proposal: crypto/x509: ability to add custom curve when parsing X509 certificate

[trung]

Per #26776, using a third party library for custom curve is advised.

However, when parsing x509 certificate (x509.ParseCetificate()), it is not possible to supply custom curve.

My proposal is to offer a configuration that can be used to supply a function to return elliptic.Curve from asn1.ObjectIdentifier to complement the default namedCurveFromIOD()

[trung]

From the current flow:

go/src/crypto/x509/x509.go

Lines 519 to 553 in fbde753

	var (
	oidNamedCurveP224 = asn1.ObjectIdentifier{1, 3, 132, 0, 33}
	oidNamedCurveP256 = asn1.ObjectIdentifier{1, 2, 840, 10045, 3, 1, 7}
	oidNamedCurveP384 = asn1.ObjectIdentifier{1, 3, 132, 0, 34}
	oidNamedCurveP521 = asn1.ObjectIdentifier{1, 3, 132, 0, 35}
	)
	func namedCurveFromOID(oid asn1.ObjectIdentifier) elliptic.Curve {
	switch {
	case oid.Equal(oidNamedCurveP224):
	return elliptic.P224()
	case oid.Equal(oidNamedCurveP256):
	return elliptic.P256()
	case oid.Equal(oidNamedCurveP384):
	return elliptic.P384()
	case oid.Equal(oidNamedCurveP521):
	return elliptic.P521()
	}
	return nil
	}
	func oidFromNamedCurve(curve elliptic.Curve) (asn1.ObjectIdentifier, bool) {
	switch curve {
	case elliptic.P224():
	return oidNamedCurveP224, true
	case elliptic.P256():
	return oidNamedCurveP256, true
	case elliptic.P384():
	return oidNamedCurveP384, true
	case elliptic.P521():
	return oidNamedCurveP521, true
	}
	return nil, false
	}

I think of injecting custom functions something like:

type OIDToCurveFunc func(oid asn1.ObjectIdentifier) elliptic.Curve
type CurveToOIDFunc func(curve elliptic.Curve) (asn1.ObjectIdentifier, bool)

var (
	CustomIOIDToCurveFunc OIDToCurveFunc
	CustomCurveToOIDFunc CurveToOIDFunc
)

func namedCurveFromOID(oid asn1.ObjectIdentifier) elliptic.Curve {
	switch {
	case oid.Equal(oidNamedCurveP224):
		return elliptic.P224()
	case oid.Equal(oidNamedCurveP256):
		return elliptic.P256()
	case oid.Equal(oidNamedCurveP384):
		return elliptic.P384()
	case oid.Equal(oidNamedCurveP521):
		return elliptic.P521()
	}
	if CustomIOIDToCurveFunc != nil {
		return CustomIOIDToCurveFunc(oid)
	}
	return nil
}

func oidFromNamedCurve(curve elliptic.Curve) (asn1.ObjectIdentifier, bool) {
	switch curve {
	case elliptic.P224():
		return oidNamedCurveP224, true
	case elliptic.P256():
		return oidNamedCurveP256, true
	case elliptic.P384():
		return oidNamedCurveP384, true
	case elliptic.P521():
		return oidNamedCurveP521, true
	}
	if CustomCurveToOIDFunc != nil {
		return CustomCurveToOIDFunc(curve)
	}
	return nil, false
}

[rsc]

Globals are probably not the right answer here. Perhaps there should be an x509.Parser that can be configured with extra hooks, a bit like there is an xml.Decoder to configure the XML parser.

Or perhaps unrecognized curves should be exposed in some general way and client code can just post-process.

/cc @FiloSottile

[trung]

Agreed with globals. There might be breaking changes once introducing x509.Parser as x509.ParseCertifcate() is used in number of places.

[FiloSottile]

crypto/x509 primarily serves the WebPKI, and there is no use of custom curves there. X.509 is a sprawling standard, so it's critical for the crypto/x509 package to stay focused on its use case.

I will keep this open to collect feedback about use cases, as things of course change over time, but we are not going to support this for now.