole. Defaults to 30 seconds.
http-proxy=value
This options is deprecated. Set the proxy to use for
HTTP and HKP keyservers. This overrides any proxy
defined in ‘dirmngr.conf’.
verbose
This option has no more function since GnuPG 2.1. Use
the dirmngr configuration options instead.
debug This option has no more function since GnuPG 2.1. Use
the dirmngr configuration options instead.
check-cert
This option has no more function since GnuPG 2.1. Use
the dirmngr configuration options instead.
ca-cert-file
This option has no more function since GnuPG 2.1. Use
the dirmngr configuration options instead.
--completes-needed n
Number of completely trusted users to introduce a new key signer
(defaults to 1).
--marginals-needed n
Number of marginally trusted users to introduce a new key signer
(defaults to 3)
--tofu-default-policy auto|good|unknown|bad|ask
The default TOFU policy (defaults to auto). For more
information about the meaning of this option, see: [trust-model-
tofu].
--max-cert-depth n
Maximum depth of a certification chain (default is 5).
--no-sig-cache
Do not cache the verification status of key signatures. Caching
gives a much better performance in key listings. However, if you
suspect that your public keyring is not save against write
modifications, you can use this option to disable the caching.
It probably does not make sense to disable it because all kind
of damage can be done if someone else has write access to your
public keyring.
--auto-check-trustdb
--no-auto-check-trustdb
If GnuPG feels that its information about the Web of Trust has
to be updated, it automatically runs the --check-trustdb command
internally. This may be a time consuming process. --no-auto-
check-trustdb disables this option.
--use-agent
--no-use-agent
This is dummy option. gpg always requires the agent.
--gpg-agent-info
This is dummy option. It has no effect when used with gpg.
--agent-program file
Specify an agent program to be used for secret key operations.
The default value is determined by running gpgconf with the
option --list-dirs. Note that the pipe symbol (|) is used for a
regression test suite hack and may thus not be used in the file
name.
--dirmngr-program file
Specify a dirmngr program to be used for keyserver access. The
default value is ‘/usr/bin/dirmngr’.
--no-autostart
Do not start the gpg-agent or the dirmngr if it has not yet been
started and its service is required. This option is mostly
useful on machines where the connection to gpg-agent has been
redirected to another machines. If dirmngr is required on the
remote machine, it may be started manually using gpgconf
--launch dirmngr.
--lock-once
Lock the databases the first time a lock is requested and do not
release the lock until the process terminates.
--lock-multiple
Release the locks every time a lock is no longer needed. Use
this to override a previous --lock-once from a config file.
--lock-never
Disable locking entirely. This option should be used only in
very special environments, where it can be assured that only one
process is accessing those files. A bootable floppy with a
stand-alone encryption system will probably use this. Improper
usage of this option may lead to data and key corruption.
--exit-on-status-write-error
This option will cause write errors on the status FD to
immediately terminate the process. That should in fact be the
default but it never worked this way and thus we need an option
to enable this, so that the change won't break applications
which close their end of a status fd connected pipe too early.
Using this option along with --enable-progress-filter may be
used to cleanly cancel long running gpg operations.
--limit-card-insert-tries n
With n greater than 0 the number of prompts asking to insert a
smartcard gets limited to N-1. Thus with a value of 1 gpg won't
at all ask to insert a card if none has been inserted at
startup. This option is useful in the configuration file in case
an application does not know about the smartcard support and
waits ad infinitum for an inserted card.
--no-random-seed-file
GnuPG uses a file to store its internal random pool over
invocations. This makes random generation faster; however
sometimes write operations are not desired. This option can be
used to achieve that with the cost of slower random generation.
--no-greeting
Suppress the initial copyright message.
--no-secmem-warning
Suppress the warning about "using insecure memory".
--no-permission-warning
Suppress the warning about unsafe file and home directory
(--homedir) permissions. Note that the permission checks that
GnuPG performs are not intended to be authoritative, but rather
they simply warn about certain common permission problems. Do
not assume that the lack of a warning means that your system is
secure.
Note that the warning for unsafe --homedir permissions cannot be
suppressed in the gpg.conf file, as this would allow an attacker
to place an unsafe gpg.conf file in place, and use this file to
suppress warnings about itself. The --homedir permissions
warning may only be suppressed on the command line.
--no-mdc-warning
Suppress the warning about missing MDC integrity protection.
--require-secmem
--no-require-secmem
Refuse to run if GnuPG cannot get secure memory. Defaults to no
(i.e. run, but give a warning).
--require-cross-certification
--no-require-cross-certification
When verifying a signature made from a subkey, ensure that the
cross certification "back signature" on the subkey is present
and valid. This protects against a subtle attack against
subkeys that can sign. Defaults to --require-cross-
certification for gpg.
--expert
--no-expert
Allow the user to do certain nonsensical or "silly" things like
signing an expired or revoked key, or certain potentially
incompatible things like generating unusual key types. This also
disables certain warning messages about potentially incompatible
actions. As the name implies, this option is for experts only.
If you don't fully understand the implications of what it allows
you to do, leave this off. --no-expert disables this option.
Key related options
--recipient name
-r Encrypt for user id name. If this option or --hidden-recipient
is not specified, GnuPG asks for the user-id unless --default-
recipient is given.
--hidden-recipient name
-R Encrypt for user ID name, but hide the key ID of this user's
key. This option helps to hide the receiver of the message and
is a limited countermeasure against traffic analysis. If this
option or --recipient is not specified, GnuPG asks for the user
ID unless --default-recipient is given.
--recipient-file file
-f This option is similar to --recipient except that it encrypts to
a key stored in the given file. file must be the name of a file
containing exactly one key. gpg assumes that the key in this
file is fully valid.
--hidden-recipient-file file
-F This option is similar to --hidden-recipient except that it
encrypts to a key stored in the given file. file must be the
name of a file containing exactly one key. gpg assumes that the
key in this file is fully valid.
--encrypt-to name
Same as --recipient but this one is intended for use in the
options file and may be used with your own user-id as an
"encrypt-to-self". These keys are only used when there are other
recipients given either by use of --recipient or by the asked
user id. No trust checking is performed for these user ids and
even disabled keys can be used.
--hidden-encrypt-to name
Same as --hidden-recipient but this one is intended for use in
the options file and may be used with your own user-id as a
hidden "encrypt-to-self". These keys are only used when there
are other recipients given either by use of --recipient or by
the asked user id. No trust checking is performed for these
user ids and even disabled keys can be used.
