crypto/x509: redundant UnknownExtKeyUsage check in verify.go

[MikhailLipanin]

Source code: crypto/x509/verify.go
In crypto/x509 package, during ExtKeyUsage check, there are condition on presence of UnknownExtKeyUsage. I don't really understand, what the purpose of this check, since in that function there only uses of cert.ExtKeyUsage field and no cert. UnknownExtKeyUsage.
So, considering this block of code: if cert has no ExtKeyUsage, but some UnknownExtKeyUsage, we also go through the remaining logic.

What the purpose of this check?

[dmitshur]

So, considering this block of code: if cert has no ExtKeyUsage, but some UnknownExtKeyUsage, we also go through the remaining logic.

It looks like the current code results in such a cert contributing to usagesRemaining-- happening because it's not skipped at the top of the loop.

CC @golang/security.

[MikhailLipanin]

@dmitshur Yes, you are right - it affects to usagesRemaining variable. But on this side - the next question is: what the purpose of variable usagesRemaining? If it marks the remaining count of EKU to check against other certs in chain, so why we go through only on ExtUsages here and don't check cert.UnknownExtKeyUsage?

Since we check the presence of UnknownExtKeyUsage here, why we do not go through them to check their presence in Certificate? Marking it as invalid (by subtracting usagesRemaining variable) does not make sense, since there will be the case, when certificate will have the EKU from requested usages, but "unknown" for the x509 package at the moment - so it will fail the current chain check.

[rolandshoemaker]

Extended key usages are specified by OIDs. When parsing a certificate, if we do not recognize the OID provided (e.g. if it does not map to one of the defined key usages), we mark it as unknown.

Since there is no way for the user to pass a custom key usage in VerifyOptions (we use integers to map to the OID forms we know about, and there is no may for the user to create new mappings), the unknown key usages inherently cannot satisfy the constraints in checkChainForKeyUsage. We also cannot ignore the unknown key usages if they are present, because a certificate which contains any usage that does not match one of the requested usages does not satisfy the algorithm and is invalid.