crypto/x509: write exact BitLength in ASN.1 encoding for certificate KeyUsage

The encoded value of the certificate KeyUsage did contain additonal padding
that was not present with other certificate generators. According to ITU-T
X.690 the BitLength value should have no padding in a DER encoding.

See discussion:
https://groups.google.com/forum/#!topic/golang-nuts/dzaJ3hMpDcs

This CL has been discussed at: http://golang.org/cl/168990043

Change-Id: I1eff3f441b0566966a2d279631901ad9287c917d
Reviewed-on: https://go-review.googlesource.com/2255Reviewed-by: Adam Langley <agl@golang.org>

src/crypto/x509/x509.go

@@ -1135,6 +1135,26 @@ func reverseBitsInAByte(in byte) byte {
 	return b3
 }
 
+// asn1BitLength returns the bit-length of bitString by considering the
+// most-significant bit in a byte to be the "first" bit. This convention
+// matches ASN.1, but differs from almost everything else.
+func asn1BitLength(bitString []byte) int {
+	bitLen := len(bitString) * 8
+
+	for i := range bitString {
+		b := bitString[len(bitString)-i-1]
+
+		for bit := uint(0); bit < 8; bit++ {
+			if (b>>bit)&1 == 1 {
+				return bitLen
+			}
+			bitLen--
+		}
+	}
+
+	return 0
+}
+
 var (
 	oidExtensionSubjectKeyId          = []int{2, 5, 29, 14}
 	oidExtensionKeyUsage              = []int{2, 5, 29, 15}
@@ -1203,7 +1223,8 @@ func buildExtensions(template *Certificate) (ret []pkix.Extension, err error) {
 			l = 2
 		}
 
-		ret[n].Value, err = asn1.Marshal(asn1.BitString{Bytes: a[0:l], BitLength: l * 8})
+		bitString := a[:l]
+		ret[n].Value, err = asn1.Marshal(asn1.BitString{Bytes: bitString, BitLength: asn1BitLength(bitString)})
 		if err != nil {
 			return
 		}

src/crypto/x509/x509_test.go

@@ -1016,6 +1016,28 @@ func TestMaxPathLen(t *testing.T) {
 	}
 }
 
+func TestASN1BitLength(t *testing.T) {
+	tests := []struct {
+		bytes  []byte
+		bitLen int
+	}{
+		{nil, 0},
+		{[]byte{0x00}, 0},
+		{[]byte{0x00, 0x00}, 0},
+		{[]byte{0xf0}, 4},
+		{[]byte{0x88}, 5},
+		{[]byte{0xff}, 8},
+		{[]byte{0xff, 0x80}, 9},
+		{[]byte{0xff, 0x81}, 16},
+	}
+
+	for i, test := range tests {
+		if got := asn1BitLength(test.bytes); got != test.bitLen {
+			t.Errorf("#%d: calculated bit-length of %d for %x, wanted %d", i, got, test.bytes, test.bitLen)
+		}
+	}
+}
+
 // This CSR was generated with OpenSSL:
 //  openssl req -out CSR.csr -new -newkey rsa:2048 -nodes -keyout privateKey.key -config openssl.cnf
 //