--no-encrypt-to
Disable the use of all --encrypt-to and --hidden-encrypt-to
keys.
--group name=value1
Sets up a named group, which is similar to aliases in email
programs. Any time the group name is a recipient (-r or
--recipient), it will be expanded to the values specified.
Multiple groups with the same name are automatically merged into
a single group.
The values are key IDs or fingerprints, but any key description
is accepted. Note that a value with spaces in it will be treated
as two different values. Note also there is only one level of
expansion --- you cannot make an group that points to another
group. When used from the command line, it may be necessary to
quote the argument to this option to prevent the shell from
treating it as multiple arguments.
--ungroup name
Remove a given entry from the --group list.
--no-groups
Remove all entries from the --group list.
--local-user name
-u Use name as the key to sign with. Note that this option
overrides --default-key.
--sender mbox
This option has two purposes. mbox must either be a complete
user id with a proper mail address or just a mail address. When
creating a signature this option tells gpg the user id of a key
used to make a signature if the key was not directly specified
by a user id. When verifying a signature the mbox is used to
restrict the information printed by the TOFU code to matching
user ids.
--try-secret-key name
For hidden recipients GPG needs to know the keys to use for
trial decryption. The key set with --default-key is always
tried first, but this is often not sufficient. This option
allows setting more keys to be used for trial decryption.
Although any valid user-id specification may be used for name it
makes sense to use at least the long keyid to avoid ambiguities.
Note that gpg-agent might pop up a pinentry for a lot keys to do
the trial decryption. If you want to stop all further trial
decryption you may use close-window button instead of the cancel
button.
--try-all-secrets
Don't look at the key ID as stored in the message but try all
secret keys in turn to find the right decryption key. This
option forces the behaviour as used by anonymous recipients
(created by using --throw-keyids or --hidden-recipient) and
might come handy in case where an encrypted message contains a
bogus key ID.
--skip-hidden-recipients
--no-skip-hidden-recipients
During decryption skip all anonymous recipients. This option
helps in the case that people use the hidden recipients feature
to hide there own encrypt-to key from others. If oneself has
many secret keys this may lead to a major annoyance because all
keys are tried in turn to decrypt something which was not really
intended for it. The drawback of this option is that it is
currently not possible to decrypt a message which includes real
anonymous recipients.
Input and Output
--armor
-a Create ASCII armored output. The default is to create the
binary OpenPGP format.
--no-armor
Assume the input data is not in ASCII armored format.
--output file
-o file
Write output to file.
--max-output n
This option sets a limit on the number of bytes that will be
generated when processing a file. Since OpenPGP supports various
levels of compression, it is possible that the plaintext of a
given message may be significantly larger than the original
OpenPGP message. While GnuPG works properly with such messages,
there is often a desire to set a maximum file size that will be
generated before processing is forced to stop by the OS limits.
Defaults to 0, which means "no limit".
--input-size-hint n
This option can be used to tell GPG the size of the input data
in bytes. n must be a positive base-10 number. This option is
only useful if the input is not taken from a file. GPG may use
thos hint to optimize its buffer allocation strategy. It is
also used by the --status-fd line ``PROGRESS'' to provide a
value for ``total'' if that is not available by other means.
--import-options parameters
This is a space or comma delimited string that gives options for
importing keys. Options can be prepended with a `no-' to give
the opposite meaning. The options are:
import-local-sigs
Allow importing key signatures marked as "local". This is
not generally useful unless a shared keyring scheme is
being used. Defaults to no.
keep-ownertrust
Normally possible still existing ownertrust values of a
key are cleared if a key is imported. This is in general
desirable so that a formerly deleted key does not
automatically gain an ownertrust values merely due to
import. On the other hand it is sometimes necessary to
re-import a trusted set of keys again but keeping already
assigned ownertrust values. This can be achieved by
using this option.
repair-pks-subkey-bug
During import, attempt to repair the damage caused by the
PKS keyserver bug (pre version 0.9.6) that mangles keys
with multiple subkeys. Note that this cannot completely
repair the damaged key as some crucial data is removed by
the keyserver, but it does at least give you back one
subkey. Defaults to no for regular --import and to yes
for keyserver --recv-keys.
import-show
Show a listing of the key as imported right before it is
stored. This can be combined with the option --dry-run
to only look at keys.
import-export
Run the entire import code but instead of storing the key
to the local keyring write it to the output. The export
options export-pka and export-dane affect the output.
This option can be used to remove all invalid parts from
a key without the need to store it.
merge-only
During import, allow key updates to existing keys, but do
not allow any new keys to be imported. Defaults to no.
import-clean
After import, compact (remove all signatures except the
self-signature) any user IDs from the new key that are
not usable. Then, remove any signatures from the new key
that are not usable. This includes signatures that were
issued by keys that are not present on the keyring. This
option is the same as running the --edit-key command
"clean" after import. Defaults to no.
import-minimal
Import the smallest key possible. This removes all
signatures except the most recent self-signature on each
user ID. This option is the same as running the --edit-
key command "minimize" after import. Defaults to no.
--import-filter name=expr
--export-filter name=expr
These options define an import/export filter which are applied
to the imported/exported keyblock right before it will be
stored/written. name defines the type of filter to use, expr
the expression to evaluate. The option can be used several
times which then appends more expression to the same name.
The available filter types are:
keep-uid
This filter will keep a user id packet and its dependent
packets in the keyblock if the expression evaluates to
true.
drop-sig
This filter drops the selected key signatures on user
ids. Self-signatures are not consideres. Currently only
implemented for --import-filter.
For the syntax of the expression see the chapter "FILTER EXPRESSIONS".
The property names for the expressions depend on the actual filter type
and are indicated in the following table.
The available properties are:
uid A string with the user id. (keep-uid)
mbox The addr-spec part of a user id with mailbox or the empty
string. (keep-uid)
primary
Boolean indicating whether the user id is the primary
one. (keep-uid)
sig_created
sig_created_d
The first is the timestamp a signature packet was
created. The second is the same but given as an ISO
string, e.g. "2016-08-17". (drop-sig)
sig_algo
A number with the public key algorithm of a signature
packet. (drop-sig)
sig_digest_algo
A number with the digest algorithm of a signature packet.
