File: gnupg.info,  Node: Agent PKDECRYPT,  Next: Agent PKSIGN,  Up: Agent Protocol

2.6.1 Decrypting a session key
------------------------------

The client asks the server to decrypt a session key.  The encrypted
session key should have all information needed to select the appropriate
secret key or to delegate it to a smartcard.

       SETKEY 

   Tell the server about the key to be used for decryption.  If this is
not used, 'gpg-agent' may try to figure out the key by trying to decrypt
the message with each key available.

       PKDECRYPT

   The agent checks whether this command is allowed and then does an
INQUIRY to get the ciphertext the client should then send the cipher
text.

         S: INQUIRE CIPHERTEXT
         C: D (xxxxxx
         C: D xxxx)
         C: END

   Please note that the server may send status info lines while reading
the data lines from the client.  The data send is a SPKI like S-Exp with
this structure:

          (enc-val
            (
              ( )
      	   ...
              ( )))

   Where algo is a string with the name of the algorithm; see the
libgcrypt documentation for a list of valid algorithms.  The number and
names of the parameters depend on the algorithm.  The agent does return
an error if there is an inconsistency.

   If the decryption was successful the decrypted data is returned by
means of "D" lines.

   Here is an example session:
        C: PKDECRYPT
        S: INQUIRE CIPHERTEXT
        C: D (enc-val elg (a 349324324)
        C: D    (b 3F444677CA)))
        C: END
        S: # session key follows
        S: S PADDING 0
        S: D (value 1234567890ABCDEF0)
        S: OK decryption successful

   The “PADDING” status line is only send if gpg-agent can tell what
kind of padding is used.  As of now only the value 0 is used to indicate
that the padding has been removed.