|GPGSM(1)||GNU Privacy Guard 2.2||GPGSM(1)|
gpgsm - CMS encryption and signing tool
gpgsm [--homedir dir] [--options file] [options] command [args]
gpgsm is a tool similar to gpg to provide digital encryption and signing services on X.509 certificates and the CMS protocol. It is mainly used as a backend for S/MIME mail processing. gpgsm includes a full featured certificate management and complies with all rules defined for the German Sphinx project.
Commands are not distinguished from options except for the fact that only one command is allowed.
- 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.
- Print warranty information. Note that you cannot abbreviate this command.
- Print a list of all available options and commands. Note that you cannot abbreviate this command.
- Perform an encryption. The keys the data is encrypted to must be set using the option --recipient.
- Perform a decryption; the type of input is automatically determined. It may either be in binary form or PEM encoded; automatic determination of base-64 encoding is not done.
- Create a digital signature. The key used is either the fist one found in the keybox or those set with the --local-user option.
- Check a signature file for validity. Depending on the arguments a detached signature may also be checked.
- Run in server mode and wait for commands on the stdin.
- --call-dirmngr command [args]
- Behave as a Dirmngr client issuing the request command with the
optional list of args. The output of the Dirmngr is printed stdout.
Please note that file names given as arguments should have an absolute
file name (i.e. commencing with /) because they are passed verbatim
to the Dirmngr and the working directory of the Dirmngr might not be the
same as the one of this client. Currently it is not possible to pass data
via stdin to the Dirmngr. command should not contain spaces.
This is command is required for certain maintaining tasks of the dirmngr where a dirmngr must be able to call back to gpgsm. See the Dirmngr manual for details.
- --call-protect-tool arguments
- Certain maintenance operations are done by an external program call gpg-protect-tool; this is usually not installed in a directory listed in the PATH variable. This command provides a simple wrapper to access this tool. arguments are passed verbatim to this command; use ‘--help’ to get a list of supported operations.
- This command allows the creation of a certificate signing request or a self-signed certificate. It is commonly used along with the --output option to save the created CSR or certificate into a file. If used with the --batch a parameter file is used to create the CSR or certificate and it is further possible to create non-self-signed certificates.
- List all available certificates stored in the local key database. Note that the displayed data might be reformatted for better human readability and illegal characters are replaced by safe substitutes.
- List all available certificates for which a corresponding a secret key is available.
- --list-external-keys pattern
- List certificates matching pattern using an external server. This utilizes the dirmngr service.
- Same as --list-keys but also prints all keys making up the chain.
- List all available certificates stored in the local key database using a format useful mainly for debugging.
- Same as --dump-keys but also prints all keys making up the chain.
- List all available certificates for which a corresponding a secret key is available using a format useful mainly for debugging.
- --dump-external-keys pattern
- List certificates matching pattern using an external server. This utilizes the dirmngr service. It uses a format useful mainly for debugging.
- This is a debugging aid to reset certain flags in the key database which are used to cache certain certificate stati. It is especially useful if a bad CRL or a weird running OCSP responder did accidentally revoke certificate. There is no security issue with this command because gpgsm always make sure that the validity of a certificate is checked right before it is used.
- --delete-keys pattern
- Delete the keys matching pattern. Note that there is no command to delete the secret part of the key directly. In case you need to do this, you should run the command gpgsm --dump-secret-keys KEYID before you delete the key, copy the string of hex-digits in the ``keygrip'' line and delete the file consisting of these hex-digits and the suffix .key from the ‘private-keys-v1.d’ directory below our GnuPG home directory (usually ‘~/.gnupg’).
- --export [pattern]
- Export all certificates stored in the Keybox or those specified by the optional pattern. Those pattern consist of a list of user ids (see: [how-to-specify-a-user-id]). When used along with the --armor option a few informational lines are prepended before each block. There is one limitation: As there is no commonly agreed upon way to pack more than one certificate into an ASN.1 structure, the binary export (i.e. without using armor) works only for the export of one certificate. Thus it is required to specify a pattern which yields exactly one certificate. Ephemeral certificate are only exported if all pattern are given as fingerprints or keygrips.
