Hi all,
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.