crypto/x509: permit serial numbers to be negative.

Some software that produces certificates doesn't encode integers
correctly and, about half the time, ends up producing certificates with
serial numbers that are actually negative.

This buggy software, sadly, appears to be common enough that we should
let these errors pass. This change allows a Certificate.SerialNumber to
be negative.

Fixes #8265.

Change-Id: Ief35dae23988fb6d5e2873e3c521366fb03c6af4
Reviewed-on: https://go-review.googlesource.com/17247
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/x509/x509.go

@@ -909,10 +909,6 @@ func parseCertificate(in *certificate) (*Certificate, error) {
 		return nil, err
 	}
 
-	if in.TBSCertificate.SerialNumber.Sign() < 0 {
-		return nil, errors.New("x509: negative serial number")
-	}
-
 	out.Version = in.TBSCertificate.Version + 1
 	out.SerialNumber = in.TBSCertificate.SerialNumber
 

src/crypto/x509/x509_test.go

@@ -343,7 +343,11 @@ func TestCreateSelfSignedCertificate(t *testing.T) {
 	for _, test := range tests {
 		commonName := "test.example.com"
 		template := Certificate{
-			SerialNumber: big.NewInt(1),
+			// SerialNumber is negative to ensure that negative
+			// values are parsed. This is due to the prevalence of
+			// buggy code that produces certificates with negative
+			// serial numbers.
+			SerialNumber: big.NewInt(-1),
 			Subject: pkix.Name{
 				CommonName:   commonName,
 				Organization: []string{"Σ Acme Co"},