crypto/x509: revert serial length restriction

This reverts CL400377, which restricted serials passed to
x509.CreateCertificate to <= 20 octets. Unfortunately this turns out to
be something _a lot_ of people get wrong. Since it's not particularly
obvious how to properly generate conformant serials, until we provide
an easier way for people to get this right, reverting this restriction
makes sense (possible solution discussed in #52444.)

Change-Id: Ia85a0ffe61e2e547abdaf1389c3e1ad29e28a2be
Reviewed-on: https://go-review.googlesource.com/c/go/+/401657
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>

src/crypto/x509/x509.go

@@ -1478,21 +1478,14 @@ func CreateCertificate(rand io.Reader, template, parent *Certificate, pub, priv
 		return nil, errors.New("x509: no SerialNumber given")
 	}
 
-	// RFC 5280 Section 4.1.2.2: serial number must positive and should not be longer
-	// than 20 octets.
+	// RFC 5280 Section 4.1.2.2: serial number must positive
 	//
-	// We cannot simply check for len(serialBytes) > 20, because encoding/asn1 may
-	// pad the slice in order to prevent the integer being mistaken for a negative
-	// number (DER uses the high bit of the left-most byte to indicate the sign.),
-	// so we need to double check the composition of the serial if it is exactly
-	// 20 bytes.
+	// We _should_ also restrict serials to <= 20 octets, but it turns out a lot of people
+	// get this wrong, in part because the encoding can itself alter the length of the
+	// serial. For now we accept these non-conformant serials.
 	if template.SerialNumber.Sign() == -1 {
 		return nil, errors.New("x509: serial number must be positive")
 	}
-	serialBytes := template.SerialNumber.Bytes()
-	if len(serialBytes) > 20 || (len(serialBytes) == 20 && serialBytes[0]&0x80 != 0) {
-		return nil, errors.New("x509: serial number exceeds 20 octets")
-	}
 
 	if template.BasicConstraintsValid && !template.IsCA && template.MaxPathLen != -1 && (template.MaxPathLen != 0 || template.MaxPathLenZero) {
 		return nil, errors.New("x509: only CAs are allowed to specify MaxPathLen")

src/crypto/x509/x509_test.go

@@ -3589,42 +3589,6 @@ func TestOmitEmptyExtensions(t *testing.T) {
 	}
 }
 
-func TestCreateCertificateLongSerial(t *testing.T) {
-	k, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	serialBytes := make([]byte, 21)
-	serialBytes[0] = 0x80
-	serialBytes[20] = 1
-	tooLong := big.NewInt(0).SetBytes(serialBytes)
-
-	tmpl := &Certificate{
-		SerialNumber: tooLong,
-		Subject: pkix.Name{
-			CommonName: ":)",
-		},
-		NotAfter:  time.Now().Add(time.Hour),
-		NotBefore: time.Now().Add(-time.Hour),
-	}
-
-	expectedErr := "x509: serial number exceeds 20 octets"
-
-	_, err = CreateCertificate(rand.Reader, tmpl, tmpl, k.Public(), k)
-	if err == nil || err.Error() != expectedErr {
-		t.Errorf("CreateCertificate returned unexpected error: want %q, got %q", expectedErr, err)
-	}
-
-	serialBytes = serialBytes[:20]
-	tmpl.SerialNumber = big.NewInt(0).SetBytes(serialBytes)
-
-	_, err = CreateCertificate(rand.Reader, tmpl, tmpl, k.Public(), k)
-	if err == nil || err.Error() != expectedErr {
-		t.Errorf("CreateCertificate returned unexpected error: want %q, got %q", expectedErr, err)
-	}
-}
-
 var negativeSerialCert = `-----BEGIN CERTIFICATE-----
 MIIBBTCBraADAgECAgH/MAoGCCqGSM49BAMCMA0xCzAJBgNVBAMTAjopMB4XDTIy
 MDQxNDIzNTYwNFoXDTIyMDQxNTAxNTYwNFowDTELMAkGA1UEAxMCOikwWTATBgcq