The current implementation of CRLs in crypto/x509 and crypto/x509/pkix is somewhat confusing, easy to misuse, and does not match the design of the rest of the package. In particular it doesn't expose the necessary information to do safe issuer verification (see grpc/grpc-go#5130 for an example of how this can go wrong.)

We could try to provide the extra information required to do safe comparisons, but unfortunately due to the design of pkix.CertificateList, which is intended to be a direct ASN.1 analog, it is not possible to add new fields, since it will inherently change how encoding/asn1 encodes/decodes related data.

In go1.15 we introduced RevocationList, a type used as input to CreateRevocationList. I propose that we introduce ParseRevocationList, and convert RevocationList into a Go representation of a CRL, similar to how Certificate is used. This results in an API much more in line with the rest of the package, and would allow us more leeway to update the representative CRL structure without having to worry about its direct ASN.1 encoding.

For the sake of slowly moving away from reliance on encoding/asn1, the new ParseRevocationList function should employ a x/crypto/cryptobyte parser.

This would deprecate the pkix.CertificateList type (and associated types), the ParseCRL and ParseDERCRL functions, and the Certificate.CheckCRLSignature method (the latter being replaced with a method on RevocationList.)

// Added (crypto/x509)
func ParseRevocationList(der []byte) (*RevocationList, error)

func (rl *RevocationList) CheckSignatureFrom(issuer *Certificate) error

type RevocationList struct {
  ...
  // New fields
  RawIssuer      []byte
  Signature      []byte
  AuthorityKeyId []byte
  Extensions     []pkix.Extension
}

// Deprecated (crypto/x509)
func ParseCRL(crlBytes []byte) (*pkix.CertificateList, error)
func ParseDERCRL(derBytes []byte) (*pkix.CertificateList, error)
func (c *Certificate) CheckCRLSignature(crl *pkix.CertificateList) error

// Deprecated (crypto/x509/pkix)
type CertificateList ...
type TBSCertificateList ...

cc @golang/security