Proposal Details
RSA signature algorithms are currently ordered as:
func algorithmsForKeyFormat(keyFormat string) []string {
switch keyFormat {
case KeyAlgoRSA:
return []string{KeyAlgoRSASHA256, KeyAlgoRSASHA512, KeyAlgoRSA}
case CertAlgoRSAv01:
return []string{CertAlgoRSASHA256v01, CertAlgoRSASHA512v01, CertAlgoRSAv01}
default:
return []string{keyFormat}
}
}This causes rsa-sha2-256 to be negotiated before rsa-sha2-512 when both are supported.
OpenSSH prefers rsa-sha2-512 over rsa-sha2-256 in its default signature algorithm ordering and negotiates rsa-sha2-512 when possible.
Suggested change
Prefer SHA-512 when available:
case KeyAlgoRSA:
return []string{KeyAlgoRSASHA512, KeyAlgoRSASHA256, KeyAlgoRSA}
case CertAlgoRSAv01:
return []string{CertAlgoRSASHA512v01, CertAlgoRSASHA256v01, CertAlgoRSAv01}This aligns Go's SSH behavior with OpenSSH while remaining fully backward compatible.
Proposal Details
RSA signature algorithms are currently ordered as:
This causes
rsa-sha2-256to be negotiated beforersa-sha2-512when both are supported.OpenSSH prefers
rsa-sha2-512overrsa-sha2-256in its default signature algorithm ordering and negotiatesrsa-sha2-512when possible.Suggested change
Prefer SHA-512 when available:
This aligns Go's SSH behavior with OpenSSH while remaining fully backward compatible.