Lutz Donnerhacke, lutz(_at_)taranis(_dot_)iks-jena(_dot_)de, writes:
Where is the symmetric algorithm of an encrypted messages defined in the
OpenPGP message format?
Do you mean, in a public key encrypted message, where is it specified what
conventional (symmetric) algorithm is used to encrypt the message itself?
This is in section 5.1:
The encrypted value "m" in the above formulas is derived from the
session key as follows. First the session key is prepended with a
one-octet algorithm identifier that specifies the conventional
encryption algorithm used to encrypt the following Symmetrically
Encrypted Data Packet.
This one-octet algorithm identifier is what tells the symmetric algorithm
that is used to encrypt the body of the message. It is only visible once
the ESK packet has been decrypted. Third parties can't tell what algorithm
was used to encrypt the message.
Hal