+ }
+
+ return true;
+
+ case SPTPS_ACK:
+
+ // We expect a handshake message to indicate transition to the new keys.
+ if(!receive_ack(s, data, len)) {
+ return false;
+ }
+
+ s->receive_record(s->handle, SPTPS_HANDSHAKE, NULL, 0);
+ s->state = SPTPS_SECONDARY_KEX;
+ return true;
+
+ // TODO: split ACK into a VERify and ACK?
+ default:
+ return error(s, EIO, "Invalid session state %d", s->state);
+ }
+}
+
+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 update_state ? error(s, EIO, "Packet is %d seqs in the future, dropped (%u)\n", seqno - s->inseqno, s->farfuture) : false;
+ }
+
+ // Unless we have seen lots of them, in which case we consider the others lost.
+ if(update_state) {
+ 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 update_state ? error(s, EIO, "Received late or replayed packet, seqno %d, last received %d\n", seqno, s->inseqno) : false;
+ }
+ } else if(update_state) {
+ // We missed some packets. Mark them in the bitmap as being late.
+ for(uint32_t 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++;
+ }