- --export-secret-key-p12 key-id
- Export the private key and the certificate identified by key-id using the PKCS#12 format. When used with the --armor option a few informational lines are prepended to the output. Note, that the PKCS#12 format is not very secure and proper transport security should be used to convey the exported key. (See: [option --p12-charset].)
- --export-secret-key-p8 key-id
- --export-secret-key-raw key-id
- Export the private key of the certificate identified by key-id with any encryption stripped. The ...-raw command exports in PKCS#1 format; the ...-p8 command exports in PKCS#8 format. When used with the --armor option a few informational lines are prepended to the output. These commands are useful to prepare a key for use on a TLS server.
- --import [files]
- Import the certificates from the PEM or binary encoded files as well as from signed-only messages. This command may also be used to import a secret key from a PKCS#12 file.
- Read information about the private keys from the smartcard and import the certificates from there. This command utilizes the gpg-agent and in turn the scdaemon.
- --change-passphrase user_id
- --passwd user_id
- Change the passphrase of the private key belonging to the certificate specified as user_id. Note, that changing the passphrase/PIN of a smartcard is not yet supported.
GPGSM features a bunch of options to control the exact behaviour and to change the default configuration.
These options are used to change the configuration and are usually found in the option file.
- --options file
- Reads configuration from file instead of from the default per-user configuration file. The default configuration file is named ‘gpgsm.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 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 named ‘gpgconf.ctl’ in the same directory as the tool ‘gpgconf.exe’. The root of the installation is then 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/usr/var/cache/gnupg’ for internal cache files.
- Outputs additional information while running. You can increase the verbosity by giving several verbose commands to gpgsm, such as ‘-vv’.
- --keyserver string
- This is a deprecated option. It was used to add an LDAP server to use for
X.509 certificate and CRL lookup. The alias --ldapserver existed
from version 2.2.28 to 2.2.33 but is now entirely ignored.
LDAP servers must be given in the configuration for dirmngr.
- --policy-file filename
- Change the default name of the policy file to filename.
- --agent-program file
- Specify an agent program to be used for secret key operations. The default value is determined by running the command gpgconf. 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 CRL checks. The default value is ‘/usr/bin/dirmngr’.
- This option is obsolete and ignored.
- Entirely disable the use of the Dirmngr.
- 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.
- Do not print a warning when the so called "secure memory" cannot be used.
- --log-file file
- When running in server mode, append all logging output to file. Use ‘socket://’ to log to socket.
- By default policy checks are enabled. These options may be used to change it.
- By default the CRL checks are enabled and the DirMngr is used to check for revoked certificates. The disable option is most useful with an off-line network connection to suppress this check and also to avoid that new certificates introduce a web bug by including a certificate specific CRL DP. The disable option also disables an issuer certificate lookup via the authorityInfoAccess property of the certificate; the --enable-issuer-key-retrieve can be used to make use of that property anyway.
- By default the CRL for trusted root certificates are checked like for any other certificates. This allows a CA to revoke its own certificates voluntary without the need of putting all ever issued certificates into a CRL. The disable option may be used to switch this extra check off. Due to the caching done by the Dirmngr, there will not be any noticeable performance gain. Note, that this also disables possible OCSP checks for trusted root certificates. A more specific way of disabling this check is by adding the ``relax'' keyword to the root CA line of the ‘trustlist.txt’
- Tell the dirmngr to reload the CRL for each request. For better performance, the dirmngr will actually optimize this by suppressing the loading for short time intervals (e.g. 30 minutes). This option is useful to make sure that a fresh CRL is available for certificates hold in the keybox. The suggested way of doing this is by using it along with the option --with-validation for a key listing command. This option should not be used in a configuration file.
- Run a CRL check even for certificates which do not have any CRL distribution point. This requires that a suitable LDAP server has been configured in Dirmngr and that the CRL can be found using the issuer. This option reverts to what GnuPG did up to version 2.2.20. This option is in general not useful.
