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. -- Colin Percival Security Officer Emeritus, FreeBSD | The power to serve Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid
