crypto/x509: convert ErrInsecureAlgorithm into a type

Change-Id: I411aeaf0cf75eb8b1c9005b622f664e9f25e4a68
Reviewed-on: https://go-review.googlesource.com/17400
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/crypto/x509/x509.go

@@ -19,6 +19,7 @@ import (
 	"encoding/asn1"
 	"encoding/pem"
 	"errors"
+	"fmt"
 	"io"
 	"math/big"
 	"net"
@@ -174,6 +175,28 @@ const (
 	ECDSAWithSHA512
 )
 
+var algoName = [...]string{
+	MD2WithRSA:      "MD2-RSA",
+	MD5WithRSA:      "MD5-RSA",
+	SHA1WithRSA:     "SHA1-RSA",
+	SHA256WithRSA:   "SHA256-RSA",
+	SHA384WithRSA:   "SHA384-RSA",
+	SHA512WithRSA:   "SHA512-RSA",
+	DSAWithSHA1:     "DSA-SHA1",
+	DSAWithSHA256:   "DSA-SHA256",
+	ECDSAWithSHA1:   "ECDSA-SHA1",
+	ECDSAWithSHA256: "ECDSA-SHA256",
+	ECDSAWithSHA384: "ECDSA-SHA384",
+	ECDSAWithSHA512: "ECDSA-SHA512",
+}
+
+func (algo SignatureAlgorithm) String() string {
+	if 0 < algo && int(algo) < len(algoName) {
+		return algoName[algo]
+	}
+	return strconv.Itoa(int(algo))
+}
+
 type PublicKeyAlgorithm int
 
 const (
@@ -541,9 +564,12 @@ type Certificate struct {
 // involves algorithms that are not currently implemented.
 var ErrUnsupportedAlgorithm = errors.New("x509: cannot verify signature: algorithm unimplemented")
 
-// ErrInsecureAlgorithm results from attempting to perform an operation that
-// involves algorithms that are deemed insecure, notably MD5.
-var ErrInsecureAlgorithm = errors.New("x509: cannot verify signature: insecure algorithm")
+// An InsecureAlgorithmError
+type InsecureAlgorithmError SignatureAlgorithm
+
+func (e InsecureAlgorithmError) Error() string {
+	return fmt.Sprintf("x509: cannot verify signature: insecure algorithm %v", SignatureAlgorithm(e))
+}
 
 // ConstraintViolationError results when a requested usage is not permitted by
 // a certificate. For example: checking a signature when the public key isn't a
@@ -656,7 +682,7 @@ func checkSignature(algo SignatureAlgorithm, signed, signature []byte, publicKey
 	case SHA512WithRSA, ECDSAWithSHA512:
 		hashType = crypto.SHA512
 	case MD2WithRSA, MD5WithRSA:
-		return ErrInsecureAlgorithm
+		return InsecureAlgorithmError(algo)
 	default:
 		return ErrUnsupportedAlgorithm
 	}

src/crypto/x509/x509_test.go

@@ -18,6 +18,7 @@ import (
 	"encoding/base64"
 	"encoding/hex"
 	"encoding/pem"
+	"fmt"
 	"internal/testenv"
 	"math/big"
 	"net"
@@ -1089,8 +1090,8 @@ func TestCriticalFlagInCSRRequestedExtensions(t *testing.T) {
 		t.Fatalf("failed to parse CSR: %s", err)
 	}
 
-	expected := []struct{
-		Id asn1.ObjectIdentifier
+	expected := []struct {
+		Id    asn1.ObjectIdentifier
 		Value []byte
 	}{
 		{oidExtensionBasicConstraints, fromBase64("MAYBAf8CAQA=")},
@@ -1203,6 +1204,23 @@ func TestVerifyEmptyCertificate(t *testing.T) {
 	}
 }
 
+func TestInsecureAlgorithmErrorString(t *testing.T) {
+	tests := []struct {
+		sa   SignatureAlgorithm
+		want string
+	}{
+		{MD2WithRSA, "x509: cannot verify signature: insecure algorithm MD2-RSA"},
+		{-1, "x509: cannot verify signature: insecure algorithm -1"},
+		{0, "x509: cannot verify signature: insecure algorithm 0"},
+		{9999, "x509: cannot verify signature: insecure algorithm 9999"},
+	}
+	for i, tt := range tests {
+		if got := fmt.Sprint(InsecureAlgorithmError(tt.sa)); got != tt.want {
+			t.Errorf("%d. mismatch.\n got: %s\nwant: %s\n", i, got, tt.want)
+		}
+	}
+}
+
 // These CSR was generated with OpenSSL:
 //  openssl req -out CSR.csr -new -sha256 -nodes -keyout privateKey.key -config openssl.cnf
 //
@@ -1249,7 +1267,7 @@ func TestMD5(t *testing.T) {
 	if err = cert.CheckSignatureFrom(cert); err == nil {
 		t.Fatalf("certificate verification succeeded incorrectly")
 	}
-	if err != ErrInsecureAlgorithm {
-		t.Fatalf("certificate verification returned %q, wanted %q", err, ErrInsecureAlgorithm)
+	if _, ok := err.(InsecureAlgorithmError); !ok {
+		t.Fatalf("certificate verification returned %v (%T), wanted InsecureAlgorithmError", err, err)
 	}
 }