- By default OCSP checks are disabled. The enable option may be used to enable OCSP checks via Dirmngr. If CRL checks are also enabled, CRLs will be used as a fallback if for some reason an OCSP request will not succeed. Note, that you have to allow OCSP requests in Dirmngr's configuration too (option --allow-ocsp) and configure Dirmngr properly. If you do not do so you will get the error code ‘Not supported’.
- If a required certificate is missing while validating the chain of certificates, try to load that certificate from an external location. This usually means that Dirmngr is employed to search for the certificate. Note that this option makes a "web bug" like behavior possible. LDAP server 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 keybox), the operator can tell both your IP address and the time when you verified the signature.
- --validation-model name
- This option changes the default validation model. The only possible values are "shell" (which is the default), "chain" which forces the use of the chain model and "steed" for a new simplified model. The chain model is also used if an option in the ‘trustlist.txt’ or an attribute of the certificate requests it. However the standard model (shell) is in that case always tried first.
- --ignore-cert-extension oid
- Add oid to the list of ignored certificate extensions. The oid is expected to be in dotted decimal form, like 18.104.22.168. This option may be used more than once. Critical flagged certificate extensions matching one of the OIDs in the list are treated as if they are actually handled and thus the certificate will not be rejected due to an unknown critical extension. Use this option with care because extensions are usually flagged as critical for a reason.
- Create PEM encoded output. Default is binary output.
- Create Base-64 encoded output; i.e. PEM without the header lines.
- Assume the input data is PEM encoded. Default is to autodetect the encoding but this is may fail.
- Assume the input data is plain base-64 encoded.
- Assume the input data is binary encoded.
- --p12-charset name
- gpgsm uses the UTF-8 encoding when encoding passphrases for PKCS#12 files. This option may be used to force the passphrase to be encoded in the specified encoding name. This is useful if the application used to import the key uses a different encoding and thus will not be able to import a file generated by gpgsm. Commonly used values for name are Latin1 and CP850. Note that gpgsm itself automagically imports any file with a passphrase encoded to the most commonly used encodings.
- --default-key user_id
- Use user_id as the standard key for signing. This key is used if no other key has been defined as a signing key. Note, that the first --local-users option also sets this key if it has not yet been set; however --default-key always overrides this.
- --local-user user_id
- -u user_id
- Set the user(s) to be used for signing. The default is the first secret key found in the database.
- --recipient name
- Encrypt to the user id name. There are several ways a user id may be given (see: [how-to-specify-a-user-id]).
- --output file
- -o file
- Write output to file. The default is to write it to stdout.
- Displays extra information with the --list-keys commands. Especially a line tagged grp is printed which tells you the keygrip of a key. This string is for example used as the file name of the secret key. Implies --with-colons.
- When doing a key listing, do a full validation check for each key and
print the result. This is usually a slow operation because it requires a
CRL lookup and other operations.
When used along with --import, a validation of the certificate to import is done and only imported if it succeeds the test. Note that this does not affect an already available certificate in the DB. This option is therefore useful to simply verify a certificate.
- For standard key listings, also print the MD5 fingerprint of the certificate.
- Include the keygrip in standard key listings. Note that the keygrip is always listed in --with-colons mode.
- Include info about the presence of a secret key in public key listings done with --with-colons.
- --include-certs n
- Using n of -2 includes all certificate except for the root cert, -1 includes all certs, 0 does not include any certs, 1 includes only the signers cert and all other positive values include up to n certificates starting with the signer cert. The default is -2.
- --cipher-algo oid
- Use the cipher algorithm with the ASN.1 object identifier oid for encryption. For convenience the strings 3DES, AES and AES256 may be used instead of their OIDs. The default is AES (2.16.822.214.171.124.4.1.2).
- --digest-algo name
- Use name as the message digest algorithm. Usually this algorithm is deduced from the respective signing certificate. This option forces the use of the given algorithm and may lead to severe interoperability problems.
