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Recently I found the sagecipher project, which uses ssh-agent's signing operation as a way to produce a symmetric key and encrypt data with it. It appears to work with ssh-ed25519 keys, but my question is: Is this guaranteed to work?

That is, are signatures always deterministic with the key and signature algorithms supported by OpenSSH, or could there be a situation where sagecipher's result is not decryptable because the tool always gets different signatures from the same input?

(Second question being, are there any obvious security problems with this method?)

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  • $\begingroup$ That's a really weird thing to do. I've never seen any other system do that. Signatures are generally never assumed to be secret, so implementations won't tend to protect them the way they would secret data. I've also not seen any evidence that it's safe (no security proof, paper, whatever) so I'd assume it's horribly broken until proven otherwise. $\endgroup$ – SAI Peregrinus Feb 1 at 1:08

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