From 79d84b226152f609197437cc2377b7bad540f4e7 Mon Sep 17 00:00:00 2001
From: Fredrik Thulin <fredrik@thulin.net>
Date: Thu, 23 Mar 2017 19:44:26 +0100
Subject: Add CRC32.

---
 cryptech_muxd | 54 ++++++++++++++++++++++++++++++++++++++++++++++--------
 slip.c        | 22 ++++++++++++++++++++++
 2 files changed, 68 insertions(+), 8 deletions(-)

diff --git a/cryptech_muxd b/cryptech_muxd
index 269ac15..3dcf449 100755
--- a/cryptech_muxd
+++ b/cryptech_muxd
@@ -58,6 +58,7 @@ import tornado.queues
 import tornado.locks
 import tornado.gen
 
+from zlib import crc32
 
 logger = logging.getLogger("cryptech_muxd")
 
@@ -89,6 +90,19 @@ def client_handle_set(msg, handle):
     return msg[:4] + struct.pack(">L", handle) + msg[8:]
 
 
+def send_checksum(msg):
+    "Add a CRC32 checksum at the end of the message."
+    crc = (~crc32(msg)) & 0xffffffff
+    return msg + struct.pack("<I", crc)
+
+def verify_checksum(msg):
+    "Verify the CRC32 checksum at the end of the message."
+    crc = crc32(msg) & 0xffffffff
+    if crc != 0xffffffff:
+        raise ValueError('Bad CRC32 in message: {} (0x{:8x})'.format(':'.join('{:02x}'.format(ord(c)) for c in msg), crc))
+    return msg[:-4]
+
+
 class SerialIOStream(tornado.iostream.BaseIOStream):
     """
     Implementation of a Tornado IOStream over a PySerial device.
@@ -164,14 +178,30 @@ class RPCIOStream(SerialIOStream):
                     q.put_nowait(None)
                 return
             logger.debug("RPC recv: %s", ":".join("{:02x}".format(ord(c)) for c in reply))
+
+            reply = slip_decode(reply)
+
+            if len(reply) < 5:
+                continue
+
+            # Check CRC
             try:
-                handle = client_handle_get(slip_decode(reply))
+                reply = verify_checksum(reply)
+            except ValueError:
+                logger.error("RPC response CRC fail: {}".format(":".join("{:02x}".format(ord(c)) for c in reply)))
+                continue
+
+            try:
+                handle = client_handle_get(reply)
             except:
                 continue
-            logger.debug("RPC queue put: handle 0x%x, qsize %s, maxsize %s",
-                         handle, self.queues[handle].qsize(), self.queues[handle].maxsize)
-            self.queues[handle].put_nowait(reply)
 
+            try:
+                logger.debug("RPC queue put: handle 0x%x, qsize %s, maxsize %s",
+                             handle, self.queues[handle].qsize(), self.queues[handle].maxsize)
+                self.queues[handle].put_nowait(slip_encode(reply))
+            except:
+                logger.debug("Invalid RPC handle: 0x{:08x} / {}".format(handle, handle))
 
 class QueuedStreamClosedError(tornado.iostream.StreamClosedError):
     "Deferred StreamClosedError passed throught a Queue."
@@ -194,7 +224,7 @@ class RPCServer(PFUnixServer):
                 query = yield stream.read_until(SLIP_END)
                 if len(query) < 9:
                     continue
-                query = slip_encode(client_handle_set(slip_decode(query), handle))
+                query = slip_encode(send_checksum(client_handle_set(slip_decode(query), handle)))
                 yield self.serial.rpc_input(query, handle, queue)
                 logger.debug("RPC queue wait, handle 0x%x", handle)
                 reply = yield queue.get()
@@ -307,21 +337,29 @@ class ProbeIOStream(SerialIOStream):
     @tornado.gen.coroutine
     def run_probe(self):
 
-        RPC_query = chr(0) * 8  # client_handle = 0, function code = RPC_FUNC_GET_VERSION
+        RPC_query = send_checksum(chr(0) * 8)  # client_handle = 0, function code = RPC_FUNC_GET_VERSION
         RPC_reply = chr(0) * 12 # opcode = RPC_FUNC_GET_VERSION, client_handle = 0, valret = HAL_OK
 
         probe_string = SLIP_END + Control_U + SLIP_END + RPC_query + SLIP_END + Control_U + Control_M
 
+        logger.debug("Probing %s with: %s", self.serial_device, ":".join("{:02x}".format(ord(c)) for c in probe_string))
+
         yield self.write(probe_string)
         yield tornado.gen.sleep(0.5)
         response = yield self.read_bytes(self.read_chunk_size, partial = True)
 
-        logger.debug("Probing %s: %r %s", self.serial_device, response, ":".join("{:02x}".format(ord(c)) for c in response))
+        logger.debug("Probing %s response: %r %s", self.serial_device, response, ":".join("{:02x}".format(ord(c)) for c in response))
 
         is_cty = any(prompt in response for prompt in ("Username:", "Password:", "cryptech>"))
 
         try:
-            is_rpc = response[response.index(SLIP_END + RPC_reply) + len(SLIP_END + RPC_reply) + 4] == SLIP_END
+            reply_idx = response.index(SLIP_END + RPC_reply)
+            reply_len = len(SLIP_END + RPC_reply)
+            logger.debug("Reply index {}, length {}".format(reply_idx, reply_len))
+            end_offs = reply_idx + reply_len + 8  # RPC_reply is followed by 4 bytes of version data and a CRC32 checksum
+            is_rpc = response[end_offs] == SLIP_END
+            logger.debug("Response[{} + {} + 4] = 0x{:x} (is_rpc {})".format(
+                reply_idx, reply_len, ord(response[end_offs]), is_rpc))
         except ValueError:
             is_rpc = False
         except IndexError:
diff --git a/slip.c b/slip.c
index b28b7e1..279b1d3 100644
--- a/slip.c
+++ b/slip.c
@@ -73,15 +73,34 @@ hal_error_t hal_slip_send_char(const uint8_t c)
     return HAL_OK;
 }
 
+static hal_error_t _send_uint32(const uint32_t val)
+{
+    uint32_t data = val;
+
+    for (int i = 0; i < 4; ++i) {
+	uint8_t *p = (uint8_t *) &data;
+	check(hal_slip_send_char(p[i]));
+    }
+
+    return HAL_OK;
+}
+
 /* Send a message with SLIP framing.
  */
 hal_error_t hal_slip_send(const uint8_t * const buf, const size_t len)
 {
+    hal_crc32_t crc;
+
     /* send an initial END character to flush out any data that may
      * have accumulated in the receiver due to line noise
      */
     check(hal_serial_send_char(END));
 
+    /* Calculate CRC32 checksum of the contents, before SLIP encoding */
+    crc = hal_crc32_init();
+    crc = hal_crc32_update(crc, buf, len);
+    crc = ~hal_crc32_finalize(crc);
+
     /* for each byte in the packet, send the appropriate character
      * sequence
      */
@@ -91,6 +110,9 @@ hal_error_t hal_slip_send(const uint8_t * const buf, const size_t len)
             return ret;
     }
 
+    /* Transmit the CRC */
+    check(_send_uint32(crc));
+
     /* tell the receiver that we're done sending the packet
      */
     check(hal_serial_send_char(END));
-- 
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