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Re: Does anyone want key-resistant tamper-evident archives?
I would certainly welcome this feature.
In a different domain - I have a similar scenario. There, I solved
it by tying the hash of public key to the content that is
signed+encrypted. This way, whenever a blob is decrypted (or
verified), the pubkey-hash in the blob is compared to the hash of
the public key used for decryption.
Colin Percival wrote:
Tarsnap is designed to detect if your data is modified: Archives are
cryptographically signed, and the signatures are verified before any
data is extracted. However, this depends on the integrity of the key:
If someone has your delete and write keys, they could delete an archive
and create a new one with the same name, and (since they have the keys)
it would cryptographically validate.
It occurs to me that we could have a stronger unforgeability property
via out-of-band (non-cryptographic) verification of the archive metadata
hash; even with the keys, it would be impossible to create a different
archive which has the same hash (unless you find a SHA256 collision). In
addition to the "stolen keys" scenario, this could be useful if you need
to prove (e.g., for auditing or legal purposes) that *you* haven't changed
an archive since the time when you created it.
Is anyone interested in having this functionality? It seems like too
obscure a use case to write code for if nobody wants it yet, but if there's
a demand then it's definitely doable.