Simplified the API a bit.

Also simplified the guts.

dsa.go

@@ -2,6 +2,7 @@ package rfc6979
 
 import (
 	"crypto/dsa"
+	"hash"
 	"math/big"
 )
 
@@ -12,7 +13,7 @@ import (
 // Note that FIPS 186-3 section 4.6 specifies that the hash should be truncated
 // to the byte-length of the subgroup. This function does not perform that
 // truncation itself.
-func SignDSA(priv *dsa.PrivateKey, hash []byte, alg HashFunc) (r, s *big.Int, err error) {
+func SignDSA(priv *dsa.PrivateKey, hash []byte, alg func() hash.Hash) (r, s *big.Int, err error) {
 	n := priv.Q.BitLen()
 	if n&7 != 0 {
 		err = dsa.ErrInvalidPublicKey

dsa_test.go

@@ -1,4 +1,4 @@
-package rfc6979
+package rfc6979_test
 
 import (
 	"crypto/dsa"
@@ -6,14 +6,17 @@ import (
 	"crypto/sha256"
 	"crypto/sha512"
 	"encoding/hex"
+	"hash"
 	"math/big"
 	"testing"
+
+	"github.com/codahale/rfc6979"
 )
 
 type dsaFixture struct {
 	name    string
 	key     *dsaKey
-	alg     HashFunc
+	alg     func() hash.Hash
 	message string
 	r, s    string
 }
@@ -239,7 +242,7 @@ func testDsaFixture(f *dsaFixture, t *testing.T) {
 		digest = digest[0:g]
 	}
 
-	r, s, err := SignDSA(f.key.key, digest, f.alg)
+	r, s, err := rfc6979.SignDSA(f.key.key, digest, f.alg)
 	if err != nil {
 		t.Error(err)
 		return

ecdsa.go

@@ -3,25 +3,10 @@ package rfc6979
 import (
 	"crypto/ecdsa"
 	"crypto/elliptic"
+	"hash"
 	"math/big"
 )
 
-// copied from crypto/ecdsa
-func hashToInt(hash []byte, c elliptic.Curve) *big.Int {
-	orderBits := c.Params().N.BitLen()
-	orderBytes := (orderBits + 7) / 8
-	if len(hash) > orderBytes {
-		hash = hash[:orderBytes]
-	}
-
-	ret := new(big.Int).SetBytes(hash)
-	excess := len(hash)*8 - orderBits
-	if excess > 0 {
-		ret.Rsh(ret, uint(excess))
-	}
-	return ret
-}
-
 // SignECDSA signs an arbitrary length hash (which should be the result of
 // hashing a larger message) using the private key, priv. It returns the
 // signature as a pair of integers.
@@ -29,7 +14,7 @@ func hashToInt(hash []byte, c elliptic.Curve) *big.Int {
 // Note that FIPS 186-3 section 4.6 specifies that the hash should be truncated
 // to the byte-length of the subgroup. This function does not perform that
 // truncation itself.
-func SignECDSA(priv *ecdsa.PrivateKey, hash []byte, alg HashFunc) (r, s *big.Int, err error) {
+func SignECDSA(priv *ecdsa.PrivateKey, hash []byte, alg func() hash.Hash) (r, s *big.Int, err error) {
 	c := priv.PublicKey.Curve
 	N := c.Params().N
 
@@ -53,3 +38,19 @@ func SignECDSA(priv *ecdsa.PrivateKey, hash []byte, alg HashFunc) (r, s *big.Int
 
 	return
 }
+
+// copied from crypto/ecdsa
+func hashToInt(hash []byte, c elliptic.Curve) *big.Int {
+	orderBits := c.Params().N.BitLen()
+	orderBytes := (orderBits + 7) / 8
+	if len(hash) > orderBytes {
+		hash = hash[:orderBytes]
+	}
+
+	ret := new(big.Int).SetBytes(hash)
+	excess := len(hash)*8 - orderBits
+	if excess > 0 {
+		ret.Rsh(ret, uint(excess))
+	}
+	return ret
+}

ecdsa_test.go

@@ -1,4 +1,4 @@
-package rfc6979
+package rfc6979_test
 
 import (
 	"crypto/ecdsa"
@@ -6,14 +6,17 @@ import (
 	"crypto/sha1"
 	"crypto/sha256"
 	"crypto/sha512"
+	"hash"
 	"math/big"
 	"testing"
+
+	"github.com/codahale/rfc6979"
 )
 
 type ecdsaFixture struct {
 	name    string
 	key     *ecdsaKey
-	alg     HashFunc
+	alg     func() hash.Hash
 	message string
 	r, s    string
 }
@@ -425,7 +428,7 @@ func testEcsaFixture(f *ecdsaFixture, t *testing.T) {
 		digest = digest[0:g]
 	}
 
-	r, s, err := SignECDSA(f.key.key, digest, f.alg)
+	r, s, err := rfc6979.SignECDSA(f.key.key, digest, f.alg)
 	if err != nil {
 		t.Error(err)
 		return

rfc6979.go

@@ -22,11 +22,8 @@ import (
 	"math/big"
 )
 
-// HashFunc is a function which provides a fresh Hash (e.g., sha256.New).
-type HashFunc func() hash.Hash
-
 // mac returns an HMAC of the given key and message.
-func (alg HashFunc) mac(k, m, buf []byte) []byte {
+func mac(alg func() hash.Hash, k, m, buf []byte) []byte {
 	h := hmac.New(alg, k)
 	h.Write(m)
 	return h.Sum(buf[:0])
@@ -76,7 +73,7 @@ func bits2octets(in []byte, q *big.Int, qlen, rolen int) []byte {
 var one = big.NewInt(1)
 
 // https://tools.ietf.org/html/rfc6979#section-3.2
-func generateSecret(q, x *big.Int, alg HashFunc, hash []byte, test func(*big.Int) bool) {
+func generateSecret(q, x *big.Int, alg func() hash.Hash, hash []byte, test func(*big.Int) bool) {
 	qlen := q.BitLen()
 	holen := alg().Size()
 	rolen := (qlen + 7) >> 3
@@ -89,25 +86,25 @@ func generateSecret(q, x *big.Int, alg HashFunc, hash []byte, test func(*big.Int
 	k := bytes.Repeat([]byte{0x00}, holen)
 
 	// Step D
-	k = alg.mac(k, append(append(v, 0x00), bx...), k)
+	k = mac(alg, k, append(append(v, 0x00), bx...), k)
 
 	// Step E
-	v = alg.mac(k, v, v)
+	v = mac(alg, k, v, v)
 
 	// Step F
-	k = alg.mac(k, append(append(v, 0x01), bx...), k)
+	k = mac(alg, k, append(append(v, 0x01), bx...), k)
 
 	// Step G
-	v = alg.mac(k, v, v)
+	v = mac(alg, k, v, v)
 
 	// Step H
 	for {
 		// Step H1
-		t := make([]byte, 0)
+		var t []byte
 
 		// Step H2
 		for len(t) < qlen/8 {
-			v = alg.mac(k, v, v)
+			v = mac(alg, k, v, v)
 			t = append(t, v...)
 		}
 
@@ -116,7 +113,7 @@ func generateSecret(q, x *big.Int, alg HashFunc, hash []byte, test func(*big.Int
 		if secret.Cmp(one) >= 0 && secret.Cmp(q) < 0 && test(secret) {
 			return
 		}
-		k = alg.mac(k, append(v, 0x00), k)
+		k = mac(alg, k, append(v, 0x00), k)
-		v = alg.mac(k, v, v)
+		v = mac(alg, k, v, v)
 	}
 }