Perry writes:
However, it can be done in advance, and you can conceivably reuse forged LEAFs.
I will point out something that I didn't quite understand myself but have since discussed with Matt Blaze in some detail -- LEAF checksums are tied to session keys. You CAN do this in advance but only if your key exchange will permit you to generate your session keys in advance, too Obviously, reusing forged LEAFs requires reusing session keys.
More precisely, as Steve's summary pointed out, it's tied to the IV, which is tied to the session key. (It makes sense - assuming the descriptions of the LEAF contents are true, the only session key component in the LEAF itself is encrypted with the chip-unique backdoor key, and tying it to the IV accomplishes key-dependence, though they could also use the session key externally from the LEAF.) Unfortunately, most Clipperphones will probably use Diffie-Hellman key exchange, since it reduces or eliminates the need for prearranged public-key management (depending on whether they're using radio or a medium that can be actively wiretapped), so precomputation will generally not be usable. I suppose some crude Diffie-Hellman implementations might always use the same half-key for every conversation, rather than generating a random one each time, and you could precompute session keys for talking to them. For email applications, however, most standards will probably use sender-generated session keys, so it would be simple enough to make secure Tessera mailers if you don't worry about subliminal channels in the hash. Bill