Add xor() cipher to fpm lock, add limitation to neo4j.

1 files changed, 40 insertions, 7 deletions

diff --git a/_log/fpm-door-lock-rf.md b/_log/fpm-door-lock-rf.md
index 8fe592a..d8822f9 100644
--- a/_log/fpm-door-lock-rf.md
+++ b/_log/fpm-door-lock-rf.md
@@ -16,21 +16,54 @@ Unreliable--constant packet loss.
 the letter, audited code many times, cross-checked with RadioHead and RFM69
 open-source drivers. No luck. 
 
-Datasheet riddled with ambiguity. 
-
 ATmega328P runs at 5V; RFM69 3.3V. Suspect logic-level converter (LLC)
 issues. High resistance. Not enough swing.
 
-2025-04: Ditched RFM69s. Switched to NRF24L01+ modules-- data pins 5V tolerant,
-no LLC required. Spent six weekends writing driver from scratch--clean-room.
-Works like a charm.
+2025-04: Ditched RFM69s. Switched to NRF24L01+ modules--data pins 5V tolerant,
+no LLC required. Spent six weekends writing a clean-room driver from scratch.
+Works like a charm. 
+
+Basic security via xor cipher–good enough for a door behind a guard post and
+gate:
+
+```
+void xor(const char *k, const char *s, char *d, uint8_t n)
+{
+  int i;
+  for (i = 0; i < n; i++)
+    d[i] = s[i] ^ k[i];
+}
+``` 
+
+Resists replay attacks by cycling the key:
+
+```
+static inline void keygen(char *buf, uint8_t n)
+{
+  int i, imax;
+  uint8_t sreg;
+  uint16_t seed; 
+  
+  sreg = SREG;
+  cli();
+  seed = TCNT1; 
+  SREG = sreg;
+
+  for (i = 0, imax = n - 1; i < imax; i++, seed++)
+    buf[i] = tab[(seed % tablen)];
+  buf[imax] = '\0';
+}
+```
+
+Protocol: FPM sends SYN. Servo responds with session key. Both xor-ed with
+static key. Session key used thereafter. Private command set authenticates
+endpoints.
 
 2025-05: Wrote FPM drivers for R503 and FPM10A. UART RX sequence was
 tricky--adopted Adafruit C++ FOSS implementation to C. R503 has built-in LEDs
 and better form factor. Chose it for the lock.
 
-2025-06: Two PCB boards for FPM (front) and servo (back) controllers. Encrypted
-RF link between them.
+2025-06: Two PCBs for FPM (front) and servo (back) controllers. 
 
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