(drop-sig)
--export-options parameters
This is a space or comma delimited string that gives options for
exporting keys. Options can be prepended with a `no-' to give
the opposite meaning. The options are:
export-local-sigs
Allow exporting key signatures marked as "local". This is
not generally useful unless a shared keyring scheme is
being used. Defaults to no.
export-attributes
Include attribute user IDs (photo IDs) while exporting.
This is useful to export keys if they are going to be
used by an OpenPGP program that does not accept attribute
user IDs. Defaults to yes.
export-sensitive-revkeys
Include designated revoker information that was marked as
"sensitive". Defaults to no.
export-clean
Compact (remove all signatures from) user IDs on the key
being exported if the user IDs are not usable. Also, do
not export any signatures that are not usable. This
includes signatures that were issued by keys that are not
present on the keyring. This option is the same as
running the --edit-key command "clean" before export
except that the local copy of the key is not modified.
Defaults to no.
export-minimal
Export the smallest key possible. This removes all
signatures except the most recent self-signature on each
user ID. This option is the same as running the --edit-
key command "minimize" before export except that the
local copy of the key is not modified. Defaults to no.
export-pka
Instead of outputting the key material output PKA records
suitable to put into DNS zone files. An ORIGIN line is
printed before each record to allow diverting the records
to the corresponding zone file.
export-dane
Instead of outputting the key material output OpenPGP
DANE records suitable to put into DNS zone files. An
ORIGIN line is printed before each record to allow
diverting the records to the corresponding zone file.
--with-colons
Print key listings delimited by colons. Note that the output
will be encoded in UTF-8 regardless of any --display-charset
setting. This format is useful when GnuPG is called from scripts
and other programs as it is easily machine parsed. The details
of this format are documented in the file ‘doc/DETAILS’, which
is included in the GnuPG source distribution.
--fixed-list-mode
Do not merge primary user ID and primary key in --with-colon
listing mode and print all timestamps as seconds since
1970-01-01. Since GnuPG 2.0.10, this mode is always used and
thus this option is obsolete; it does not harm to use it though.
--legacy-list-mode
Revert to the pre-2.1 public key list mode. This only affects
the human readable output and not the machine interface (i.e.
--with-colons). Note that the legacy format does not convey
suitable information for elliptic curves.
--with-fingerprint
Same as the command --fingerprint but changes only the format of
the output and may be used together with another command.
--with-subkey-fingerprint
If a fingerprint is printed for the primary key, this option
forces printing of the fingerprint for all subkeys. This could
also be achieved by using the --with-fingerprint twice but by
using this option along with keyid-format "none" a compact
fingerprint is printed.
--with-icao-spelling
Print the ICAO spelling of the fingerprint in addition to the
hex digits.
--with-keygrip
Include the keygrip in the key listings. In --with-colons mode
this is implicitly enable for secret keys.
--with-wkd-hash
Print a Web Key Directory indentifier along with each user ID in
key listings. This is an experimental feature and semantics may
change.
--with-secret
Include info about the presence of a secret key in public key
listings done with --with-colons.
OpenPGP protocol specific options.
-t, --textmode
--no-textmode
Treat input files as text and store them in the OpenPGP
canonical text form with standard "CRLF" line endings. This also
sets the necessary flags to inform the recipient that the
encrypted or signed data is text and may need its line endings
converted back to whatever the local system uses. This option is
useful when communicating between two platforms that have
different line ending conventions (UNIX-like to Mac, Mac to
Windows, etc). --no-textmode disables this option, and is the
default.
--force-v3-sigs
--no-force-v3-sigs
--force-v4-certs
--no-force-v4-certs
These options are obsolete and have no effect since GnuPG 2.1.
--force-mdc
Force the use of encryption with a modification detection code.
This is always used with the newer ciphers (those with a
blocksize greater than 64 bits), or if all of the recipient keys
indicate MDC support in their feature flags.
--disable-mdc
Disable the use of the modification detection code. Note that by
using this option, the encrypted message becomes vulnerable to a
message modification attack.
--disable-signer-uid
By default the user ID of the signing key is embedded in the
data signature. As of now this is only done if the signing key
has been specified with local-user using a mail address. This
information can be helpful for verifier to locate the key; see
option --auto-key-retrieve.
--personal-cipher-preferences string
Set the list of personal cipher preferences to string. Use gpg
--version to get a list of available algorithms, and use none to
set no preference at all. This allows the user to safely
override the algorithm chosen by the recipient key preferences,
as GPG will only select an algorithm that is usable by all
recipients. The most highly ranked cipher in this list is also
used for the --symmetric encryption command.
--personal-digest-preferences string
Set the list of personal digest preferences to string. Use gpg
--version to get a list of available algorithms, and use none to
set no preference at all. This allows the user to safely
override the algorithm chosen by the recipient key preferences,
as GPG will only select an algorithm that is usable by all
recipients. The most highly ranked digest algorithm in this
list is also used when signing without encryption (e.g.
--clearsign or --sign).
--personal-compress-preferences string
Set the list of personal compression preferences to string. Use
gpg --version to get a list of available algorithms, and use
none to set no preference at all. This allows the user to
safely override the algorithm chosen by the recipient key
preferences, as GPG will only select an algorithm that is usable
by all recipients. The most highly ranked compression algorithm
in this list is also used when there are no recipient keys to
consider (e.g. --symmetric).
--s2k-cipher-algo name
Use name as the cipher algorithm for symmetric encryption with a
passphrase if --personal-cipher-preferences and --cipher-algo
are not given. The default is AES-128.
--s2k-digest-algo name
Use name as the digest algorithm used to mangle the passphrases
for symmetric encryption. The default is SHA-1.
--s2k-mode n
Selects how passphrases for symmetric encryption are mangled. If
n is 0 a plain passphrase (which is in general not recommended)
will be used, a 1 adds a salt (which should not be used) to the
passphrase and a 3 (the default) iterates the whole process a
number of times (see --s2k-count).
--s2k-count n
Specify how many times the passphrases mangling for symmetric
encryption is repeated. This value may range between 1024 and
65011712 inclusive. The default is inquired from gpg-agent.
Note that not all values in the 1024-65011712 range are legal
and if an illegal value is selected, GnuPG will round up to the
nearest legal value. This option is only meaningful if --s2k-
mode is set to the default of 3.
