Implement sptps_verify_datagram().

Implementation of sptps_verify_datagram() was left as a TODO. This
causes problems when using SPTPS in tinc, because this function is
used in try_mac(), which itself is used in try_harder() to locate
nodes sending UDP packets from unexpected addresses. In the current
state this function always returns true, resulting in UDP addresses
of random nodes getting changed which makes UDP communication
fragile and unreliable. In addition, this makes UDP communication
impossible through port translation and local discovery.

This commit adds the missing implementation, thus fixing the issue.

diff --git a/src/sptps.c b/src/sptps.c
index 7d6293c..3fbd854 100644 (file)
--- a/src/sptps.c
+++ b/src/sptps.c
@@ -370,14 +370,74 @@ static bool receive_handshake(sptps_t *s, const char *data, uint16_t len) {
        }
 }
 
+static bool sptps_check_seqno(sptps_t *s, uint32_t seqno, bool update_state) {
+       // Replay protection using a sliding window of configurable size.
+       // s->inseqno is expected sequence number
+       // seqno is received sequence number
+       // s->late[] is a circular buffer, a 1 bit means a packet has not been received yet
+       // The circular buffer contains bits for sequence numbers from s->inseqno - s->replaywin * 8 to (but excluding) s->inseqno.
+       if(s->replaywin) {
+               if(seqno != s->inseqno) {
+                       if(seqno >= s->inseqno + s->replaywin * 8) {
+                               // Prevent packets that jump far ahead of the queue from causing many others to be dropped.
+                               bool farfuture = s->farfuture < s->replaywin >> 2;
+                               if (update_state)
+                                       s->farfuture++;
+                               if(farfuture)
+                                       return error(s, EIO, "Packet is %d seqs in the future, dropped (%u)\n", seqno - s->inseqno, s->farfuture);
+
+                               // Unless we have seen lots of them, in which case we consider the others lost.
+                               warning(s, "Lost %d packets\n", seqno - s->inseqno);
+                               if (update_state) {
+                                       // Mark all packets in the replay window as being late.
+                                       memset(s->late, 255, s->replaywin);
+                               }
+                       } else if (seqno < s->inseqno) {
+                               // If the sequence number is farther in the past than the bitmap goes, or if the packet was already received, drop it.
+                               if((s->inseqno >= s->replaywin * 8 && seqno < s->inseqno - s->replaywin * 8) || !(s->late[(seqno / 8) % s->replaywin] & (1 << seqno % 8)))
+                                       return error(s, EIO, "Received late or replayed packet, seqno %d, last received %d\n", seqno, s->inseqno);
+                       } else if (update_state) {
+                               // We missed some packets. Mark them in the bitmap as being late.
+                               for(int i = s->inseqno; i < seqno; i++)
+                                       s->late[(i / 8) % s->replaywin] |= 1 << i % 8;
+                       }
+               }
+
+               if (update_state) {
+                       // Mark the current packet as not being late.
+                       s->late[(seqno / 8) % s->replaywin] &= ~(1 << seqno % 8);
+                       s->farfuture = 0;
+               }
+       }
+
+       if (update_state) {
+               if(seqno >= s->inseqno)
+                       s->inseqno = seqno + 1;
+
+               if(!s->inseqno)
+                       s->received = 0;
+               else
+                       s->received++;
+       }
+
+       return true;
+}
+
 // Check datagram for valid HMAC
 bool sptps_verify_datagram(sptps_t *s, const char *data, size_t len) {
        if(!s->instate || len < 21)
                return error(s, EIO, "Received short packet");
 
-       // TODO: just decrypt without updating the replay window
+       uint32_t seqno;
+       memcpy(&seqno, data, 4);
+       seqno = ntohl(seqno);
 
-       return true;
+       char buffer[len];
+       size_t outlen;
+       if(!chacha_poly1305_decrypt(s->incipher, seqno, data + 4, len - 4, buffer, &outlen))
+               return false;
+
+       return sptps_check_seqno(s, seqno, false);
 }
 
 // Receive incoming data, datagram version.
@@ -412,45 +472,8 @@ static bool sptps_receive_data_datagram(sptps_t *s, const char *data, size_t len
        if(!chacha_poly1305_decrypt(s->incipher, seqno, data + 4, len - 4, buffer, &outlen))
                return error(s, EIO, "Failed to decrypt and verify packet");
 
-       // Replay protection using a sliding window of configurable size.
-       // s->inseqno is expected sequence number
-       // seqno is received sequence number
-       // s->late[] is a circular buffer, a 1 bit means a packet has not been received yet
-       // The circular buffer contains bits for sequence numbers from s->inseqno - s->replaywin * 8 to (but excluding) s->inseqno.
-       if(s->replaywin) {
-               if(seqno != s->inseqno) {
-                       if(seqno >= s->inseqno + s->replaywin * 8) {
-                               // Prevent packets that jump far ahead of the queue from causing many others to be dropped.
-                               if(s->farfuture++ < s->replaywin >> 2)
-                                       return error(s, EIO, "Packet is %d seqs in the future, dropped (%u)\n", seqno - s->inseqno, s->farfuture);
-
-                               // Unless we have seen lots of them, in which case we consider the others lost.
-                               warning(s, "Lost %d packets\n", seqno - s->inseqno);
-                               // Mark all packets in the replay window as being late.
-                               memset(s->late, 255, s->replaywin);
-                       } else if (seqno < s->inseqno) {
-                               // If the sequence number is farther in the past than the bitmap goes, or if the packet was already received, drop it.
-                               if((s->inseqno >= s->replaywin * 8 && seqno < s->inseqno - s->replaywin * 8) || !(s->late[(seqno / 8) % s->replaywin] & (1 << seqno % 8)))
-                                       return error(s, EIO, "Received late or replayed packet, seqno %d, last received %d\n", seqno, s->inseqno);
-                       } else {
-                               // We missed some packets. Mark them in the bitmap as being late.
-                               for(int i = s->inseqno; i < seqno; i++)
-                                       s->late[(i / 8) % s->replaywin] |= 1 << i % 8;
-                       }
-               }
-
-               // Mark the current packet as not being late.
-               s->late[(seqno / 8) % s->replaywin] &= ~(1 << seqno % 8);
-               s->farfuture = 0;
-       }
-
-       if(seqno >= s->inseqno)
-               s->inseqno = seqno + 1;
-
-       if(!s->inseqno)
-               s->received = 0;
-       else
-               s->received++;
+       if(!sptps_check_seqno(s, seqno, true))
+               return false;
 
        // Append a NULL byte for safety.
        buffer[len - 20] = 0;