Add the Rekey feature as described in spec

Signed-off-by: David Stainton <dstainton415@gmail.com>

noise_test.go

@@ -464,3 +464,78 @@ func (NoiseSuite) TestHandshakeRollback(c *C) {
 	expected, _ := hex.DecodeString("07a37cbc142093c8b755dc1b10e86cb426374ad16aa853ed0bdfc0b2b86d1c7c5e4dc9545d41b3280f4586a5481829e1e24ec5a0")
 	c.Assert(msg, DeepEquals, expected)
 }
+
+func (NoiseSuite) TestRekey(c *C) {
+	rng := new(RandomInc)
+
+	clientStaticKeypair := DH25519.GenerateKeypair(rng)
+	clientConfig := Config{}
+	clientConfig.CipherSuite = NewCipherSuite(DH25519, CipherChaChaPoly, HashBLAKE2b)
+	clientConfig.Random = rng
+	clientConfig.Pattern = HandshakeNN
+	clientConfig.Initiator = true
+	clientConfig.Prologue = []byte{0}
+	clientConfig.StaticKeypair = clientStaticKeypair
+	clientConfig.EphemeralKeypair = DH25519.GenerateKeypair(rng)
+	clientHs := NewHandshakeState(clientConfig)
+
+	serverStaticKeypair := DH25519.GenerateKeypair(rng)
+	serverConfig := Config{}
+	serverConfig.CipherSuite = NewCipherSuite(DH25519, CipherChaChaPoly, HashBLAKE2b)
+	serverConfig.Random = rng
+	serverConfig.Pattern = HandshakeNN
+	serverConfig.Initiator = false
+	serverConfig.Prologue = []byte{0}
+	serverConfig.StaticKeypair = serverStaticKeypair
+	serverConfig.EphemeralKeypair = DH25519.GenerateKeypair(rng)
+	serverHs := NewHandshakeState(serverConfig)
+
+	clientHsMsg, _, _ := clientHs.WriteMessage(nil, nil)
+	c.Assert(32, Equals, len(clientHsMsg))
+
+	serverHsResult, _, _, err := serverHs.ReadMessage(nil, clientHsMsg)
+	c.Assert(err, IsNil)
+	c.Assert(0, Equals, len(serverHsResult))
+
+	serverHsMsg, csR0, csR1 := serverHs.WriteMessage(nil, nil)
+	c.Assert(48, Equals, len(serverHsMsg))
+
+	clientHsResult, csI0, csI1, err := clientHs.ReadMessage(nil, serverHsMsg)
+	c.Assert(err, IsNil)
+	c.Assert(0, Equals, len(clientHsResult))
+
+	clientMessage := []byte("hello")
+	msg := csI0.Encrypt(nil, nil, clientMessage)
+	res, err := csR0.Decrypt(nil, nil, msg)
+	c.Assert(string(clientMessage), Equals, string(res))
+
+	oldK := csI0.k
+	csI0.Rekey()
+	c.Assert(oldK, Not(Equals), csI0.k)
+	csR0.Rekey()
+
+	clientMessage = []byte("hello again")
+	msg = csI0.Encrypt(nil, nil, clientMessage)
+	res, err = csR0.Decrypt(nil, nil, msg)
+	c.Assert(string(clientMessage), Equals, string(res))
+
+	serverMessage := []byte("bye")
+	msg = csR1.Encrypt(nil, nil, serverMessage)
+	res, err = csI1.Decrypt(nil, nil, msg)
+	c.Assert(string(serverMessage), Equals, string(res))
+
+	csR1.Rekey()
+	csI1.Rekey()
+
+	serverMessage = []byte("bye bye")
+	msg = csR1.Encrypt(nil, nil, serverMessage)
+	res, err = csI1.Decrypt(nil, nil, msg)
+	c.Assert(string(serverMessage), Equals, string(res))
+
+	// only rekey one side, test for failure
+	csR1.Rekey()
+	serverMessage = []byte("bye again")
+	msg = csR1.Encrypt(nil, nil, serverMessage)
+	res, err = csI1.Decrypt(nil, nil, msg)
+	c.Assert(string(serverMessage), Not(Equals), string(res))
+}

state.go

@@ -11,6 +11,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math"
 )
 
 // A CipherState provides symmetric encryption and decryption after a successful
@@ -61,6 +62,14 @@ func (s *CipherState) Cipher() Cipher {
 	return s.c
 }
 
+func (s *CipherState) Rekey() {
+	var zeros [32]byte
+	var out []byte
+	out = s.c.Encrypt(out, math.MaxUint64, []byte{}, zeros[:])
+	copy(s.k[:], out[:32])
+	s.c = s.cs.Cipher(s.k)
+}
+
 type symmetricState struct {
 	CipherState
 	hasK bool