crypto/x509: exempt broken Entrust certificate from checks.

Entrust have issued a root certificate that's not marked as valid for
signing certificates.

This results in Go programs failing to validate certificates that
chain up to this root (i.e. gateway.push.apple.com:2195).

Although this is clearly a mistake on Entrust's part, it seems that we
will have to bodge around it.

R=golang-dev, dave, r
CC=golang-dev
https://golang.org/cl/6346064

src/pkg/crypto/x509/x509.go

@@ -433,6 +433,55 @@ func (c *Certificate) Equal(other *Certificate) bool {
 	return bytes.Equal(c.Raw, other.Raw)
 }
 
+// Entrust have a broken root certificate (CN=Entrust.net Certification
+// Authority (2048)) which isn't marked as a CA certificate and is thus invalid
+// according to PKIX.
+// We recognise this certificate by its SubjectPublicKeyInfo and exempt it
+// from the Basic Constraints requirement.
+// See http://www.entrust.net/knowledge-base/technote.cfm?tn=7869
+//
+// TODO(agl): remove this hack once their reissued root is sufficiently
+// widespread.
+var entrustBrokenSPKI = []byte{
+	0x30, 0x82, 0x01, 0x22, 0x30, 0x0d, 0x06, 0x09,
+	0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x01, 0x01,
+	0x01, 0x05, 0x00, 0x03, 0x82, 0x01, 0x0f, 0x00,
+	0x30, 0x82, 0x01, 0x0a, 0x02, 0x82, 0x01, 0x01,
+	0x00, 0x97, 0xa3, 0x2d, 0x3c, 0x9e, 0xde, 0x05,
+	0xda, 0x13, 0xc2, 0x11, 0x8d, 0x9d, 0x8e, 0xe3,
+	0x7f, 0xc7, 0x4b, 0x7e, 0x5a, 0x9f, 0xb3, 0xff,
+	0x62, 0xab, 0x73, 0xc8, 0x28, 0x6b, 0xba, 0x10,
+	0x64, 0x82, 0x87, 0x13, 0xcd, 0x57, 0x18, 0xff,
+	0x28, 0xce, 0xc0, 0xe6, 0x0e, 0x06, 0x91, 0x50,
+	0x29, 0x83, 0xd1, 0xf2, 0xc3, 0x2a, 0xdb, 0xd8,
+	0xdb, 0x4e, 0x04, 0xcc, 0x00, 0xeb, 0x8b, 0xb6,
+	0x96, 0xdc, 0xbc, 0xaa, 0xfa, 0x52, 0x77, 0x04,
+	0xc1, 0xdb, 0x19, 0xe4, 0xae, 0x9c, 0xfd, 0x3c,
+	0x8b, 0x03, 0xef, 0x4d, 0xbc, 0x1a, 0x03, 0x65,
+	0xf9, 0xc1, 0xb1, 0x3f, 0x72, 0x86, 0xf2, 0x38,
+	0xaa, 0x19, 0xae, 0x10, 0x88, 0x78, 0x28, 0xda,
+	0x75, 0xc3, 0x3d, 0x02, 0x82, 0x02, 0x9c, 0xb9,
+	0xc1, 0x65, 0x77, 0x76, 0x24, 0x4c, 0x98, 0xf7,
+	0x6d, 0x31, 0x38, 0xfb, 0xdb, 0xfe, 0xdb, 0x37,
+	0x02, 0x76, 0xa1, 0x18, 0x97, 0xa6, 0xcc, 0xde,
+	0x20, 0x09, 0x49, 0x36, 0x24, 0x69, 0x42, 0xf6,
+	0xe4, 0x37, 0x62, 0xf1, 0x59, 0x6d, 0xa9, 0x3c,
+	0xed, 0x34, 0x9c, 0xa3, 0x8e, 0xdb, 0xdc, 0x3a,
+	0xd7, 0xf7, 0x0a, 0x6f, 0xef, 0x2e, 0xd8, 0xd5,
+	0x93, 0x5a, 0x7a, 0xed, 0x08, 0x49, 0x68, 0xe2,
+	0x41, 0xe3, 0x5a, 0x90, 0xc1, 0x86, 0x55, 0xfc,
+	0x51, 0x43, 0x9d, 0xe0, 0xb2, 0xc4, 0x67, 0xb4,
+	0xcb, 0x32, 0x31, 0x25, 0xf0, 0x54, 0x9f, 0x4b,
+	0xd1, 0x6f, 0xdb, 0xd4, 0xdd, 0xfc, 0xaf, 0x5e,
+	0x6c, 0x78, 0x90, 0x95, 0xde, 0xca, 0x3a, 0x48,
+	0xb9, 0x79, 0x3c, 0x9b, 0x19, 0xd6, 0x75, 0x05,
+	0xa0, 0xf9, 0x88, 0xd7, 0xc1, 0xe8, 0xa5, 0x09,
+	0xe4, 0x1a, 0x15, 0xdc, 0x87, 0x23, 0xaa, 0xb2,
+	0x75, 0x8c, 0x63, 0x25, 0x87, 0xd8, 0xf8, 0x3d,
+	0xa6, 0xc2, 0xcc, 0x66, 0xff, 0xa5, 0x66, 0x68,
+	0x55, 0x02, 0x03, 0x01, 0x00, 0x01,
+}
+
 // CheckSignatureFrom verifies that the signature on c is a valid signature
 // from parent.
 func (c *Certificate) CheckSignatureFrom(parent *Certificate) (err error) {
@@ -441,8 +490,10 @@ func (c *Certificate) CheckSignatureFrom(parent *Certificate) (err error) {
 	// certificate, or the extension is present but the cA boolean is not
 	// asserted, then the certified public key MUST NOT be used to verify
 	// certificate signatures."
-	if parent.Version == 3 && !parent.BasicConstraintsValid ||
-		parent.BasicConstraintsValid && !parent.IsCA {
+	// (except for Entrust, see comment above entrustBrokenSPKI)
+	if (parent.Version == 3 && !parent.BasicConstraintsValid ||
+		parent.BasicConstraintsValid && !parent.IsCA) &&
+		!bytes.Equal(c.RawSubjectPublicKeyInfo, entrustBrokenSPKI) {
 		return ConstraintViolationError{}
 	}