From b45784e07b0fd05f45b1e6d4ca5da580c7b9471b Mon Sep 17 00:00:00 2001
From: Russ Magee <rmagee@gmail.com>
Date: Sun, 15 Apr 2018 12:58:24 -0700
Subject: [PATCH] Minimal hmac channel verification w/close on tampering

---
 hkexnet.go       | 34 ++++++++++++++++------------------
 hkexsh/hkexsh.go |  5 ++++-
 2 files changed, 20 insertions(+), 19 deletions(-)

diff --git a/hkexnet.go b/hkexnet.go
index 63a318b..0405a86 100644
--- a/hkexnet.go
+++ b/hkexnet.go
@@ -298,10 +298,10 @@ func (c Conn) Read(b []byte) (n int, err error) {
 			break
 		}
 
-		var hmacIn uint8
+		var hmacIn [4]uint8
 		var payloadLen uint32
 
-		// Read the hmac LSB and payload len first
+		// Read the hmac and payload len first
 		err = binary.Read(c.c, binary.BigEndian, &hmacIn)
 		// Normal client 'exit' from interactive session will cause
 		// (on server side) err.Error() == "<iface/addr info ...>: use of closed network connection"
@@ -314,14 +314,6 @@ func (c Conn) Read(b []byte) (n int, err error) {
 			return 0, err
 		}
 
-		//if err != nil {
-		//	if err.Error() != "EOF" {
-		//		log.Println("Error was:", err.Error())
-		//	} else {
-		//		return 0, err
-		//	}
-		//}
-
 		err = binary.Read(c.c, binary.BigEndian, &payloadLen)
 		if err != nil {
 			if err.Error() != "EOF" {
@@ -348,7 +340,7 @@ func (c Conn) Read(b []byte) (n int, err error) {
 			}
 		}
 
-		log.Printf("  <:ctext:\r\n%s\r\n", hex.Dump(payloadBytes[:n])) //EncodeToString(b[:n])) // print only used portion
+		log.Printf("  <:ctext:\r\n%s\r\n", hex.Dump(payloadBytes[:n]))
 
 		db := bytes.NewBuffer(payloadBytes[:n]) //copying payloadBytes to db
 		// The StreamReader acts like a pipe, decrypting
@@ -358,7 +350,7 @@ func (c Conn) Read(b []byte) (n int, err error) {
 		// The caller isn't necessarily reading the full payload so we need
 		// to decrypt ot an intermediate buffer, draining it on demand of caller
 		decryptN, err := rs.Read(payloadBytes)
-		log.Printf("  <-ptext:\r\n%s\r\n", hex.Dump(payloadBytes[:n])) //EncodeToString(b[:n]))
+		log.Printf("  <-ptext:\r\n%s\r\n", hex.Dump(payloadBytes[:n]))
 		if err != nil {
 			panic(err)
 		}
@@ -367,8 +359,14 @@ func (c Conn) Read(b []byte) (n int, err error) {
 
 		// Re-calculate hmac, compare with received value
 		c.rm.Write(payloadBytes)
-		hTmp := c.rm.Sum(nil)[0]
-		log.Printf("<%04x) HMAC:(i)%02x (c)%02x\r\n", decryptN, hmacIn, hTmp)
+		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 {
+			fmt.Println("** ALERT - hmac mismatch, possible channel tampering **")
+			c.Close()
+		}
 	}
 	retN := c.dBuf.Len()
 	if retN > len(b) {
@@ -386,18 +384,18 @@ func (c Conn) Read(b []byte) (n int, err error) {
 // See go doc io.Writer
 func (c Conn) Write(b []byte) (n int, err error) {
 	//log.Printf("[Encrypting...]\r\n")
-	var hmacOut uint8
+	var hmacOut []uint8
 	var payloadLen uint32
 
-	log.Printf("  :>ptext:\r\n%s\r\n", hex.Dump(b)) //EncodeToString(b))
+	log.Printf("  :>ptext:\r\n%s\r\n", hex.Dump(b))
 
 	payloadLen = uint32(len(b))
 
 	// Calculate hmac on payload
 	c.wm.Write(b)
-	hmacOut = uint8(c.wm.Sum(nil)[0])
+	hmacOut = c.wm.Sum(nil)[0:4]
 
-	log.Printf("  (%04x> HMAC(o):%02x\r\n", payloadLen, hmacOut)
+	log.Printf("  (%04x> HMAC(o):%s\r\n", payloadLen, hex.EncodeToString(hmacOut))
 
 	var wb bytes.Buffer
 	// The StreamWriter acts like a pipe, forwarding whatever is
diff --git a/hkexsh/hkexsh.go b/hkexsh/hkexsh.go
index cd969b6..e0657d7 100644
--- a/hkexsh/hkexsh.go
+++ b/hkexsh/hkexsh.go
@@ -80,8 +80,9 @@ func main() {
 	// Set stdin in raw mode if it's an interactive session
 	// TODO: send flag to server side indicating this
 	//  affects shell command used
+	var oldState *hkexsh.State
 	if isatty.IsTerminal(os.Stdin.Fd()) {
-		oldState, err := hkexsh.MakeRaw(int(os.Stdin.Fd()))
+		oldState, err = hkexsh.MakeRaw(int(os.Stdin.Fd()))
 		if err != nil {
 			panic(err)
 		}
@@ -155,6 +156,7 @@ func main() {
 		if inerr != nil {
 			if inerr.Error() != "EOF" {
 				fmt.Println(inerr)
+				_ = hkexsh.Restore(int(os.Stdin.Fd()), oldState) // Best effort.
 				os.Exit(1)
 			}
 		}
@@ -178,6 +180,7 @@ func main() {
 				log.Println(outerr)
 				if outerr.Error() != "EOF" {
 					fmt.Println(outerr)
+					_ = hkexsh.Restore(int(os.Stdin.Fd()), oldState) // Best effort.
 					os.Exit(2)
 				}
 			}