Re: Possible Error/Oversight in Encrypted Session Key Pkt format

[Hal Finney <hal@xxxxxxxx>]

Lindsay Mathieson, <lindsay@xxxxxxxxxxxxxx>, writes:
> For Encrypted Session Key Packets (Section 5.1) the OpenPGP spec states:
>
> 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.  *Then a two-octet checksum is appended which is
> equal to the sum of the preceding octets, including the algorithm
> identifier and session key, modulo 65536.*
>
> The problem is with the checksum, the spec states that it includes the
> algorithm identifier, whereas in PGP 2.x it actually doesn't, it only
> includes the session key itself. I'm not sure is this is the case for PGP
> 5.x though. Also it might be handy to mention that the checksum is in MSB
> ordering.
>
> So perhaps if it was changed to:
>
> Then a two-octet checksum in MSB order is appended, which is
> equal to the sum of the preceding session key octets, modulo 65536.

You are correct, I misread the code when I provided this information to
Jon.  The alg ID is not included in the checksum in 5.X or in 2.X.  We
should correct the draft spec.

Hal Finney
hal@xxxxxxx
hal@xxxxxxxx