ssh: Add the hmac-sha2-256-etm@openssh.com algorithm

Fixes golang/go#17676

Change-Id: I96c51431b174898a6bc0f6bec7f4561d5d64819f

ssh/cipher.go

@@ -135,6 +135,7 @@ const prefixLen = 5
 type streamPacketCipher struct {
 	mac    hash.Hash
 	cipher cipher.Stream
+	etm    bool
 
 	// The following members are to avoid per-packet allocations.
 	prefix      [prefixLen]byte
@@ -150,7 +151,14 @@ func (s *streamPacketCipher) readPacket(seqNum uint32, r io.Reader) ([]byte, err
 		return nil, err
 	}
 
-	s.cipher.XORKeyStream(s.prefix[:], s.prefix[:])
+	var encryptedPaddingLength [1]byte
+	if s.mac != nil && s.etm {
+		copy(encryptedPaddingLength[:], s.prefix[4:5])
+		s.cipher.XORKeyStream(s.prefix[4:5], s.prefix[4:5])
+	} else {
+		s.cipher.XORKeyStream(s.prefix[:], s.prefix[:])
+	}
+
 	length := binary.BigEndian.Uint32(s.prefix[0:4])
 	paddingLength := uint32(s.prefix[4])
 
@@ -159,7 +167,12 @@ func (s *streamPacketCipher) readPacket(seqNum uint32, r io.Reader) ([]byte, err
 		s.mac.Reset()
 		binary.BigEndian.PutUint32(s.seqNumBytes[:], seqNum)
 		s.mac.Write(s.seqNumBytes[:])
-		s.mac.Write(s.prefix[:])
+		if s.etm {
+			s.mac.Write(s.prefix[:4])
+			s.mac.Write(encryptedPaddingLength[:])
+		} else {
+			s.mac.Write(s.prefix[:])
+		}
 		macSize = uint32(s.mac.Size())
 	}
 
@@ -184,10 +197,17 @@ func (s *streamPacketCipher) readPacket(seqNum uint32, r io.Reader) ([]byte, err
 	}
 	mac := s.packetData[length-1:]
 	data := s.packetData[:length-1]
+
+	if s.mac != nil && s.etm {
+		s.mac.Write(data)
+	}
+
 	s.cipher.XORKeyStream(data, data)
 
 	if s.mac != nil {
-		s.mac.Write(data)
+		if !s.etm {
+			s.mac.Write(data)
+		}
 		s.macResult = s.mac.Sum(s.macResult[:0])
 		if subtle.ConstantTimeCompare(s.macResult, mac) != 1 {
 			return nil, errors.New("ssh: MAC failure")
@@ -203,7 +223,13 @@ func (s *streamPacketCipher) writePacket(seqNum uint32, w io.Writer, rand io.Rea
 		return errors.New("ssh: packet too large")
 	}
 
-	paddingLength := packetSizeMultiple - (prefixLen+len(packet))%packetSizeMultiple
+	aadlen := 0
+	if s.mac != nil && s.etm {
+		// packet length is not encrypted for EtM modes
+		aadlen = 4
+	}
+
+	paddingLength := packetSizeMultiple - (prefixLen+len(packet)-aadlen)%packetSizeMultiple
 	if paddingLength < 4 {
 		paddingLength += packetSizeMultiple
 	}
@@ -220,15 +246,37 @@ func (s *streamPacketCipher) writePacket(seqNum uint32, w io.Writer, rand io.Rea
 		s.mac.Reset()
 		binary.BigEndian.PutUint32(s.seqNumBytes[:], seqNum)
 		s.mac.Write(s.seqNumBytes[:])
+
+		if s.etm {
+			// For EtM algorithms, the packet length must stay unencrypted,
+			// but the following data (padding length) must be encrypted
+			s.cipher.XORKeyStream(s.prefix[4:5], s.prefix[4:5])
+		}
+
 		s.mac.Write(s.prefix[:])
-		s.mac.Write(packet)
-		s.mac.Write(padding)
+
+		if !s.etm {
+			// For non-EtM algorithms, the algorithm is applied on unencrypted data
+			s.mac.Write(packet)
+			s.mac.Write(padding)
+		}
+	}
+
+	if !(s.mac != nil && s.etm) {
+		// For EtM algorithms, the padding length has already been encrypted
+		// and the packet length must remain unencrypted
+		s.cipher.XORKeyStream(s.prefix[:], s.prefix[:])
 	}
 
-	s.cipher.XORKeyStream(s.prefix[:], s.prefix[:])
 	s.cipher.XORKeyStream(packet, packet)
 	s.cipher.XORKeyStream(padding, padding)
 
