Add EthAddress method for Ethereum-compatible address derivation

Adds EthAddress() methods to both PrivateKey and PublicKey types
to compute Ethereum addresses from secp256k1 keys using Keccak256.

go.mod

@@ -1,6 +1,6 @@
 module github.com/luxfi/crypto
 
-go 1.25.5
+go 1.25.4
 
 require (
 	filippo.io/age v1.2.1

secp256k1/keys.go

@@ -191,6 +191,11 @@ func (k *PrivateKey) Address() ids.ShortID {
 	return k.PublicKey().Address()
 }
 
+// EthAddress returns the Ethereum address derived from the private key
+func (k *PrivateKey) EthAddress() [20]byte {
+	return k.PublicKey().EthAddress()
+}
+
 // Address returns the address of the public key as an ids.ShortID
 func (k *PublicKey) Address() ids.ShortID {
 	// Use traditional Lux address format (SHA256 + RIPEMD160)
@@ -201,6 +206,21 @@ func (k *PublicKey) Address() ids.ShortID {
 	return addr
 }
 
+// EthAddress returns the Ethereum address derived from the public key
+// This is computed as the last 20 bytes of the Keccak256 hash of the uncompressed public key
+func (k *PublicKey) EthAddress() [20]byte {
+	// Get uncompressed public key bytes (excluding the 0x04 prefix)
+	pkBytes := k.Bytes()
+	
+	// Compute Keccak256 hash
+	hash := Keccak256(pkBytes)
+	
+	// Take the last 20 bytes as the address
+	var addr [20]byte
+	copy(addr[:], hash[12:])
+	return addr
+}
+
 // Bytes returns the public key bytes
 func (k *PublicKey) Bytes() []byte {
 	return k.bytes