+
+ // Check HMAC.
+ uint16_t netlen = htons(len - 21);
+
+ char buffer[len + 23];
+
+ memcpy(buffer, &netlen, 2);
+ memcpy(buffer + 2, data, len);
+
+ if(!digest_verify(s->indigest, buffer, len - 14, buffer + len - 14))
+ return error(s, EIO, "Invalid HMAC");
+
+ // 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);
+ memset(s->late, 0, 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++;
+
+ // Decrypt.
+ memcpy(&seqno, buffer + 2, 4);
+ if(!cipher_set_counter(s->incipher, &seqno, sizeof seqno))
+ return false;
+ if(!cipher_counter_xor(s->incipher, buffer + 6, len - 4, buffer + 6))
+ return false;
+
+ // Append a NULL byte for safety.
+ buffer[len - 14] = 0;
+
+ uint8_t type = buffer[6];
+
+ if(type < SPTPS_HANDSHAKE) {
+ if(!s->instate)
+ return error(s, EIO, "Application record received before handshake finished");
+ if(!s->receive_record(s->handle, type, buffer + 7, len - 21))
+ return false;
+ } else if(type == SPTPS_HANDSHAKE) {
+ if(!receive_handshake(s, buffer + 7, len - 21))
+ return false;
+ } else {
+ return error(s, EIO, "Invalid record type %d", type);
+ }
+
+ return true;