Compliance options
These options control what GnuPG is compliant to. Only one of these
options may be active at a time. Note that the default setting of this
is nearly always the correct one. See the INTEROPERABILITY WITH OTHER
OPENPGP PROGRAMS section below before using one of these options.
--gnupg
Use standard GnuPG behavior. This is essentially OpenPGP
behavior (see --openpgp), but with some additional workarounds
for common compatibility problems in different versions of PGP.
This is the default option, so it is not generally needed, but
it may be useful to override a different compliance option in
the gpg.conf file.
--openpgp
Reset all packet, cipher and digest options to strict OpenPGP
behavior. Use this option to reset all previous options like
--s2k-*, --cipher-algo, --digest-algo and --compress-algo to
OpenPGP compliant values. All PGP workarounds are disabled.
--rfc4880
Reset all packet, cipher and digest options to strict RFC-4880
behavior. Note that this is currently the same thing as
--openpgp.
--rfc4880bis
Enable experimental features from proposed updates to RFC-4880.
This option can be used in addition to the other compliance
options. Warning: The behavior may change with any GnuPG
release and created keys or data may not be usable with future
GnuPG versions.
--rfc2440
Reset all packet, cipher and digest options to strict RFC-2440
behavior.
--pgp6 Set up all options to be as PGP 6 compliant as possible. This
restricts you to the ciphers IDEA (if the IDEA plugin is
installed), 3DES, and CAST5, the hashes MD5, SHA1 and RIPEMD160,
and the compression algorithms none and ZIP. This also disables
--throw-keyids, and making signatures with signing subkeys as
PGP 6 does not understand signatures made by signing subkeys.
This option implies --disable-mdc --escape-from-lines.
--pgp7 Set up all options to be as PGP 7 compliant as possible. This is
identical to --pgp6 except that MDCs are not disabled, and the
list of allowable ciphers is expanded to add AES128, AES192,
AES256, and TWOFISH.
--pgp8 Set up all options to be as PGP 8 compliant as possible. PGP 8
is a lot closer to the OpenPGP standard than previous versions
of PGP, so all this does is disable --throw-keyids and set
--escape-from-lines. All algorithms are allowed except for the
SHA224, SHA384, and SHA512 digests.
Doing things one usually doesn't want to do.
-n
--dry-run
Don't make any changes (this is not completely implemented).
--list-only
Changes the behaviour of some commands. This is like --dry-run
but different in some cases. The semantic of this command may be
extended in the future. Currently it only skips the actual
decryption pass and therefore enables a fast listing of the
encryption keys.
-i
--interactive
Prompt before overwriting any files.
--debug-level level
Select the debug level for investigating problems. level may be
a numeric value or by a keyword:
none No debugging at all. A value of less than 1 may be used
instead of the keyword.
basic Some basic debug messages. A value between 1 and 2 may
be used instead of the keyword.
advanced
More verbose debug messages. A value between 3 and 5 may
be used instead of the keyword.
expert Even more detailed messages. A value between 6 and 8 may
be used instead of the keyword.
guru All of the debug messages you can get. A value greater
than 8 may be used instead of the keyword. The creation
of hash tracing files is only enabled if the keyword is
used.
How these messages are mapped to the actual debugging flags is not
specified and may change with newer releases of this program. They are
however carefully selected to best aid in debugging.
--debug flags
Set debugging flags. All flags are or-ed and flags may be given
in C syntax (e.g. 0x0042) or as a comma separated list of flag
names. To get a list of all supported flags the single word
"help" can be used.
--debug-all
Set all useful debugging flags.
--debug-iolbf
Set stdout into line buffered mode. This option is only honored
when given on the command line.
--faked-system-time epoch
This option is only useful for testing; it sets the system time
back or forth to epoch which is the number of seconds elapsed
since the year 1970. Alternatively epoch may be given as a full
ISO time string (e.g. "20070924T154812").
--enable-progress-filter
Enable certain PROGRESS status outputs. This option allows
frontends to display a progress indicator while gpg is
processing larger files. There is a slight performance overhead
using it.
--status-fd n
Write special status strings to the file descriptor n. See the
file DETAILS in the documentation for a listing of them.
--status-file file
Same as --status-fd, except the status data is written to file
file.
--logger-fd n
Write log output to file descriptor n and not to STDERR.
--log-file file
--logger-file file
Same as --logger-fd, except the logger data is written to file
file. Note that --log-file is only implemented for GnuPG-2.
--attribute-fd n
Write attribute subpackets to the file descriptor n. This is
most useful for use with --status-fd, since the status messages
are needed to separate out the various subpackets from the
stream delivered to the file descriptor.
--attribute-file file
Same as --attribute-fd, except the attribute data is written to
file file.
--comment string
--no-comments
Use string as a comment string in clear text signatures and
ASCII armored messages or keys (see --armor). The default
behavior is not to use a comment string. --comment may be
repeated multiple times to get multiple comment strings. --no-
comments removes all comments. It is a good idea to keep the
length of a single comment below 60 characters to avoid problems
with mail programs wrapping such lines. Note that comment
lines, like all other header lines, are not protected by the
signature.
--emit-version
--no-emit-version
Force inclusion of the version string in ASCII armored output.
If given once only the name of the program and the major number
is emitted, given twice the minor is also emitted, given triple
the micro is added, and given quad an operating system
identification is also emitted. --no-emit-version (default)
disables the version line.
--sig-notation name=value
--cert-notation name=value
-N, --set-notation name=value
Put the name value pair into the signature as notation data.
name must consist only of printable characters or spaces, and
must contain a '@' character in the form
keyname@domain.example.com (substituting the appropriate keyname
and domain name, of course). This is to help prevent pollution
of the IETF reserved notation namespace. The --expert flag
overrides the '@' check. value may be any printable string; it
will be encoded in UTF8, so you should check that your
--display-charset is set correctly. If you prefix name with an
exclamation mark (!), the notation data will be flagged as
critical (rfc4880:5.2.3.16). --sig-notation sets a notation for
data signatures. --cert-notation sets a notation for key
signatures (certifications). --set-notation sets both.
There are special codes that may be used in notation names. "%k"
will be expanded into the key ID of the key being signed, "%K"
into the long key ID of the key being signed, "%f" into the
fingerprint of the key being signed, "%s" into the key ID of the
key making the signature, "%S" into the long key ID of the key
making the signature, "%g" into the fingerprint of the key
making the signature (which might be a subkey), "%p" into the
fingerprint of the primary key of the key making the signature,
"%c" into the signature count from the OpenPGP smartcard, and
"%%" results in a single "%". %k, %K, and %f are only meaningful
when making a key signature (certification), and %c is only
meaningful when using the OpenPGP smartcard.
