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2.6.2 Signing a Hash

The client ask the agent to sign a given hash value. A default key will be chosen if no key has been set. To set a key a client first uses:

        SIGKEY <keyGrip>

This can be used multiple times to create multiple signature, the list of keys is reset with the next PKSIGN command or a RESET. The server test whether the key is a valid key to sign something and responds with okay.

        SETHASH --hash=<name>|<algo> <hexstring>

The client can use this command to tell the server about the data <hexstring> (which usually is a hash) to be signed. <algo> is the decimal encoded hash algorithm number as used by Libgcrypt. Either <algo> or –hash=<name> must be given. Valid names for <name> are:

sha1
sha256
rmd160
md5
tls-md5sha1

The actual signing is done using

        PKSIGN <options>

Options are not yet defined, but my later be used to choose among different algorithms. The agent does then some checks, asks for the passphrase and as a result the server returns the signature as an SPKI like S-expression in "D" lines:

          (sig-val
            (<algo>
              (<param_name1> <mpi>)
      	   ...
              (<param_namen> <mpi>)))

The operation is affected by the option

        OPTION use-cache-for-signing=0|1

The default of 1 uses the cache. Setting this option to 0 will lead gpg-agent to ignore the passphrase cache. Note, that there is also a global command line option for gpg-agent to globally disable the caching.

Here is an example session:

        C: SIGKEY <keyGrip>
        S: OK key available
        C: SIGKEY <keyGrip>
        S: OK key available
        C: PKSIGN
        S: # I did ask the user whether he really wants to sign
        S: # I did ask the user for the passphrase
        S: INQUIRE HASHVAL
        C: D ABCDEF012345678901234
        C: END
        S: # signature follows
        S: D (sig-val rsa (s 45435453654612121212))
        S: OK