>> In the event of an e4 parameter change the server MUST have the body of >> the message for a revoke. End of story. > i agree, otherwise it isn't possible to recompute fingerprints.
Dumb question: why cannot this be done on the client's machine? I suppose that there's a contundent reason for not to do that...
First, none of this matters, it can be solved with the overlap, which bypasses the need for recomputation.
So, bearing in mind recomputation can be avoided, for academic purposes only, it's highly impractical to handle recomputation on the client end.
Do you understand how e4 works?
e4 calculates the SHA1 hash of a range of characters in the email. It's got a starting-byte index and an ending-byte index as parameters. When these parameters change, the hash becomes different. If you want to carry forward old reports to the new hash-range, you have to recompute the hash from scratch.
To handle the work of recomputation on the client side would involve one of two options:
1) precompute all possible hashes at the time of submission. Not practical because there's millions of sensible e4 combinations, and it would take hours to compute them all, and you'd end up with at least a 24mb submission. ((20bytes hash + 4 bytes parameters) * 1 million combinations)
2) force the clients to maintain an archive of submitted messages. Force them to immediately respond to a server-initiated request for recompute of all the archived messages (cannot wait till later, thus you would have to allow the razor server to initiate a connection to your system for this.) Not practical for client-side security reasons. Do you want a hole in your firewall? Do you want your client burning CPU at the beck and call of a razor server? Not practical for client-side storage reasons: Do you want to offer mid-term storage service for emails to the razor client? Say all revokes for the past month?
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