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The ssh_signkey vtable has grown a new method ssh_key_invalid(), which checks whether the key is going to be usable for constructing a signature at all. Currently the only way this can fail is if it's an RSA key so short that there isn't room to put all the PKCS#1 formatting in the signature preimage integer, but the return value is an arbitrary error message just in case more reasons are needed later. This is tested separately rather than at key-creation time because of the signature flags system: an RSA key of intermediate length could be valid for SHA-1 signing but not for SHA-512. So really this method should be called at the point where you've decided what sig flags you want to use, and you're checking if _those flags_ are OK. On the verification side, there's no need for a separate check. If someone presents us with an RSA key so short that it's impossible to encode a valid signature using it, then we simply regard all signatures as invalid.