There is one mention (in the first paragraph of 5.3) of a Conventional
Encrypted Session Key packet.
But otherwise, the old 2.6.2 "pgp -c" type encryption is left unaddressed.
We don't discuss command line arguments in this spec. The old style
conventional encryption is a bare Symmetrically Encrypted Data Packet
(section 5.7):
The conventional cipher used may be specified in an Encrypted Session
Key or Conventional Encrypted Session Key packet which precedes the
Symmetrically Encrypted Data Packet. In that case, the cipher
algorithm octet is prepended to the session key before it is encrypted.
If no packets of these types precede the encrypted data, the IDEA
algorithm is used with the session key calculated as the MD5 hash of
the passphrase.
The last sentence is what is used for the old pgp -c case.
To do -c encryption with something other than IDEA and simple hashing
you use a Conventional ESK packet follwed by the Symmetrically Encrypted
packet.
Hal Finney