Session Key Questions

Werner Koch wk at
Wed Sep 17 16:41:38 CEST 2008

On Wed, 17 Sep 2008 15:52, kevhilton at said:

> 1. How is the session key generated?  How is its entropy randomness
> determined?  Is there a specific algorithm used to generate the key?

It is a random number of course:

   This random number generator is modelled after the one described in
   Peter Gutmann's paper: "Software Generation of Practically Strong
   Random Numbers". See also chapter 6 in his book "Cryptographic
   Security Architecture", New York, 2004, ISBN 0-387-95387-6.

> 2. Once generated, Im confused how its used.  When I use the gpg
> --show-session-key option I receive:
> gpg: session key:
> `9:EB7DFF392EA4EDBC90A8836F82462CD0E0B5AB22D49141941CE252311ECD2D9C'

That one is the encrypted using the public key algorithm (RSA or
Elgamal) and prepended to the messaage as described in rfc4880.

> 3. Is it possible to decrypt a gnupg encrypted message if I know the
> decrypted session key?  How could this be accomplished?

Yes, use:

  --override-session-key string

    Don't use the public key but the session key string.  The format of
    this string is the same as the one printed by --show-session-key.
    This option is normally not used but comes handy in case someone
    forces you to reveal the content of an encrypted message; using this
    option you can do this without handing out the secret key.



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