crypto/ecdsa: add SignASN1, VerifyASN1

Update the Example in the crypto/ecdsa package for signing
and verifying signatures to use these new functions.

This also changes (*PrivateKey).Sign to use
x/crypto/cryptobyte/asn1 instead of encoding/asn1
to marshal the signature.

Fixes #20544

Change-Id: I3423cfc4d7f9e1748fbed5a631438c8a3b280df4
Reviewed-on: https://go-review.googlesource.com/c/go/+/217940
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

src/crypto/ecdsa/ecdsa.go

@@ -33,10 +33,12 @@ import (
 	"crypto/elliptic"
 	"crypto/internal/randutil"
 	"crypto/sha512"
-	"encoding/asn1"
 	"errors"
 	"io"
 	"math/big"
+
+	"golang.org/x/crypto/cryptobyte"
+	"golang.org/x/crypto/cryptobyte/asn1"
 )
 
 // A invertible implements fast inverse mod Curve.Params().N
@@ -66,10 +68,6 @@ type PrivateKey struct {
 	D *big.Int
 }
 
-type ecdsaSignature struct {
-	R, S *big.Int
-}
-
 // Public returns the public key corresponding to priv.
 func (priv *PrivateKey) Public() crypto.PublicKey {
 	return &priv.PublicKey
@@ -88,7 +86,12 @@ func (priv *PrivateKey) Sign(rand io.Reader, digest []byte, opts crypto.SignerOp
 		return nil, err
 	}
 
-	return asn1.Marshal(ecdsaSignature{r, s})
+	var b cryptobyte.Builder
+	b.AddASN1(asn1.SEQUENCE, func(b *cryptobyte.Builder) {
+		b.AddASN1BigInt(r)
+		b.AddASN1BigInt(s)
+	})
+	return b.Bytes()
 }
 
 var one = new(big.Int).SetInt64(1)
@@ -238,6 +241,15 @@ func Sign(rand io.Reader, priv *PrivateKey, hash []byte) (r, s *big.Int, err err
 	return
 }
 
+// SignASN1 signs a hash (which should be the result of hashing a larger message)
+// using the private key, priv. If the hash is longer than the bit-length of the
+// private key's curve order, the hash will be truncated to that length. It
+// returns the ASN.1 encoded signature. The security of the private key
+// depends on the entropy of rand.
+func SignASN1(rand io.Reader, priv *PrivateKey, hash []byte) ([]byte, error) {
+	return priv.Sign(rand, hash, nil)
+}
+
 // Verify verifies the signature in r, s of hash using the public key, pub. Its
 // return value records whether the signature is valid.
 func Verify(pub *PublicKey, hash []byte, r, s *big.Int) bool {
@@ -282,6 +294,24 @@ func Verify(pub *PublicKey, hash []byte, r, s *big.Int) bool {
 	return x.Cmp(r) == 0
 }
 
+// VerifyASN1 verifies the ASN.1 encoded signature, sig, of hash using the
+// public key, pub. Its return value records whether the signature is valid.
+func VerifyASN1(pub *PublicKey, hash, sig []byte) bool {
+	var (
+		r, s  = &big.Int{}, &big.Int{}
+		inner cryptobyte.String
+	)
+	input := cryptobyte.String(sig)
+	if !input.ReadASN1(&inner, asn1.SEQUENCE) ||
+		!input.Empty() ||
+		!inner.ReadASN1Integer(r) ||
+		!inner.ReadASN1Integer(s) ||
+		!inner.Empty() {
+		return false
+	}
+	return Verify(pub, hash, r, s)
+}
+
 type zr struct {
 	io.Reader
 }

src/crypto/ecdsa/ecdsa_test.go

@@ -131,6 +131,36 @@ func TestSignAndVerify(t *testing.T) {
 	testSignAndVerify(t, elliptic.P521(), "p521")
 }
 
+func testSignAndVerifyASN1(t *testing.T, c elliptic.Curve, tag string) {
+	priv, _ := GenerateKey(c, rand.Reader)
+
+	hashed := []byte("testing")
+	sig, err := SignASN1(rand.Reader, priv, hashed)
+	if err != nil {
+		t.Errorf("%s: error signing: %s", tag, err)
+		return
+	}
+
+	if !VerifyASN1(&priv.PublicKey, hashed, sig) {
+		t.Errorf("%s: VerifyASN1 failed", tag)
+	}
+
+	hashed[0] ^= 0xff
+	if VerifyASN1(&priv.PublicKey, hashed, sig) {
+		t.Errorf("%s: VerifyASN1 always works!", tag)
+	}
+}
+
+func TestSignAndVerifyASN1(t *testing.T) {
+	testSignAndVerifyASN1(t, elliptic.P224(), "p224")
+	if testing.Short() {
+		return
+	}
+	testSignAndVerifyASN1(t, elliptic.P256(), "p256")
+	testSignAndVerifyASN1(t, elliptic.P384(), "p384")
+	testSignAndVerifyASN1(t, elliptic.P521(), "p521")
+}
+
 func testNonceSafety(t *testing.T, c elliptic.Curve, tag string) {
 	priv, _ := GenerateKey(c, rand.Reader)
 

src/crypto/ecdsa/example_test.go

@@ -21,12 +21,12 @@ func Example() {
 	msg := "hello, world"
 	hash := sha256.Sum256([]byte(msg))
 
-	r, s, err := ecdsa.Sign(rand.Reader, privateKey, hash[:])
+	sig, err := ecdsa.SignASN1(rand.Reader, privateKey, hash[:])
 	if err != nil {
 		panic(err)
 	}
-	fmt.Printf("signature: (0x%x, 0x%x)\n", r, s)
+	fmt.Printf("signature: %x\n", sig)
 
-	valid := ecdsa.Verify(&privateKey.PublicKey, hash[:], r, s)
+	valid := ecdsa.VerifyASN1(&privateKey.PublicKey, hash[:], sig)
 	fmt.Println("signature verified:", valid)
 }

src/go/build/deps_test.go

@@ -384,7 +384,10 @@ var pkgDeps = map[string][]string{
 	// Mathematical crypto: dependencies on fmt (L4) and math/big.
 	// We could avoid some of the fmt, but math/big imports fmt anyway.
 	"crypto/dsa": {"L4", "CRYPTO", "math/big"},
-	"crypto/ecdsa":    {"L4", "CRYPTO", "crypto/elliptic", "math/big", "encoding/asn1"},
+	"crypto/ecdsa": {
+		"L4", "CRYPTO", "crypto/elliptic", "math/big",
+		"golang.org/x/crypto/cryptobyte", "golang.org/x/crypto/cryptobyte/asn1",
+	},
 	"crypto/elliptic": {"L4", "CRYPTO", "math/big"},
 	"crypto/rsa":      {"L4", "CRYPTO", "crypto/rand", "math/big"},