Linux Blog


Section: GNU Privacy Guard (1)
Updated: 2007-12-03
Index Return to Main Contents


gpg-agent - Secret key management for GnuPG  


gpg-agent [--homedir dir] [--options file] [options]
gpg-agent [--homedir dir] [--options file] [options] --server
gpg-agent [--homedir dir] [--options file] [options] --daemon [command_line]



gpg-agent is a daemon to manage secret (private) keys independently from any protocol. It is used as a backend for gpg and gpgsm as well as for a couple of other utilities.

The usual way to run the agent is from the ~/.xsession file:

eval `gpg-agent --daemon`

If you don't use an X server, you can also put this into your regular startup file ~/.profile or .bash_profile. It is best not to run multiple instance of the gpg-agent, so you should make sure that only one is running: gpg-agent uses an environment variable to inform clients about the communication parameters. You can write the content of this environment variable to a file so that you can test for a running agent. This short script may do the job:

if test -f $HOME/.gpg-agent-info &&    kill -0 `cut -d: -f 2 $HOME/.gpg-agent-info` 2>/dev/null; then
     GPG_AGENT_INFO=`cat $HOME/.gpg-agent-info`
     export GPG_AGENT_INFO   
     eval `gpg-agent --daemon`
     echo $GPG_AGENT_INFO >$HOME/.gpg-agent-info

Note that the new option --write-env-file may be used instead.

You should always add the following lines to your .bashrc or whatever initialization file is used for all shell invocations:

export GPG_TTY

It is important that this environment variable always reflects the output of the tty command. For W32 systems this option is not required.

Please make sure that a proper pinentry program has been installed under the default filename (which is system dependant) or use the option pinentry-pgm to specify the full name of that program. It is often useful to install a symbolic link from the actual used pinentry (e.g. `/usr/bin/pinentry-gtk') to the expected one (e.g. `/usr/bin/pinentry').



Commands are not distinguished from options execpt for the fact that only one one command is allowed.

Print the program version and licensing information. Not that you can abbreviate this command.

Print a usage message summarizing the most useful command-line options. Not that you can abbreviate this command.

Print a list of all available options and commands. Not that you can abbreviate this command.

Run in server mode and wait for commands on the stdin. The default mode is to create a socket and listen for commands there.

--daemon [command line]
Run the program in the background. This option is required to prevent it from being accidently running in the background. A common way to do this is:

$ eval `gpg-agent --daemon`



--options file
Reads configuration from file instead of from the default per-user configuration file. The default configuration file is named `gpg-agent.conf' and expected in the `.gnupg' directory directly below the home directory of the user.

--homedir dir
Set the name of the home directory to dir. If his 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 W32 systems) by means on the Registry entry HKCU\Software\GNU\GnuPG:HomeDir.

Outputs additional information while running. You can increase the verbosity by giving several verbose commands to gpgsm, such as '-vv'.

Try to be as quiet as possible.

Don't invoke a pinentry or do any other thing requiring human interaction.

--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.

--debug-level level
Select the debug level for investigating problems. level may be one of:

   .TP none
   no debugging at all.
   .TP basic
   some basic debug messages
   .TP advanced
   more verbose debug messages
   .TP expert
   even more detailed messages
   .TP guru
   all of the debug messages you can get

How these messages are mapped to the actual debugging flags is not specified and may change with newer releaes of this program. They are however carefully selected to best aid in debugging.

--debug flags
This option is only useful for debugging and the behaviour may change at any time without notice. FLAGS are bit encoded and may be given in usual C-Syntax. The currently defined bits are:

0 (1)
X.509 or OpenPGP protocol related data
1 (2)
values of big number integers
2 (4)
low level crypto operations
5 (32)
memory allocation
6 (64)
7 (128)
show memory statistics.
9 (512)
write hashed data to files named dbgmd-000*
10 (1024)
trace Assuan protocol
12 (4096)
bypass all certificate validation

Same as --debug=0xffffffff

--debug-wait n
When running in server mode, wait n seconds before entering the actual processing loop and print the pid. This gives time to attach a debugger.

Don't detach the process from the console. This is manly usefule for debugging.

Format the info output in daemon mode for use with the standard Bourne shell respective the C-shell . The default is to guess it based on the environment variable SHELL which is in almost all cases sufficient.

--write-env-file file
Often it is required to connect to the agent from a process not being an inferior of gpg-agent and thus the environment variable with the socket name is not available. To help setting up those variables in other sessions, this option may be used to write the information into file. If file is not specified the default name `${HOME}/.gpg-agent-info' will be used. The format is suitable to be evaluated by a Bourne shell like in this simple example:

eval `cat file`
eval `cut -d= -f 1 < file | xargs echo export`

Tell the pinentryo not to grab the keyboard and mouse. This option should in general not be used to avaoid X-sniffing attacks.