+	if s.mac != nil && s.etm {
+		// For EtM algorithms, packet and padding must be encrypted
+		s.mac.Write(packet)
+		s.mac.Write(padding)
+	}
+
 	if _, err := w.Write(s.prefix[:]); err != nil {
 		return err
 	}

ssh/cipher_test.go

@@ -26,39 +26,41 @@ func TestPacketCiphers(t *testing.T) {
 	defer delete(cipherModes, aes128cbcID)
 
 	for cipher := range cipherModes {
-		kr := &kexResult{Hash: crypto.SHA1}
-		algs := directionAlgorithms{
-			Cipher:      cipher,
-			MAC:         "hmac-sha1",
-			Compression: "none",
-		}
-		client, err := newPacketCipher(clientKeys, algs, kr)
-		if err != nil {
-			t.Errorf("newPacketCipher(client, %q): %v", cipher, err)
-			continue
-		}
-		server, err := newPacketCipher(clientKeys, algs, kr)
-		if err != nil {
-			t.Errorf("newPacketCipher(client, %q): %v", cipher, err)
-			continue
-		}
-
-		want := "bla bla"
-		input := []byte(want)
-		buf := &bytes.Buffer{}
-		if err := client.writePacket(0, buf, rand.Reader, input); err != nil {
-			t.Errorf("writePacket(%q): %v", cipher, err)
-			continue
-		}
-
-		packet, err := server.readPacket(0, buf)
-		if err != nil {
-			t.Errorf("readPacket(%q): %v", cipher, err)
-			continue
-		}
-
-		if string(packet) != want {
-			t.Errorf("roundtrip(%q): got %q, want %q", cipher, packet, want)
+		for mac := range macModes {
+			kr := &kexResult{Hash: crypto.SHA1}
+			algs := directionAlgorithms{
+				Cipher:      cipher,
+				MAC:         mac,
+				Compression: "none",
+			}
+			client, err := newPacketCipher(clientKeys, algs, kr)
+			if err != nil {
+				t.Errorf("newPacketCipher(client, %q, %q): %v", cipher, mac, err)
+				continue
+			}
+			server, err := newPacketCipher(clientKeys, algs, kr)
+			if err != nil {
+				t.Errorf("newPacketCipher(client, %q, %q): %v", cipher, mac, err)
+				continue
+			}
+
+			want := "bla bla"
+			input := []byte(want)
+			buf := &bytes.Buffer{}
+			if err := client.writePacket(0, buf, rand.Reader, input); err != nil {
+				t.Errorf("writePacket(%q, %q): %v", cipher, mac, err)
+				continue
+			}
+
+			packet, err := server.readPacket(0, buf)
+			if err != nil {
+				t.Errorf("readPacket(%q, %q): %v", cipher, mac, err)
+				continue
+			}
+
+			if string(packet) != want {
+				t.Errorf("roundtrip(%q, %q): got %q, want %q", cipher, mac, packet, want)
+			}
 		}
 	}
 }

ssh/common.go

@@ -56,7 +56,7 @@ var supportedHostKeyAlgos = []string{
 // This is based on RFC 4253, section 6.4, but with hmac-md5 variants removed
 // because they have reached the end of their useful life.
 var supportedMACs = []string{
-	"hmac-sha2-256", "hmac-sha1", "hmac-sha1-96",
+	"hmac-sha2-256-etm@openssh.com", "hmac-sha2-256", "hmac-sha1", "hmac-sha1-96",
 }
 
 var supportedCompressions = []string{compressionNone}

ssh/mac.go

@@ -15,6 +15,7 @@ import (
 
 type macMode struct {
 	keySize int
+	etm     bool
 	new     func(key []byte) hash.Hash
 }
 
@@ -45,13 +46,16 @@ func (t truncatingMAC) Size() int {
 func (t truncatingMAC) BlockSize() int { return t.hmac.BlockSize() }
 
 var macModes = map[string]*macMode{
-	"hmac-sha2-256": {32, func(key []byte) hash.Hash {
+	"hmac-sha2-256-etm@openssh.com": {32, true, func(key []byte) hash.Hash {
 		return hmac.New(sha256.New, key)
 	}},
-	"hmac-sha1": {20, func(key []byte) hash.Hash {
+	"hmac-sha2-256": {32, false, func(key []byte) hash.Hash {
+		return hmac.New(sha256.New, key)
+	}},
+	"hmac-sha1": {20, false, func(key []byte) hash.Hash {
 		return hmac.New(sha1.New, key)
 	}},
-	"hmac-sha1-96": {20, func(key []byte) hash.Hash {
+	"hmac-sha1-96": {20, false, func(key []byte) hash.Hash {
 		return truncatingMAC{12, hmac.New(sha1.New, key)}
 	}},
 }

ssh/transport.go

@@ -267,6 +267,7 @@ func newPacketCipher(d direction, algs directionAlgorithms, kex *kexResult) (pac
 
 	c := &streamPacketCipher{
 		mac: macModes[algs.MAC].new(macKey),
+		etm: macModes[algs.MAC].etm,
 	}
 	c.macResult = make([]byte, c.mac.Size())