Erron Criddle writes:
Regarding the 2 octet checksum (that is encrypted), how do we encrypt:
encrypt (Secret MPI's) + encrypt (2 octet checksum)
or:
encrypt (Secret MPI's + 2 octet checksum)
I'm not sure what distinction you are trying to draw here. Presumably
"+" refers to concatenation? With CFB mode you have the effect of a
stream cipher, hence there is no difference between encrypt(a+b) and
encrypt(a)+encrypt(b).
Also, I am also assuming a 16 bit checksum is:
16 bit checksum = 16 most significant bits (sum of Secret MPI's ASCII
values MOD 65536)
I'm not sure what you mean by the 16 "most significant" bits of a value
which is mod 2^16. That's just a 16 bit value.
These are not ASCII characters (ASCII is a mapping between typographic
symbols like letters, and binary values). They are simple binary octets.
this also assumes a big endian system.
Yes, the 16 bit value is taken in big endian form, i.e. high byte,
then low byte. This is the general convention in OpenPGP, as noted in
section 3.1.
Hal Finney