Initial commit

README.md

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+x25519 Elliptic curve Diffie-Hellman key exchange over Curve25519
+
+Doc at https://godoc.org/github.com/riobard/go-x25519

x25519.go

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+// Package x25519 implements Elliptic Curve Diffie-Hellman (ECDH) function over Curve25519.
+// Details at https://cr.yp.to/ecdh.html and https://tools.ietf.org/html/rfc7748
+package x25519
+
+import (
+	"crypto/rand"
+	"io"
+
+	"golang.org/x/crypto/curve25519"
+)
+
+// KeySize is the size of keys in bytes used in this package.
+const KeySize = 32
+
+// PublicKey is the type of Curve25519 public keys
+type PublicKey []byte
+
+// SecretKey is the type of Curve25519 secret keys
+type SecretKey []byte
+
+// Public returns the PublicKey corresponding to the SecretKey.
+func (k SecretKey) Public() PublicKey {
+	var sk, pk [KeySize]byte
+	copy(sk[:], k)
+	curve25519.ScalarBaseMult(&pk, &sk)
+	return pk[:]
+}
+
+// GenerateKey generates a public/secret key pair using entropy from random, or crypto/rand.Reader
+// if random is nil.
+func GenerateKey(random io.Reader) (PublicKey, SecretKey, error) {
+	var pk, sk [KeySize]byte
+	if random == nil {
+		random = rand.Reader
+	}
+	if _, err := io.ReadFull(random, sk[:]); err != nil {
+		return nil, nil, err
+	}
+	// NOTE: clamping is not necessary because curve25519.ScaleMult will do it anyway.
+	curve25519.ScalarBaseMult(&pk, &sk)
+	return pk[:], sk[:], nil
+}
+
+// ComputeSecret computes the shared secret between our secret key and their public key.
+func ComputeSecret(our SecretKey, their PublicKey) []byte {
+	var shared, sk, pk [KeySize]byte
+	copy(sk[:], our)
+	copy(pk[:], their)
+	curve25519.ScalarMult(&shared, &sk, &pk)
+	return shared[:]
+}

x25519_test.go

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+package x25519
+
+import (
+	"bytes"
+	"encoding/hex"
+	"testing"
+)
+
+var (
+	// Test vector from https://tools.ietf.org/html/rfc7748#section-6.1
+	aliceSK      = "77076d0a7318a57d3c16c17251b26645df4c2f87ebc0992ab177fba51db92c2a"
+	alicePK      = "8520f0098930a754748b7ddcb43ef75a0dbf3a0d26381af4eba4a98eaa9b4e6a"
+	bobSK        = "5dab087e624a8a4b79e17f8b83800ee66f3bb1292618b6fd1c2f8b27ff88e0eb"
+	bobPK        = "de9edb7d7b7dc1b4d35b61c2ece435373f8343c85b78674dadfc7e146f882b4f"
+	sharedSecret = "4a5d9d5ba4ce2de1728e3bf480350f25e07e21c947d19e3376f09b3c1e161742"
+)
+
+func TestGenerateKey(t *testing.T) {
+	ask, err := hex.DecodeString(aliceSK)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	apk := SecretKey(ask).Public()
+	if alicePK != hex.EncodeToString(apk[:]) {
+		t.Fatal("public key failed")
+	}
+
+	bsk, err := hex.DecodeString(bobSK)
+	if err != nil {
+		t.Fatal(err)
+	}
+	bpk := SecretKey(bsk).Public()
+
+	if bobPK != hex.EncodeToString(bpk) {
+		t.Fatal("public key failed")
+	}
+
+	s1 := hex.EncodeToString(ComputeSecret(ask, bpk))
+	s2 := hex.EncodeToString(ComputeSecret(bsk, apk))
+	if s1 != s2 {
+		t.Fatal("shared secret failed")
+	}
+	if s1 != sharedSecret {
+		t.Fatal("shared secret failed")
+	}
+}
+
+func TestGenerateKey2(t *testing.T) {
+	ourPK, ourSK, _ := GenerateKey(nil)
+	theirPK, theirSK, _ := GenerateKey(nil)
+
+	t.Logf("our secret = %0x", ourSK)
+	t.Logf("our public = %0x", ourPK)
+	t.Logf("their secret = %0x", theirSK)
+	t.Logf("their public = %0x", theirPK)
+
+	s1 := ComputeSecret(ourSK, theirPK)
+	s2 := ComputeSecret(theirSK, ourPK)
+	if !bytes.Equal(s1, s2) {
+		t.Fatal("computed shared secrets differs")
+	}
+	t.Logf("shared secret = %0x", s1)
+}