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?)
