Add Group.HashFunc() to return hash function (#59)

Signed-off-by: bytemare <3641580+bytemare@users.noreply.github.com>

README.md

@@ -41,6 +41,7 @@ type Group interface {
     NewScalar() Scalar
     NewElement() Element
     Base() Element
+	HashFunc() crypto.Hash
     HashToScalar(input, dst []byte) Scalar
     HashToGroup(input, dst []byte) Element
     EncodeToGroup(input, dst []byte) Element

groups.go

@@ -13,6 +13,7 @@
 package crypto
 
 import (
+	"crypto"
 	"errors"
 	"fmt"
 	"sync"
@@ -113,6 +114,11 @@ func checkDST(dst []byte) {
 	}
 }
 
+// HashFunc returns the RFC9380 associated hash function of the group.
+func (g Group) HashFunc() crypto.Hash {
+	return g.get().HashFunc()
+}
+
 // HashToScalar returns a safe mapping of the arbitrary input to a Scalar.
 // The DST must not be empty or nil, and is recommended to be longer than 16 bytes.
 func (g Group) HashToScalar(input, dst []byte) *Scalar {

internal/edwards25519/group.go

@@ -10,6 +10,8 @@
 package edwards25519
 
 import (
+	"crypto"
+
 	ed "filippo.io/edwards25519"
 
 	"github.com/bytemare/crypto/internal"
@@ -43,6 +45,11 @@ func (g Group) Base() internal.Element {
 	return &Element{*ed.NewGeneratorPoint()}
 }
 
+// HashFunc returns the RFC9380 associated hash function of the group.
+func (g Group) HashFunc() crypto.Hash {
+	return crypto.SHA512
+}
+
 // HashToScalar returns a safe mapping of the arbitrary input to a Scalar.
 // The DST must not be empty or nil, and is recommended to be longer than 16 bytes.
 func (g Group) HashToScalar(input, dst []byte) internal.Scalar {

internal/group.go

@@ -9,6 +9,8 @@
 // Package internal defines simple and abstract APIs to group Elements and Scalars.
 package internal
 
+import "crypto"
+
 // Group abstracts operations in a prime-order group.
 type Group interface {
 	// NewScalar returns a new scalar set to 0.
@@ -20,6 +22,9 @@ type Group interface {
 	// Base returns the group's base point a.k.a. canonical generator.
 	Base() Element
 
+	// HashFunc returns the RFC9380 associated hash function of the group.
+	HashFunc() crypto.Hash
+
 	// HashToScalar returns a safe mapping of the arbitrary input to a Scalar.
 	// The DST must not be empty or nil, and is recommended to be longer than 16 bytes.
 	HashToScalar(input, dst []byte) Scalar

internal/nist/group.go

@@ -95,6 +95,11 @@ func (g Group[P]) newPoint(p P) *Element[P] {
 	}
 }
 
+// HashFunc returns the RFC9380 associated hash function of the group.
+func (g Group[P]) HashFunc() crypto.Hash {
+	return g.curve.hash
+}
+
 // HashToScalar returns a safe mapping of the arbitrary input to a Scalar.
 // The DST must not be empty or nil, and is recommended to be longer than 16 bytes.
 func (g Group[P]) HashToScalar(input, dst []byte) internal.Scalar {

internal/ristretto/ristretto.go

@@ -54,6 +54,11 @@ func (g Group) Base() internal.Element {
 	return &Element{*ristretto255.NewElement().Base()}
 }
 
+// HashFunc returns the RFC9380 associated hash function of the group.
+func (g Group) HashFunc() crypto.Hash {
+	return crypto.SHA512
+}
+
 // HashToScalar returns a safe mapping of the arbitrary input to a Scalar.
 // The DST must not be empty or nil, and is recommended to be longer than 16 bytes.
 func (g Group) HashToScalar(input, dst []byte) internal.Scalar {

internal/secp256k1/group.go

@@ -10,6 +10,8 @@
 package secp256k1
 
 import (
+	"crypto"
+
 	"github.com/bytemare/secp256k1"
 
 	"github.com/bytemare/crypto/internal"
@@ -50,6 +52,11 @@ func (g Group) Base() internal.Element {
 	return newElement().Base()
 }
 
+// HashFunc returns the RFC9380 associated hash function of the group.
+func (g Group) HashFunc() crypto.Hash {
+	return crypto.SHA256
+}
+
 // HashToScalar returns a safe mapping of the arbitrary input to a Scalar.
 // The DST must not be empty or nil, and is recommended to be longer than 16 bytes.
 func (g Group) HashToScalar(input, dst []byte) internal.Scalar {

tests/groups_test.go

@@ -141,6 +141,14 @@ func TestGroup_ElementLength(t *testing.T) {
 	})
 }
 
+func TestHashFunc(t *testing.T) {
+	testAll(t, func(group *testGroup) {
+		if group.group.HashFunc() != group.hash {
+			t.Error(errExpectedEquality)
+		}
+	})
+}
+
 func TestHashToScalar(t *testing.T) {
 	testAll(t, func(group *testGroup) {
 		sv := decodeScalar(t, group.group, group.hashToCurve.hashToScalar)

tests/table_test.go

@@ -9,9 +9,10 @@
 package group_test
 
 import (
+	"crypto"
 	"testing"
 
-	"github.com/bytemare/crypto"
+	group "github.com/bytemare/crypto"
 )
 
 func testAll(t *testing.T, f func(*testGroup)) {
@@ -47,7 +48,8 @@ type testGroup struct {
 	hashToCurve   testHashToCurve
 	elementLength int
 	scalarLength  int
-	group         crypto.Group
+	group         group.Group
+	hash          crypto.Hash
 }
 
 var testTable = []*testGroup{
@@ -85,6 +87,7 @@ var testTable = []*testGroup{
 		32,
 		32,
 		1,
+		crypto.SHA512,
 	},
 	{
 		[15]string{
@@ -120,6 +123,7 @@ var testTable = []*testGroup{
 		33,
 		32,
 		3,
+		crypto.SHA256,
 	},
 	{
 		[15]string{
@@ -155,6 +159,7 @@ var testTable = []*testGroup{
 		49,
 		48,
 		4,
+		crypto.SHA384,
 	},
 	{
 		[15]string{
@@ -190,6 +195,7 @@ var testTable = []*testGroup{
 		67,
 		66,
 		5,
+		crypto.SHA512,
 	},
 	{
 		[15]string{
@@ -225,6 +231,7 @@ var testTable = []*testGroup{
 		32,
 		32,
 		6,
+		crypto.SHA512,
 	},
 	{
 		[15]string{
@@ -260,5 +267,6 @@ var testTable = []*testGroup{
 		33,
 		32,
 		7,
+		crypto.SHA256,
 	},
 }

tests/utils_test.go

@@ -14,6 +14,7 @@ import (
 	"encoding/hex"
 	"errors"
 	"fmt"
+	"strings"
 	"testing"
 
 	"github.com/bytemare/crypto"
@@ -166,8 +167,8 @@ func testDecodingHexFails(t *testing.T, thing1, thing2 serde) {
 
 	if err := thing2.DecodeHex(string(malformed)); err == nil {
 		t.Fatal("expected error on malformed string")
-	} else {
-		t.Log(err)
+	} else if !strings.HasSuffix(err.Error(), "DecodeHex: encoding/hex: invalid byte: U+005F '_'") {
+		t.Fatalf("unexpected error: %q", err)
 	}
 }