- --extra-digest-algo name
- Sometimes signatures are broken in that they announce a different digest algorithm than actually used. gpgsm uses a one-pass data processing model and thus needs to rely on the announced digest algorithms to properly hash the data. As a workaround this option may be used to tell gpgsm to also hash the data using the algorithm name; this slows processing down a little bit but allows verification of such broken signatures. If gpgsm prints an error like ``digest algo 8 has not been enabled'' you may want to try this option, with ‘SHA256’ for name.
- --compliance string
- Set the compliance mode. Valid values are shown when using "help" for string.
- --min-rsa-length n
- This option adjusts the compliance mode "de-vs" for stricter key size requirements. For example, a value of 3000 turns rsa2048 and dsa2048 keys into non-VS-NfD compliant keys.
- To check that data has been encrypted according to the rules of the current compliance mode, a gpgsm user needs to evaluate the status lines. This is allows frontends to handle compliance check in a more flexible way. However, for scripted use the required evaluation of the status-line requires quite some effort; this option can be used instead to make sure that the gpgsm process exits with a failure if the compliance rules are not fulfilled. Note that this option has currently an effect only in "de-vs" mode.
- --ignore-cert-with-oid oid
- Add oid to the list of OIDs to be checked while reading certificates from smartcards. The oid is expected to be in dotted decimal form, like 126.96.36.199. This option may be used more than once. As of now certificates with an extended key usage matching one of those OIDs are ignored during a --learn-card operation and not imported. This option can help to keep the local key database clear of unneeded certificates stored on smartcards.
- --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").
- Include ephemeral flagged keys in the output of key listings. Note that they are included anyway if the key specification for a listing is given as fingerprint or keygrip.
- --compatibility-flags flags
- Set compatibility flags to work around problems due to non-compliant certificates or data. The flags are given as a comma separated list of flag names and are OR-ed together. The special flag "none" clears the list and allows to start over with an empty list. To get a list of available flags the sole word "help" can be used.
- --debug-level level
- Select the debug level for investigating problems. level may be a numeric value or by a keyword:
- No debugging at all. A value of less than 1 may be used instead of the keyword.
- Some basic debug messages. A value between 1 and 2 may be used instead of the keyword.
- More verbose debug messages. A value between 3 and 5 may be used instead of the keyword.
- Even more detailed messages. A value between 6 and 8 may be used instead of the keyword.
- 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
- This option is only useful for debugging and the behaviour may change at any time without notice; using --debug-levels is the preferred method to select the debug verbosity. 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
Note, that all flags set using this option may get overridden by --debug-level.
- Same as --debug=0xffffffff
- Usually gpgsm tries to avoid dumping core by well written code and by disabling core dumps for security reasons. However, bugs are pretty durable beasts and to squash them it is sometimes useful to have a core dump. This option enables core dumps unless the Bad Thing happened before the option parsing.
- This is actually not a debugging option but only useful as such. It lets gpgsm bypass all certificate chain validation checks.
- This is actually not a debugging option but only useful as such. It lets gpgsm ignore all notAfter dates, this is used by the regression tests.
- --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.
- --pinentry-mode mode
- Set the pinentry mode to mode. Allowed values for mode are:
- Use the default of the agent, which is ask.
- Force the use of the Pinentry.
- Emulate use of Pinentry's cancel button.
- Return a Pinentry error (``No Pinentry'').
- Redirect Pinentry queries to the caller. Note that in contrast to Pinentry the user is not prompted again if he enters a bad password.
- --request-origin origin
- Tell gpgsm to assume that the operation ultimately originated at origin. Depending on the origin certain restrictions are applied and the Pinentry may include an extra note on the origin. Supported values for origin are: local which is the default, remote to indicate a remote origin or browser for an operation requested by a web browser.
- Suppress the import of common certificates on keybox creation.
All the long options may also be given in the configuration file after stripping off the two leading dashes.
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 <firstname.lastname@example.org>
- By exact match on an email address.