--sig-policy-url string
--cert-policy-url string
--set-policy-url string
Use string as a Policy URL for signatures (rfc4880:5.2.3.20).
If you prefix it with an exclamation mark (!), the policy URL
packet will be flagged as critical. --sig-policy-url sets a
policy url for data signatures. --cert-policy-url sets a policy
url for key signatures (certifications). --set-policy-url sets
both.
The same %-expandos used for notation data are available here as
well.
--sig-keyserver-url string
Use string as a preferred keyserver URL for data signatures. If
you prefix it with an exclamation mark (!), the keyserver URL
packet will be flagged as critical.
The same %-expandos used for notation data are available here as
well.
--set-filename string
Use string as the filename which is stored inside messages.
This overrides the default, which is to use the actual filename
of the file being encrypted. Using the empty string for string
effectively removes the filename from the output.
--for-your-eyes-only
--no-for-your-eyes-only
Set the `for your eyes only' flag in the message. This causes
GnuPG to refuse to save the file unless the --output option is
given, and PGP to use a "secure viewer" with a claimed Tempest-
resistant font to display the message. This option overrides
--set-filename. --no-for-your-eyes-only disables this option.
--use-embedded-filename
--no-use-embedded-filename
Try to create a file with a name as embedded in the data. This
can be a dangerous option as it enables overwriting files.
Defaults to no.
--cipher-algo name
Use name as cipher algorithm. Running the program with the
command --version yields a list of supported algorithms. If this
is not used the cipher algorithm is selected from the
preferences stored with the key. In general, you do not want to
use this option as it allows you to violate the OpenPGP
standard. --personal-cipher-preferences is the safe way to
accomplish the same thing.
--digest-algo name
Use name as the message digest algorithm. Running the program
with the command --version yields a list of supported
algorithms. In general, you do not want to use this option as it
allows you to violate the OpenPGP standard. --personal-digest-
preferences is the safe way to accomplish the same thing.
--compress-algo name
Use compression algorithm name. "zlib" is RFC-1950 ZLIB
compression. "zip" is RFC-1951 ZIP compression which is used by
PGP. "bzip2" is a more modern compression scheme that can
compress some things better than zip or zlib, but at the cost of
more memory used during compression and decompression.
"uncompressed" or "none" disables compression. If this option is
not used, the default behavior is to examine the recipient key
preferences to see which algorithms the recipient supports. If
all else fails, ZIP is used for maximum compatibility.
ZLIB may give better compression results than ZIP, as the
compression window size is not limited to 8k. BZIP2 may give
even better compression results than that, but will use a
significantly larger amount of memory while compressing and
decompressing. This may be significant in low memory situations.
Note, however, that PGP (all versions) only supports ZIP
compression. Using any algorithm other than ZIP or "none" will
make the message unreadable with PGP. In general, you do not
want to use this option as it allows you to violate the OpenPGP
standard. --personal-compress-preferences is the safe way to
accomplish the same thing.
--cert-digest-algo name
Use name as the message digest algorithm used when signing a
key. Running the program with the command --version yields a
list of supported algorithms. Be aware that if you choose an
algorithm that GnuPG supports but other OpenPGP implementations
do not, then some users will not be able to use the key
signatures you make, or quite possibly your entire key.
--disable-cipher-algo name
Never allow the use of name as cipher algorithm. The given name
will not be checked so that a later loaded algorithm will still
get disabled.
--disable-pubkey-algo name
Never allow the use of name as public key algorithm. The given
name will not be checked so that a later loaded algorithm will
still get disabled.
--throw-keyids
--no-throw-keyids
Do not put the recipient key IDs into encrypted messages. This
helps to hide the receivers of the message and is a limited
countermeasure against traffic analysis. ([Using a little social
engineering anyone who is able to decrypt the message can check
whether one of the other recipients is the one he suspects.])
On the receiving side, it may slow down the decryption process
because all available secret keys must be tried. --no-throw-
keyids disables this option. This option is essentially the same
as using --hidden-recipient for all recipients.
--not-dash-escaped
This option changes the behavior of cleartext signatures so that
they can be used for patch files. You should not send such an
armored file via email because all spaces and line endings are
hashed too. You can not use this option for data which has 5
dashes at the beginning of a line, patch files don't have this.
A special armor header line tells GnuPG about this cleartext
signature option.
--escape-from-lines
--no-escape-from-lines
Because some mailers change lines starting with "From " to
">From " it is good to handle such lines in a special way when
creating cleartext signatures to prevent the mail system from
breaking the signature. Note that all other PGP versions do it
this way too. Enabled by default. --no-escape-from-lines
disables this option.
--passphrase-repeat n
Specify how many times gpg will request a new passphrase be
repeated. This is useful for helping memorize a passphrase.
Defaults to 1 repetition.
--passphrase-fd n
Read the passphrase from file descriptor n. Only the first line
will be read from file descriptor n. If you use 0 for n, the
passphrase will be read from STDIN. This can only be used if
only one passphrase is supplied.
Note that this passphrase is only used if the option --batch has
also been given. This is different from GnuPG version 1.x.
--passphrase-file file
Read the passphrase from file file. Only the first line will be
read from file file. This can only be used if only one
passphrase is supplied. Obviously, a passphrase stored in a file
is of questionable security if other users can read this file.
Don't use this option if you can avoid it. Note that this
passphrase is only used if the option --batch has also been
given. This is different from GnuPG version 1.x.
--passphrase string
Use string as the passphrase. This can only be used if only one
passphrase is supplied. Obviously, this is of very questionable
security on a multi-user system. Don't use this option if you
can avoid it. Note that this passphrase is only used if the
option --batch has also been given. This is different from
GnuPG version 1.x.
--pinentry-mode mode
Set the pinentry mode to mode. Allowed values for mode are:
default
Use the default of the agent, which is ask.
ask Force the use of the Pinentry.
cancel Emulate use of Pinentry's cancel button.
error Return a Pinentry error (``No Pinentry'').
loopback
Redirect Pinentry queries to the caller. Note that in
contrast to Pinentry the user is not prompted again if he
enters a bad password.
