How to decrypt a message using only session key?

Question

I can view a session key to the encrypted data using option gpg --show-session-key <file>.
Knowing the session key, how can i decrypt the data without using my private key?

Answer 1

To encrypt a file for a recipient:

$ gpg -ea -r [email protected] < file > file.gpg

To get the session key for an encrypted file when you have the private key of the recipient:

$ gpg --show-session-key < file.gpg
...
gpg: session key: `9:901D6ED579AFF935F9F157A5198BCE48B50AD87345DEADBA06F42C5D018C78CC'
...

To use this session key to decrypt the file, without needing the private key:

$ gpg --override-session-key 9:901D6ED579AFF935F9F157A5198BCE48B50AD87345DEADBA06F42C5D018C78CC -d < file.gpg

This information was obtained by simply having a look at the man page for gpg:

--show-session-key
   Display the session key used for one message. 
   See --override-session-key for the counterpart of this option
   ...
--override-session-key string
  Don't use the public key but the session key string.