primitives/sr25519: Enable generating secret keys from ed25519 keys

oasisprotocol · Aug 18, 2023 · 3abeae8 · 3abeae8
1 parent db37f07
commit 3abeae8
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Showing 2 changed files with 59 additions and 0 deletions.
diff --git a/primitives/sr25519/keys.go b/primitives/sr25519/keys.go
@@ -252,6 +252,34 @@ func NewSecretKeyFromBytes(b []byte) (*SecretKey, error) {
 	return &sk, nil
 }
 
+// NewSecretKeyFromEd25519Bytes constructs a SecretKey from the byte representation
+// of an expanded Ed25519 key.
+func NewSecretKeyFromEd25519Bytes(b []byte) (*SecretKey, error) {
+	if len(b) != SecretKeySize {
+		return nil, fmt.Errorf("sr25519: invalid expanded ed25519 key length")
+	}
+
+	// Check the scalar is valid, s == clamp(s)
+	s := b[:SecretKeyScalarSize]
+	if s[0] != (s[0]&0b1111_1000) || (s[31] != ((s[31] & 0b0011_1111) | 0b0100_0000)) {
+		return nil, fmt.Errorf("sr25519: ed25519 scalar invalid")
+	}
+
+	scalar, err := scalarDivideByCofactor(b[:SecretKeyScalarSize])
+	if err != nil {
+		return nil, err
+	}
+
+	var nonce [SecretKeyNonceSize]byte
+	copy(nonce[:], b[SecretKeyScalarSize:])
+
+	sk := &SecretKey{
+		key:   scalar,
+		nonce: nonce,
+	}
+	return sk, nil
+}
+
 // GenerateSecretKey generates a SecretKey using entropy from rng.
 // If rng is nil, crypto/rand.Reader will be used.
 func GenerateSecretKey(rng io.Reader) (*SecretKey, error) {

diff --git a/primitives/sr25519/keys_test.go b/primitives/sr25519/keys_test.go
@@ -34,6 +34,7 @@ package sr25519
 import (
 	"bytes"
 	"crypto/rand"
+	"encoding/hex"
 	"testing"
 
 	"github.com/oasisprotocol/curve25519-voi/curve/scalar"
@@ -197,6 +198,36 @@ func TestSecretKey(t *testing.T) {
 			t.Fatalf("sk == sk2")
 		}
 	})
+	t.Run("Ed25519Bytes", func(t *testing.T) {
+		ed25519Bytes, _ := hex.DecodeString("28b0ae221c6bb06856b287f60d7ea0d98552ea5a16db16956849aa371db3eb51fd190cce74df356432b410bd64682309d6dedb27c76845daf388557cbac3ca34")
+		sk, err := NewSecretKeyFromEd25519Bytes(ed25519Bytes)
+		if err != nil {
+			t.Fatalf("NewSecretKeyFromEd25519Bytes: %v", err)
+		}
+
+		pkBytes, err := sk.PublicKey().MarshalBinary()
+		if err != nil {
+			t.Fatalf("sk.MarshalBinary: %v", err)
+		}
+		expectedPkBytes, _ := hex.DecodeString("46ebddef8cd9bb167dc30878d7113b7e168e6f0646beffd77d69d39bad76b47a")
+		if !bytes.Equal(pkBytes, expectedPkBytes) {
+			t.Fatalf("public key bytes differ: %v != %v", hex.EncodeToString(pkBytes), hex.EncodeToString(expectedPkBytes))
+		}
+
+		skBytes := make([]byte, scalar.ScalarSize)
+		if sk.key.ToBytes(skBytes) != nil {
+			t.Fatalf("sk.ToBytes: %v", err)
+		}
+		expectedSkBytes, _ := hex.DecodeString("05d65584630d16cd4af6d0bec10f34bb504a5dcb62dba2122d49f5a663763d0a")
+		if !bytes.Equal(skBytes, expectedSkBytes) {
+			t.Fatalf("secret key bytes differ (%v != %v)", hex.EncodeToString(skBytes), hex.EncodeToString(expectedSkBytes))
+		}
+
+		expectedNonce, _ := hex.DecodeString("fd190cce74df356432b410bd64682309d6dedb27c76845daf388557cbac3ca34")
+		if !bytes.Equal(sk.nonce[:], expectedNonce) {
+			t.Fatalf("nonce bytes differ: %v != %v", hex.EncodeToString(sk.nonce[:]), hex.EncodeToString(expectedNonce))
+		}
+	})
 }
 
 func TestPublicKey(t *testing.T) {