--command-fd n
This is a replacement for the deprecated shared-memory IPC mode.
If this option is enabled, user input on questions is not
expected from the TTY but from the given file descriptor. It
should be used together with --status-fd. See the file
doc/DETAILS in the source distribution for details on how to use
it.
--command-file file
Same as --command-fd, except the commands are read out of file
file
--allow-non-selfsigned-uid
--no-allow-non-selfsigned-uid
Allow the import and use of keys with user IDs which are not
self-signed. This is not recommended, as a non self-signed user
ID is trivial to forge. --no-allow-non-selfsigned-uid disables.
--allow-freeform-uid
Disable all checks on the form of the user ID while generating a
new one. This option should only be used in very special
environments as it does not ensure the de-facto standard format
of user IDs.
--ignore-time-conflict
GnuPG normally checks that the timestamps associated with keys
and signatures have plausible values. However, sometimes a
signature seems to be older than the key due to clock problems.
This option makes these checks just a warning. See also
--ignore-valid-from for timestamp issues on subkeys.
--ignore-valid-from
GnuPG normally does not select and use subkeys created in the
future. This option allows the use of such keys and thus
exhibits the pre-1.0.7 behaviour. You should not use this option
unless there is some clock problem. See also --ignore-time-
conflict for timestamp issues with signatures.
--ignore-crc-error
The ASCII armor used by OpenPGP is protected by a CRC checksum
against transmission errors. Occasionally the CRC gets mangled
somewhere on the transmission channel but the actual content
(which is protected by the OpenPGP protocol anyway) is still
okay. This option allows GnuPG to ignore CRC errors.
--ignore-mdc-error
This option changes a MDC integrity protection failure into a
warning. This can be useful if a message is partially corrupt,
but it is necessary to get as much data as possible out of the
corrupt message. However, be aware that a MDC protection
failure may also mean that the message was tampered with
intentionally by an attacker.
--allow-weak-digest-algos
Signatures made with known-weak digest algorithms are normally
rejected with an ``invalid digest algorithm'' message. This
option allows the verification of signatures made with such weak
algorithms. MD5 is the only digest algorithm considered weak by
default. See also --weak-digest to reject other digest
algorithms.
--weak-digest name
Treat the specified digest algorithm as weak. Signatures made
over weak digests algorithms are normally rejected. This option
can be supplied multiple times if multiple algorithms should be
considered weak. See also --allow-weak-digest-algos to disable
rejection of weak digests. MD5 is always considered weak, and
does not need to be listed explicitly.
--no-default-keyring
Do not add the default keyrings to the list of keyrings. Note
that GnuPG will not operate without any keyrings, so if you use
this option and do not provide alternate keyrings via --keyring
or --secret-keyring, then GnuPG will still use the default
public or secret keyrings.
--no-keyring
Do not add use any keyrings even if specified as options.
--skip-verify
Skip the signature verification step. This may be used to make
the decryption faster if the signature verification is not
needed.
--with-key-data
Print key listings delimited by colons (like --with-colons) and
print the public key data.
--fast-list-mode
Changes the output of the list commands to work faster; this is
achieved by leaving some parts empty. Some applications don't
need the user ID and the trust information given in the
listings. By using this options they can get a faster listing.
The exact behaviour of this option may change in future
versions. If you are missing some information, don't use this
option.
--no-literal
This is not for normal use. Use the source to see for what it
might be useful.
--set-filesize
This is not for normal use. Use the source to see for what it
might be useful.
--show-session-key
Display the session key used for one message. See --override-
session-key for the counterpart of this option.
We think that Key Escrow is a Bad Thing; however the user should
have the freedom to decide whether to go to prison or to reveal
the content of one specific message without compromising all
messages ever encrypted for one secret key.
You can also use this option if you receive an encrypted message
which is abusive or offensive, to prove to the administrators of
the messaging system that the ciphertext transmitted corresponds
to an inappropriate plaintext so they can take action against
the offending user.
--override-session-key string
Don't use the public key but the session key string. The format
of this string is the same as the one printed by --show-session-
key. This option is normally not used but comes handy in case
someone forces you to reveal the content of an encrypted
message; using this option you can do this without handing out
the secret key.
--ask-sig-expire
--no-ask-sig-expire
When making a data signature, prompt for an expiration time. If
this option is not specified, the expiration time set via
--default-sig-expire is used. --no-ask-sig-expire disables this
option.
--default-sig-expire
The default expiration time to use for signature expiration.
Valid values are "0" for no expiration, a number followed by the
letter d (for days), w (for weeks), m (for months), or y (for
years) (for example "2m" for two months, or "5y" for five
years), or an absolute date in the form YYYY-MM-DD. Defaults to
"0".
--ask-cert-expire
--no-ask-cert-expire
When making a key signature, prompt for an expiration time. If
this option is not specified, the expiration time set via
--default-cert-expire is used. --no-ask-cert-expire disables
this option.
--default-cert-expire
The default expiration time to use for key signature expiration.
Valid values are "0" for no expiration, a number followed by the
letter d (for days), w (for weeks), m (for months), or y (for
years) (for example "2m" for two months, or "5y" for five
years), or an absolute date in the form YYYY-MM-DD. Defaults to
"0".
--allow-secret-key-import
This is an obsolete option and is not used anywhere.
--allow-multiple-messages
--no-allow-multiple-messages
Allow processing of multiple OpenPGP messages contained in a
single file or stream. Some programs that call GPG are not
prepared to deal with multiple messages being processed
together, so this option defaults to no. Note that versions of
GPG prior to 1.4.7 always allowed multiple messages.
Warning: Do not use this option unless you need it as a
temporary workaround!
--enable-special-filenames
This options enables a mode in which filenames of the form
‘-&n’, where n is a non-negative decimal number, refer to the
file descriptor n and not to a file with that name.
--no-expensive-trust-checks
Experimental use only.
--preserve-permissions
Don't change the permissions of a secret keyring back to user
read/write only. Use this option only if you really know what
you are doing.
--default-preference-list string
Set the list of default preferences to string. This preference
list is used for new keys and becomes the default for "setpref"
in the edit menu.
--default-keyserver-url name
Set the default keyserver URL to name. This keyserver will be
used as the keyserver URL when writing a new self-signature on a
key, which includes key generation and changing preferences.