- This is indicated by enclosing the email address in the usual way with left and right angles.
- 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).
- 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 has been reordered and modified for better readability; use --with-colons to print the raw (but standard escaped) RFC-2253 string.
- 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 RFC-2253 encoded DN of the issuer. This should return the Root cert of the issuer. See note above.
- 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.
- 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.
- 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.
- . 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
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.
$ gpgsm -er email@example.com <plaintext >ciphertext
There are a few configuration files to control certain aspects of gpgsm's operation. Unless noted, they are expected in the current home directory (see: [option --homedir]).
- This is the standard configuration file read by gpgsm 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: [gpgsm-option --options]). You should backup this file.
- This is a list of allowed CA policies. This file should list the object
identifiers of the policies line by line. Empty lines and lines starting
with a hash mark are ignored. Policies missing in this file and not marked
as critical in the certificate will print only a warning; certificates
with policies marked as critical and not listed in this file will fail the
signature verification. You should backup this file.
For example, to allow only the policy 2.289.9.9, the file should look like this:
# Allowed policies 2.289.9.9
- This is the list of root certificates used for qualified certificates.
They are defined as certificates capable of creating legally binding
signatures in the same way as handwritten signatures are. Comments start
with a hash mark and empty lines are ignored. Lines do have a length limit
but this is not a serious limitation as the format of the entries is fixed
and checked by gpgsm: A non-comment line starts with optional
whitespace, followed by exactly 40 hex characters, white space and a
lowercased 2 letter country code. Additional data delimited with by a
white space is current ignored but might late be used for other purposes.
Note that even if a certificate is listed in this file, this does not mean that the certificate is trusted; in general the certificates listed in this file need to be listed also in ‘trustlist.txt’.
This is a global file an installed in the data directory (e.g. ‘/usr/share/gnupg/qualified.txt’). GnuPG installs a suitable file with root certificates as used in Germany. As new Root-CA certificates may be issued over time, these entries may need to be updated; new distributions of this software should come with an updated list but it is still the responsibility of the Administrator to check that this list is correct.
Every time gpgsm uses a certificate for signing or verification this file will be consulted to check whether the certificate under question has ultimately been issued by one of these CAs. If this is the case the user will be informed that the verified signature represents a legally binding (``qualified'') signature. When creating a signature using such a certificate an extra prompt will be issued to let the user confirm that such a legally binding signature shall really be created.
Because this software has not yet been approved for use with such certificates, appropriate notices will be shown to indicate this fact.
- This is plain text file with a few help entries used with pinentry as well as a large list of help items for gpg and gpgsm. The standard file has English help texts; to install localized versions use filenames like ‘help.LL.txt’ with LL denoting the locale. GnuPG comes with a set of predefined help files in the data directory (e.g. ‘/usr/share/gnupg/gnupg/help.de.txt’) and allows overriding of any help item by help files stored in the system configuration directory (e.g. ‘/etc/gnupg/help.de.txt’). For a reference of the help file's syntax, please see the installed ‘help.txt’ file.
- This file is a collection of common certificates used to populated a newly created ‘pubring.kbx’. An administrator may replace this file with a custom one. The format is a concatenation of PEM encoded X.509 certificates. This global file is installed in the data directory (e.g. ‘/usr/share/gnupg/com-certs.pem’).
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 gpgsm creates and maintains a few other files; they all live in the current home directory (see: [option --homedir]). Only gpgsm may modify these files.
- This a database file storing the certificates as well as meta information. For debugging purposes the tool kbxutil may be used to show the internal structure of this file. You should backup this file.
- This content of this file is used to maintain the internal state of the random number generator across invocations. The same file is used by other programs of this software too.
- If this file exists gpgsm will first try to connect to this socket for accessing gpg-agent before starting a new gpg-agent instance. Under Windows this socket (which in reality be a plain file describing a regular TCP listening port) is the standard way of connecting the gpg-agent.
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
should give you access to the complete manual including a menu structure and an index.