--log-file file
Append all logging output to file. This is very helpful in seeing what the agent actually does.

Allow clients to mark keys as trusted, i.e. put them into the `trustlist.txt' file. This is by default not allowed to make it harder for users to inadvertly accept Root-CA keys.

This option will let gpg-agent bypass the passphrase cache for all signing operation. Note that there is also a per-session option to control this behaviour but this command line option takes precedence.

--default-cache-ttl n
Set the time a cache entry is valid to n seconds. The default are 600 seconds.

--default-cache-ttl-ssh n
Set the time a cache entry used for SSH keys is valid to n seconds. The default are 1800 seconds.

--max-cache-ttl n
Set the maximum time a cache entry is valid to n seconds. After this time a cache entry will get expired even if it has been accessed recently. The default are 2 hours (7200 seconds).

--max-cache-ttl-ssh n
Set the maximum time a cache entry used for SSH keys is valid to n seconds. After this time a cache entry will get expired even if it has been accessed recently. The default are 2 hours (7200 seconds).

Enforce the passphrase constraints by not allowing the user to bypass them using the ``Take it anyway'' button.

--min-passphrase-len n
Set the minimal length of a passphrase. When entering a new passphrase shorter than this value a warning will be displayed. Defaults to 8.

--min-passphrase-nonalpha n
Set the minimal number of digits or special characters required in a passphrase. When entering a new passphrase with less than this number of digits or special characters a warning will be displayed. Defaults to 1.

--check-passphrase-pattern file
Check the passphrase against the pattern given in file. When entering a new passphrase matching one of these pattern a warning will be displayed. file should be an absolute filename. The default is not to use any pattern file.

Security note: It is known that checking a passphrase against a list of pattern or even against a complete dictionary is not very effective to enforce good passphrases. Users will soon figure up ways to bypass such a policy. A better policy is to educate users on good security behavior and optional to run a passphrase cracker regularly on all users passphrases t catch the very simple ones.

--max-passphrase-days n
Ask the user to change the passphrase if n days have passed since the last change. With --enforce-passphrase-constraints set the user may not bypass this check.

This option does nothing yet.

--pinentry-program filename
Use program filename as the PIN entry. The default is installation dependend and can be shown with the --version command.

--pinentry-touch-file filename
By default the file name of the socket gpg-agent is listening for requests is passed to Pinentry, so that it can touch that file before exiting (it does this only in curses mode). This option changes the file passed to Pinentry to filename. The special name /dev/null may be used to completely disable this feature. Note that Pinentry will not create that file, it will only change the modification and access time.

--scdaemon-program filename
Use program filename as the Smartcard daemon. The default is installation dependend and can be shown with the --version command.

Do not make use of the scdaemon tool. This option has the effect of disabling the ability to do smartcard operations. Note, that enabling this option at runtime does not kill an already forked scdaemon.

By enabling this option gpg-agent will listen on the socket named `S.gpg-agent', located in the home directory, and not create a random socket below a temporary directory. Tools connecting to gpg-agent should first try to connect to the socket given in environment variable GPG_AGENT_INFO and the fall back to this socket. This option may not be used if the home directory is mounted as a remote file system. Note, that --use-standard-socket is the default on Windows systems.

--display string
--ttyname string
--ttytype string
--lc-type string
--lc-messages string
These options are used with the server mode to pass localization information.

Ignore requests to change change the current tty respective the X window system's DISPLAY variable. This is useful to lock the pinentry to pop up at the tty or display you started the agent.


Enable emulation of the OpenSSH Agent protocol.

In this mode of operation, the agent does not only implement the gpg-agent protocol, but also the agent protocol used by OpenSSH (through a seperate socket). Consequently, it should possible to use the gpg-agent as a drop-in replacement for the well known ssh-agent.

SSH Keys, which are to be used through the agent, need to be added to the gpg-agent initially through the ssh-add utility. When a key is added, ssh-add will ask for the password of the provided key file and send the unprotected key material to the agent; this causes the gpg-agent to ask for a passphrase, which is to be used for encrypting the newly received key and storing it in a gpg-agent specific directory.

Once, a key has been added to the gpg-agent this way, the gpg-agent will be ready to use the key.

Note: in case the gpg-agent receives a signature request, the user might need to be prompted for a passphrase, which is necessary for decrypting the stored key. Since the ssh-agent protocol does not contain a mechanism for telling the agent on which display/terminal it is running, gpg-agent's ssh-support will use the TTY or X display where gpg-agent has been started. To switch this display to the current one, the follwing command may be used:

echo UPDATESTARTUPTTY | gpg-connect-agent

All the long options may also be given in the configuration file after stripping off the two leading dashes.



