+// Receive incoming data, datagram version.
+static bool sptps_receive_data_datagram(sptps_t *s, const char *data, size_t len) {
+ if(len < (s->instate ? 21 : 5))
+ return error(s, EIO, "Received short packet");
+
+ uint32_t seqno;
+ memcpy(&seqno, data, 4);
+ seqno = ntohl(seqno);
+
+ if(!s->instate) {
+ if(seqno != s->inseqno) {
+ fprintf(stderr, "Received invalid packet seqno: %d != %d\n", seqno, s->inseqno);
+ return error(s, EIO, "Invalid packet seqno");
+ }
+
+ s->inseqno = seqno + 1;
+
+ uint8_t type = data[4];
+
+ if(type != SPTPS_HANDSHAKE)
+ return error(s, EIO, "Application record received before handshake finished");
+
+ return receive_handshake(s, data + 5, len - 5);
+ }
+
+ if(seqno < s->inseqno) {
+ fprintf(stderr, "Received late or replayed packet: %d < %d\n", seqno, s->inseqno);
+ return true;
+ }
+
+ if(seqno > s->inseqno)
+ fprintf(stderr, "Missed %d packets\n", seqno - s->inseqno);
+
+ s->inseqno = seqno + 1;
+
+ uint16_t netlen = htons(len - 21);
+
+ char buffer[len + 23];
+
+ memcpy(buffer, &netlen, 2);
+ memcpy(buffer + 2, data, len);
+
+ memcpy(&seqno, buffer + 2, 4);
+
+ // Check HMAC and decrypt.
+ if(!digest_verify(&s->indigest, buffer, len - 14, buffer + len - 14))
+ return error(s, EIO, "Invalid HMAC");
+
+ cipher_set_counter(&s->incipher, &seqno, sizeof seqno);
+ 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 {
+ return error(s, EIO, "Invalid record type");
+ }
+
+ return true;
+}