MD5(Timestamp+message)
That's an idea, thanks. If both timestamp and contents of two messages coincide, then the messages are, indeed, same. (Though I think that SHA1 is better in terms of collisions, and I won't employ SHA2 there because cryptographic integrity will be protected by SSL.)

Also, I still need a way to distinguish the messages as they are delivered to different users (I need some time to prepare an answer for you, BrowserUK).

Then you sync by comparing lists of the hash, which is conveniently fixed length and easily ordered.
So, part of the question is comparing two sorted arrays of 20-byte numbers. Thank you, now I understand better how tools like rsync work. The open part of the question is: which shortcuts I can apply to avoid exchanging full lists of messages? (for me, the worst case would be (1e5 * 20) = 2M of data per sync attempt, which is okay if performed rarely, but quite a lot - and may require resuming - on a EGDE connection)

In reply to Re^2: [OT] Merging messages between users on different machines by Anonymous Monk
in thread [OT] Merging messages between users on different machines by Anonymous Monk

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