From c56d4d9ad9b62267014fe208c50c92673e9ade28 Mon Sep 17 00:00:00 2001
From: Russ Magee <rmagee@gmail.com>
Date: Sun, 15 Apr 2018 13:29:06 -0700
Subject: [PATCH] hmac tampering indication implemented -- local and
 remote-side

---
 hkexnet.go | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/hkexnet.go b/hkexnet.go
index 0405a86..2849781 100644
--- a/hkexnet.go
+++ b/hkexnet.go
@@ -17,6 +17,7 @@ import (
 	"crypto/cipher"
 	"encoding/binary"
 	"encoding/hex"
+	"errors"
 	"fmt"
 	"hash"
 	"io"
@@ -298,9 +299,18 @@ func (c Conn) Read(b []byte) (n int, err error) {
 			break
 		}
 
+		var ctrlStatOp uint8
 		var hmacIn [4]uint8
 		var payloadLen uint32
 
+		// Read ctrl/status opcode (for now, set nonzero on hmac mismatch)
+		err = binary.Read(c.c, binary.BigEndian, &ctrlStatOp)
+		if ctrlStatOp != 0 {
+			// Other side indicated channel tampering, close channel
+			c.Close()
+			return 1, errors.New("** ALERT - remote end detected HMAC mismatch - possible channel tampering **")
+		}
+
 		// Read the hmac and payload len first
 		err = binary.Read(c.c, binary.BigEndian, &hmacIn)
 		// Normal client 'exit' from interactive session will cause
@@ -362,10 +372,10 @@ func (c Conn) Read(b []byte) (n int, err error) {
 		hTmp := c.rm.Sum(nil)[0:4]
 		log.Printf("<%04x) HMAC:(i)%s (c)%02x\r\n", decryptN, hex.EncodeToString([]byte(hmacIn[0:])), hTmp)
 
-		// Puke if hmac didn't match, corrupted channel
-		if !bytes.Equal(hTmp, []byte(hmacIn[0:])) || hmacIn[0] > 0xf8 {
+		// Log alert if hmac didn't match, corrupted channel
+		if !bytes.Equal(hTmp, []byte(hmacIn[0:])) /*|| hmacIn[0] > 0xf8*/ {
 			fmt.Println("** ALERT - hmac mismatch, possible channel tampering **")
-			c.Close()
+			_, _ = c.c.Write([]byte{0x1})
 		}
 	}
 	retN := c.dBuf.Len()
@@ -406,7 +416,11 @@ func (c Conn) Write(b []byte) (n int, err error) {
 		panic(err)
 	}
 	log.Printf("  ->ctext:\r\n%s\r\n", hex.Dump(wb.Bytes()))
-
+	
+	var ctrlStatOp byte
+	ctrlStatOp = 0x00
+	_ = binary.Write(c.c, binary.BigEndian, &ctrlStatOp)
+	
 	// Write hmac LSB, payloadLen followed by payload
 	_ = binary.Write(c.c, binary.BigEndian, hmacOut)
 	_ = binary.Write(c.c, binary.BigEndian, payloadLen)