--list-config
Display various internal configuration parameters of GnuPG. This
option is intended for external programs that call GnuPG to
perform tasks, and is thus not generally useful. See the file
‘doc/DETAILS’ in the source distribution for the details of
which configuration items may be listed. --list-config is only
usable with --with-colons set.
--list-gcrypt-config
Display various internal configuration parameters of Libgcrypt.
--gpgconf-list
This command is similar to --list-config but in general only
internally used by the gpgconf tool.
--gpgconf-test
This is more or less dummy action. However it parses the
configuration file and returns with failure if the configuration
file would prevent gpg from startup. Thus it may be used to run
a syntax check on the configuration file.
Deprecated options
--show-photos
--no-show-photos
Causes --list-keys, --list-sigs, --list-public-keys, --list-
secret-keys, and verifying a signature to also display the photo
ID attached to the key, if any. See also --photo-viewer. These
options are deprecated. Use --list-options [no-]show-photos
and/or --verify-options [no-]show-photos instead.
--show-keyring
Display the keyring name at the head of key listings to show
which keyring a given key resides on. This option is deprecated:
use --list-options [no-]show-keyring instead.
--always-trust
Identical to --trust-model always. This option is deprecated.
--show-notation
--no-show-notation
Show signature notations in the --list-sigs or --check-sigs
listings as well as when verifying a signature with a notation
in it. These options are deprecated. Use --list-options
[no-]show-notation and/or --verify-options [no-]show-notation
instead.
--show-policy-url
--no-show-policy-url
Show policy URLs in the --list-sigs or --check-sigs listings as
well as when verifying a signature with a policy URL in it.
These options are deprecated. Use --list-options [no-]show-
policy-url and/or --verify-options [no-]show-policy-url instead.
EXAMPLES
gpg -se -r Bob file
sign and encrypt for user Bob
gpg --clearsign file
make a clear text signature
gpg -sb file
make a detached signature
gpg -u 0x12345678 -sb file
make a detached signature with the key 0x12345678
gpg --list-keys user_ID
show keys
gpg --fingerprint user_ID
show fingerprint
gpg --verify pgpfile
gpg --verify sigfile
Verify the signature of the file but do not output the data. The
second form is used for detached signatures, where sigfile is
the detached signature (either ASCII armored or binary) and are
the signed data; if this is not given, the name of the file
holding the signed data is constructed by cutting off the
extension (".asc" or ".sig") of sigfile or by asking the user
for the filename.
HOW TO SPECIFY A USER ID
There are different ways to specify a user ID to GnuPG. Some of them
are only valid for gpg others are only good for gpgsm. Here is the
entire list of ways to specify a key:
By key Id.
This format is deduced from the length of the string and its
content or 0x prefix. The key Id of an X.509 certificate are the
low 64 bits of its SHA-1 fingerprint. The use of key Ids is
just a shortcut, for all automated processing the fingerprint
should be used.
When using gpg an exclamation mark (!) may be appended to force
using the specified primary or secondary key and not to try and
calculate which primary or secondary key to use.
The last four lines of the example give the key ID in their long
form as internally used by the OpenPGP protocol. You can see the
long key ID using the option --with-colons.
234567C4
0F34E556E
01347A56A
0xAB123456
234AABBCC34567C4
0F323456784E56EAB
01AB3FED1347A5612
0x234AABBCC34567C4
By fingerprint.
This format is deduced from the length of the string and its
content or the 0x prefix. Note, that only the 20 byte version
fingerprint is available with gpgsm (i.e. the SHA-1 hash of the
certificate).
When using gpg an exclamation mark (!) may be appended to force
using the specified primary or secondary key and not to try and
calculate which primary or secondary key to use.
The best way to specify a key Id is by using the fingerprint.
This avoids any ambiguities in case that there are duplicated
key IDs.
1234343434343434C434343434343434
123434343434343C3434343434343734349A3434
0E12343434343434343434EAB3484343434343434
0xE12343434343434343434EAB3484343434343434
gpgsm also accepts colons between each pair of hexadecimal digits
because this is the de-facto standard on how to present X.509
fingerprints. gpg also allows the use of the space separated SHA-1
fingerprint as printed by the key listing commands.
By exact match on OpenPGP user ID.
This is denoted by a leading equal sign. It does not make sense
for X.509 certificates.
=Heinrich Heine <heinrichh@uni-duesseldorf.de>
By exact match on an email address.
This is indicated by enclosing the email address in the usual
way with left and right angles.
<heinrichh@uni-duesseldorf.de>
By partial match on an email address.
This is indicated by prefixing the search string with an @.
This uses a substring search but considers only the mail address
(i.e. inside the angle brackets).
@heinrichh
By exact match on the subject's DN.
This is indicated by a leading slash, directly followed by the
RFC-2253 encoded DN of the subject. Note that you can't use the
string printed by "gpgsm --list-keys" because that one as been
reordered and modified for better readability; use --with-colons
to print the raw (but standard escaped) RFC-2253 string
/CN=Heinrich Heine,O=Poets,L=Paris,C=FR
By exact match on the issuer's DN.
This is indicated by a leading hash mark, directly followed by a
slash and then directly followed by the rfc2253 encoded DN of
the issuer. This should return the Root cert of the issuer.
See note above.
#/CN=Root Cert,O=Poets,L=Paris,C=FR
By exact match on serial number and issuer's DN.
This is indicated by a hash mark, followed by the hexadecimal
representation of the serial number, then followed by a slash
and the RFC-2253 encoded DN of the issuer. See note above.
#4F03/CN=Root Cert,O=Poets,L=Paris,C=FR
By keygrip
This is indicated by an ampersand followed by the 40 hex digits
of a keygrip. gpgsm prints the keygrip when using the command
--dump-cert. It does not yet work for OpenPGP keys.
&D75F22C3F86E355877348498CDC92BD21010A480
By substring match.
This is the default mode but applications may want to explicitly
indicate this by putting the asterisk in front. Match is not
case sensitive.
Heine
*Heine
. and + prefixes
These prefixes are reserved for looking up mails anchored at the
end and for a word search mode. They are not yet implemented
and using them is undefined.
Please note that we have reused the hash mark identifier which
was used in old GnuPG versions to indicate the so called local-
id. It is not anymore used and there should be no conflict when
used with X.509 stuff.
Using the RFC-2253 format of DNs has the drawback that it is not
possible to map them back to the original encoding, however we
don't have to do this because our key database stores this
encoding as meta data.
