crypto/x509: CreateCertificate should maybe check the signatures it generates

[rolandshoemaker]

x509.CreateCertificate, and all of the other CreateXXX methods in the package, appear to accept zero length signatures returned by the signer. It'd require a rather broken signer to produce this, but it seems like something the package should be checking for rather than blindly accepting the returned signature. Example.

Off the top of my head I don't think there is anything that explicitly disallows this, but none of the supported signature algorithms should produce a zero length signature, so it should be safe to just disallow it.

[cagedmantis]

/cc @FiloSottile

[FiloSottile]

I'm not sure why special-case empty signatures. There is definitely an argument to be made about verifying the signature while generating, because Signers produce all sorts of invalid signatures (empty, PKCS #1 v1.5 when asked for PSS, wrong private key for the public key...) but I feel like we should either do a full verification or just accept that we trust the Signer's output.

[rolandshoemaker]

Seems reasonable, in general I'd be strongly supportive of actually fully verifying the signer output so that we're not creating malformed certificates. That said I opened this because this was surprising behavior to me, this is the base broken case for a signer that requires no actual knowledge of the signature algorithm to detect, all of the supported signature schemes should produce a non-zero signature, so preventing it would be very simple and remove an entire class of signer brokenness ¯\_(ツ)_/¯.

[FiloSottile]

@rolandshoemaker I'd like to see a CL that introduces a signature verification in CreateCertificate with a benchmark. If the numbers are not too bad, we can fix the whole class and avoid a lot of pain to a lot of people (and even maybe save some RSA keys from the CRT in case of bugs/faults).

[FiloSottile]

See also #37845

[gopherbot]

Change https://golang.org/cl/259697 mentions this issue: crypto/x509: add signature verification to CreateCertificate