KAT/docs/psa.md

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PSA (Peugeot/Citroën) Protocol

Rust module: src/protocols/psa.rs
Reference: REFERENCES/ProtoPirate/protocols/psa.c

Overview

PSA uses Manchester at 250/500 µs symbol (125/250 µs sub-symbol for preamble). 128 bits total: key1 (64) + validation (16) + key2/rest (48); decode uses key1 + 16-bit validation. TEA decrypt/encrypt with fixed key schedules; mode 0x23 adds an XOR layer. Two modes: seed 0x23 (TEA + XOR), seed 0xF3/0x36 (TEA, BF2 key schedule).

Timing

Parameter	Value	Notes
Symbol short	250 µs	±100 µs
Symbol long	500 µs	±100 µs
Preamble	125/250 µs sub-symbols
Min bits	128

Encoding

Manchester; preamble uses 125/250 µs; then 250/500 µs symbols. TEA encrypt; mode 0x23 adds XOR.

Frame Layout (128 bits)

• key1: 64 bits
• validation: 16 bits
• key2/rest: 48 bits

TEA decrypt key1 (and validation); mode 0x23: XOR with BF1 key schedule; mode 0x36: TEA with BF2 key schedule. Serial/button/counter extracted from decrypted key1.

Decoder Steps

1. WaitEdge — Wait for edge/preamble.
2. CountPattern — Count preamble pattern (125/250 µs).
3. DecodeManchester — Manchester decode 128 bits; TEA decrypt; apply mode (0x23 XOR or 0x36); extract fields.
4. End — End marker (e.g. 1000 µs); return decode.

Encoder

Supported; preamble, Manchester 128 bits, TEA (+ XOR for 0x23).

Frequencies

433.92 MHz.