FILTER EXPRESSIONS
The options --import-filter and --export-filter use expressions with
this syntax (square brackets indicate an optional part and curly braces
a repetition, white space between the elements are allowed):
[lc] {[{flag}] PROPNAME op VALUE [lc]}
The name of a property (PROPNAME) may only consist of letters, digits
and underscores. The description for the filter type describes which
properties are defined. If an undefined property is used it evaluates
to the empty string. Unless otherwise noted, the VALUE must always be
given and may not be the empty string. No quoting is defined for the
value, thus the value may not contain the strings && or ||, which are
used as logical connection operators. The flag -- can be used to
remove this restriction.
Numerical values are computed as long int; standard C notation applies.
lc is the logical connection operator; either && for a conjunction or
|| for a disjunction. A conjunction is assumed at the begin of an
expression. Conjunctions have higher precedence than disjunctions. If
VALUE starts with one of the characters used in any op a space after
the op is required.
The supported operators (op) are:
=~ Substring must match.
!~ Substring must not match.
= The full string must match.
<> The full string must not match.
== The numerical value must match.
!= The numerical value must not match.
<= The numerical value of the field must be LE than the value.
< The numerical value of the field must be LT than the value.
>= The numerical value of the field must be GT than the value.
>= The numerical value of the field must be GE than the value.
-n True if value is not empty (no value allowed).
-z True if value is empty (no value allowed).
-t Alias for "PROPNAME != 0" (no value allowed).
-f Alias for "PROPNAME == 0" (no value allowed).
Values for flag must be space separated. The supported flags are:
-- VALUE spans to the end of the expression.
-c The string match in this part is done case-sensitive.
The filter options concatenate several specifications for a filter of
the same type. For example the four options in this example:
--import-option keep-uid="uid =~ Alfa"
--import-option keep-uid="&& uid !~ Test"
--import-option keep-uid="|| uid =~ Alpha"
--import-option keep-uid="uid !~ Test"
which is equivalent to
--import-option \
keep-uid="uid =~ Alfa" && uid !~ Test" || uid =~ Alpha" && "uid !~ Test"
imports only the user ids of a key containing the strings "Alfa" or
"Alpha" but not the string "test".
FILES
There are a few configuration files to control certain aspects of gpg's
operation. Unless noted, they are expected in the current home
directory (see: [option --homedir]).
gpg.conf
This is the standard configuration file read by gpg on startup.
It may contain any valid long option; the leading two dashes may
not be entered and the option may not be abbreviated. This
default name may be changed on the command line (see: [gpg-
option --options]). You should backup this file.
Note that on larger installations, it is useful to put predefined files
into the directory ‘/etc/skel/.gnupg’ so that newly created users start
up with a working configuration. For existing users a small helper
script is provided to create these files (see: [addgnupghome]).
For internal purposes gpg creates and maintains a few other files; They
all live in in the current home directory (see: [option --homedir]).
Only the gpg program may modify these files.
~/.gnupg
This is the default home directory which is used if neither the
environment variable GNUPGHOME nor the option --homedir is
given.
~/.gnupg/pubring.gpg
The public keyring. You should backup this file.
~/.gnupg/pubring.gpg.lock
The lock file for the public keyring.
~/.gnupg/pubring.kbx
The public keyring using a different format. This file is
sharred with gpgsm. You should backup this file.
~/.gnupg/pubring.kbx.lock
The lock file for ‘pubring.kbx’.
~/.gnupg/secring.gpg
A secret keyring as used by GnuPG versions before 2.1. It is
not used by GnuPG 2.1 and later.
~/.gnupg/secring.gpg.lock
The lock file for the secret keyring.
~/.gnupg/.gpg-v21-migrated
File indicating that a migration to GnuPG 2.1 has been done.
~/.gnupg/trustdb.gpg
The trust database. There is no need to backup this file; it is
better to backup the ownertrust values (see: [option --export-
ownertrust]).
~/.gnupg/trustdb.gpg.lock
The lock file for the trust database.
~/.gnupg/random_seed
A file used to preserve the state of the internal random pool.
~/.gnupg/openpgp-revocs.d/
This is the directory where gpg stores pre-generated revocation
certificates. The file name corresponds to the OpenPGP
fingerprint of the respective key. It is suggested to backup
those certificates and if the primary private key is not stored
on the disk to move them to an external storage device. Anyone
who can access theses files is able to revoke the corresponding
key. You may want to print them out. You should backup all
files in this directory and take care to keep this backup closed
away.
/usr/share/gnupg/options.skel
The skeleton options file.
Operation is further controlled by a few environment variables:
HOME Used to locate the default home directory.
GNUPGHOME
If set directory used instead of "~/.gnupg".
GPG_AGENT_INFO
This variable is obsolete; it was used by GnuPG versions before
2.1.
PINENTRY_USER_DATA
This value is passed via gpg-agent to pinentry. It is useful to
convey extra information to a custom pinentry.
COLUMNS
LINES Used to size some displays to the full size of the screen.
LANGUAGE
Apart from its use by GNU, it is used in the W32 version to
override the language selection done through the Registry. If
used and set to a valid and available language name (langid),
the file with the translation is loaded from
gpgdir/gnupg.nls/langid.mo. Here gpgdir is the directory out of
which the gpg binary has been loaded. If it can't be loaded the
Registry is tried and as last resort the native Windows locale
system is used.
BUGS
On older systems this program should be installed as setuid(root). This is necessary to lock memory pages. Locking memory pages prevents the operating system from writing memory pages (which may contain passphrases or other sensitive material) to disk. If you get no warning message about insecure memory your operating system supports locking without being root. The program drops root privileges as soon as locked memory is allocated. Note also that some systems (especially laptops) have the ability to ``suspend to disk'' (also known as ``safe sleep'' or ``hibernate''). This writes all memory to disk before going into a low power or even powered off mode. Unless measures are taken in the operating system to protect the saved memory, passphrases or other sensitive material may be recoverable from it later. Before you report a bug you should first search the mailing list archives for similar problems and second check whether such a bug has already been reported to our bug tracker at http://bugs.gnupg.org .
SEE ALSO
gpgv(1), gpgsm(1), gpg-agent(1) The full documentation for this tool is maintained as a Texinfo manual. If GnuPG and the info program are properly installed at your site, the command info gnupg should give you access to the complete manual including a menu structure and an index.
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
