Re: [Razor-users] Revoke submits whole e-mail?

[Matt K. <mke...@ev...>]

At 12:39 PM 10/25/2004, Luis Alberto Pab=F3n Flores wrote:
> >> 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=20
bypasses the need for recomputation.

So, bearing in mind recomputation can be avoided, for academic purposes=20
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=
=20
a starting-byte index and an ending-byte index as parameters. When these=20
parameters change, the hash becomes different. If you want to carry forward=
=20
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=
=20
two options:

1) precompute all possible hashes at the time of submission. Not practical=
=20
because there's millions of sensible e4 combinations, and it would take=20
hours to compute them all, and you'd end up with at least a 24mb=20
submission. ((20bytes hash + 4 bytes parameters) * 1 million combinations)

2) force the clients to maintain an archive of submitted messages. Force=20
them to immediately respond to a server-initiated request for recompute of=
=20
all the archived messages (cannot wait till later, thus you would have to=20
allow the razor server to initiate a connection to your system for this.)=20
Not practical for client-side security reasons. Do you want a hole in your=
=20
firewall? Do you want your client burning CPU at the beck and call of a=20
razor server? Not practical for client-side storage reasons: Do you want to=
=20
offer mid-term storage service for emails to the razor client? Say all=20
revokes for the past month?