return standard compressed encoding length for nist infinity points (#36

) Signed-off-by: bytemare <3641580+bytemare@users.noreply.github.com>
bytemare · Mar 18, 2023 · de52f48 · de52f48
1 parent 409260c
commit de52f48
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Showing 3 changed files with 42 additions and 4 deletions.
diff --git a/internal/nist/element.go b/internal/nist/element.go
@@ -15,6 +15,12 @@ import (
 	"github.com/bytemare/crypto/internal"
 )
 
+const (
+	p256CompressedEncodingLength = 33
+	p384CompressedEncodingLength = 49
+	p521CompressedEncodingLength = 67
+)
+
 // Element implements the Element interface for group elements over NIST curves.
 type Element[Point nistECPoint[Point]] struct {
 	p   Point
@@ -159,13 +165,34 @@ func (e *Element[P]) Copy() internal.Element {
 
 // Encode returns the compressed byte encoding of the element.
 func (e *Element[P]) Encode() []byte {
+	if e.IsIdentity() {
+		return encodeInfinity(e)
+	}
+
 	return e.p.BytesCompressed()
 }
 
+func encodeInfinity[Point nistECPoint[Point]](element *Element[Point]) []byte {
+	_, err := element.p.BytesX()
+	var encodedLength int
+
+	switch err.Error()[:4] {
+	case "P256":
+		encodedLength = p256CompressedEncodingLength
+	case "P384":
+		encodedLength = p384CompressedEncodingLength
+	case "P521":
+		encodedLength = p521CompressedEncodingLength
+	}
+
+	return make([]byte, encodedLength)
+}
+
 // XCoordinate returns the encoded x coordinate of the element.
 func (e *Element[P]) XCoordinate() []byte {
 	if e.IsIdentity() {
-		return e.new().BytesCompressed()
+		inf := encodeInfinity(e)
+		return inf[:len(inf)-1]
 	}
 
 	b, err := e.p.BytesX()

diff --git a/tests/element_test.go b/tests/element_test.go
@@ -9,6 +9,7 @@
 package group_test
 
 import (
+	"encoding/hex"
 	"testing"
 
 	"github.com/bytemare/crypto"
@@ -119,6 +120,16 @@ func TestElement_WrongInput(t *testing.T) {
 
 func TestElement_EncodedLength(t *testing.T) {
 	testAll(t, func(t2 *testing.T, group *testGroup) {
+		id := group.id.NewElement().Identity().Encode()
+		if len(id) != group.elementLength {
+			t.Fatalf("Encode() of the identity element is expected to return %d bytes, but returned %d bytes", group.elementLength, len(id))
+		}
+
+		encodedID := hex.EncodeToString(id)
+		if encodedID != group.identity {
+			t.Fatalf("Encode() of the identity element is unexpected.\n\twant: %v\n\tgot : %v", group.identity, encodedID)
+		}
+
 		encodedElement := group.id.NewElement().Base().Multiply(group.id.NewScalar().Random()).Encode()
 		if len(encodedElement) != group.elementLength {
 			t.Fatalf("Encode() is expected to return %d bytes, but returned %d bytes", group.elementLength, encodedElement)

diff --git a/tests/utils_test.go b/tests/utils_test.go
@@ -47,7 +47,7 @@ func testGroups() []*testGroup {
 			"P256_XMD:SHA-256_SSWU_RO_",
 			"P256_XMD:SHA-256_SSWU_NU_",
 			"036b17d1f2e12c4247f8bce6e563a440f277037d812deb33a0f4a13945d898c296",
-			"00",
+			"000000000000000000000000000000000000000000000000000000000000000000",
 			33,
 			32,
 			3,
@@ -57,7 +57,7 @@ func testGroups() []*testGroup {
 			"P384_XMD:SHA-384_SSWU_RO_",
 			"P384_XMD:SHA-384_SSWU_NU_",
 			"03aa87ca22be8b05378eb1c71ef320ad746e1d3b628ba79b9859f741e082542a385502f25dbf55296c3a545e3872760ab7",
-			"00",
+			"00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000",
 			49,
 			48,
 			4,
@@ -67,7 +67,7 @@ func testGroups() []*testGroup {
 			"P521_XMD:SHA-512_SSWU_RO_",
 			"P521_XMD:SHA-512_SSWU_NU_",
 			"0200c6858e06b70404e9cd9e3ecb662395b4429c648139053fb521f828af606b4d3dbaa14b5e77efe75928fe1dc127a2ffa8de3348b3c1856a429bf97e7e31c2e5bd66",
-			"00",
+			"00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000",
 			67,
 			66,
 			5,