The usual way to invoke gpg-agent is

$ eval `gpg-agent --daemon`

An alternative way is by replacing ssh-agent with gpg-agent. If for example ssh-agent is started as part of the Xsession intialization you may simply replace ssh-agent by a script like:


exec /usr/local/bin/gpg-agent --enable-ssh-support --daemon       --write-env-file ${HOME}/.gpg-agent-info "$@"

and add something like (for Bourne shells)

  if [ -f "${HOME}/.gpg-agent-info" ]; then
    . "${HOME}/.gpg-agent-info"
    export GPG_AGENT_INFO
    export SSH_AUTH_SOCK
    export SSH_AGENT_PID

to your shell initialization file (e.g. `~/.bashrc').



There are a few configuration files needed for the operation of the agent. By default they may all be found in the current home directory (see: [option --homedir]).


  This is the standard configuration file read by gpg-agent 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 file is also read after a SIGHUP however only a few
  options will actually have an effect.  This default name may be
  changed on the command line (see: [option --options]).


  This is the list of trusted keys.  Comment lines, indicated by a leading
  hash mark, as well as empty lines are ignored.  To mark a key as trusted
  you need to enter its fingerprint followed by a space and a capital
  letter S.  Colons may optionally be used to separate the bytes of
  a fingerprint; this allows to cut and paste the fingerprint from a key
  listing output.
  Here is an example where two keys are marked as ultimately trusted:
  .RS 2
  # CN=Wurzel ZS 3,O=Intevation GmbH,C=DE
  A6935DD34EF3087973C706FC311AA2CCF733765B S
  # CN=PCA-1-Verwaltung-02/O=PKI-1-Verwaltung/C=DE
  DC:BD:69:25:48:BD:BB:7E:31:6E:BB:80:D3:00:80:35:D4:F8:A6:CD S 
Before entering a key into this file, you need to ensure its
authenticity.  How to do this depends on your organisation; your
administrator might have already entered those keys which are deemed
trustworthy enough into this file.  Places where to look for the
fingerprint of a root certificate are letters received from the CA or
the website of the CA (after making 100% sure that this is indeed the
website of that CA).  You may want to consider allowing interactive
updates of this file by using the see: [option --allow-mark-trusted].
This is however not as secure as maintaining this file manually.  It is
even advisable to change the permissions to read-only so that this file
can't be changed inadvertently.

As a special feature a line include-default will include a global
list of trusted certificates (e.g. `/etc/gnupg/trustlist.txt').
This global list is also used if the local list is not available.

It is possible to add further flags after the S for use by the

Relax checking of some root certificate requirements. This is for example required if the certificate is missing the basicConstraints attribute (despite that it is a MUST for CA certificates).
If validation of a certificate finally issued by a CA with this flag set fails, try again using the chain validation model.
This file is used when support for the secure shell agent protocol has been enabled (see: [option --enable-ssh-support]). Only keys present in this file are used in the SSH protocol. The ssh-add tool y be used to add new entries to this file; you may also add them manually. Comment lines, indicated by a leading hash mark, as well as empty lines are ignored. An entry starts with optional white spaces, followed by the keygrip of the key given as 40 hex digits, optionally followed by the caching TTL in seconds and another optional field for arbitrary flags. A ! may be prepended to the keygrip to disable this entry. The follwoing example lists exactly one key. Note that keys available through a OpenPGP smartcard in the active smartcard reader are implictly added to this list; i.e. there is no need to list them. .RS 2 # Key added on 2005-02-25 15:08:29 5A6592BF45DC73BD876874A28FD4639282E29B52 0 .fi
This is the directory where gpg-agent stores the private keys. Each key is stored in a file with the name made up of the keygrip and the suffix `key'. 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 the a small helper script is provied to create these files (see: [addgnupghome]).


A running gpg-agent may be controlled by signals, i.e. using the kill command to send a signal to the process.

Here is a list of supported signals:

This signal flushes all chached passphrases and if the program has been started with a configuration file, the configuration file is read again. Only certain options are honored: quiet, verbose, debug, debug-all, debug-level, no-grab, pinentry-program, default-cache-ttl, max-cache-ttl, ignore-cache-for-signing, allow-mark-trusted and disable-scdaemon. scdaemon-program is also supported but due to the current implementation, which calls the scdaemon only once, it is not of much use unless you manually kill the scdaemon.

Shuts down the process but waits until all current requests are fulfilled. If the process has received 3 of these signals and requests are still pending, a shutdown is forced.

Shuts down the process immediately.

Dump internal information to the log file.

This signal is used for internal purposes.



gpg2(1), gpgsm(1), gpg-connect-agent(1), scdaemon(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.




Random Man Pages: