KAT/docs/psa2.md

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PSA2 Protocol

Rust module: src/protocols/psa2.rs Reference: Flipper-ARF lib/subghz/protocols/psa2.c

Overview

PSA2 (Peugeot/Citroën — internal name "PSA OLD") is the OLDER PSA variant, distinct from KAT's existing psa (modified-TEA/XEA) decoder. Manchester encoding: 250/500 µs symbol (Pattern 1, standard rate) or 125/250 µs (Pattern 2, half rate), using the canonical Flipper manchester_advance table. 128-bit frame = key1 (64 bits) + key2/validation word: the decoder collects 64 bits → key1, then 16 more (to 80 bits) → the 16-bit validation field / key2_low.

Crypto: TEA (Tiny Encryption Algorithm) with a dual brute-force fallback (BF1 0x23000000–0x24000000, BF2 0xF3000000–0xF4000000) and a mode23/mode36 selector, validated via a nibble checksum.

Performance: the live decoder only ever runs the cheap O(1) mode23 XOR path (direct_xor_decrypt) per frame — it NEVER runs the TEA brute force. The bounded TEA brute force (decrypt_full, ~16.7M iters per range) is ported faithfully but is reserved for a deferred-decrypt UI action, mirroring the C exactly. Beyond the C's nibble-checksum gate, KAT adds two false-positive suppressors required by its feed-all-decoders model: a key2_high precondition and a valid-button check (PSA2 buttons are Lock=0, Unlock=1, Trunk=2 only). Emission is gated strictly on a successful, field-bearing mode23 decrypt.

Timing

Parameter	Value	Notes
Short (P1)	250 µs	±100 µs (te_delta)
Long (P1)	500 µs	±100 µs
Short (P2)	125 µs	±50 µs (half rate)
Long (P2)	250 µs	±50 µs
End marker	1000 µs (P1) / 500 µs (P2)
Min bits	128	key1 (64) + validation (16) collected
Preamble	>0x46 pairs (P1) / >0x45 (P2)

Frame Layout (128 bits = key1 64-bit + key2/validation)

The decoder collects 64 bits → key1, then 16 bits → the validation field (key2_low). After the mode23 XOR decrypt (fields from extract_fields_mode23):

• serial: (buf[2]<<16) | (buf[3]<<8) | buf[4] (24-bit)
• button: buf[8] & 0xF — Lock=0, Unlock=1, Trunk=2
• counter: (buf[5]<<8) | buf[6] (16-bit)
• crc: buf[7]

data = (key1_high << 32) | key1_low (64 bits).

RF

• Encoding: Manchester (canonical Flipper table; standard 250/500 µs or half-rate 125/250 µs)
• RF modulation: AM (OOK)
• Encryption: TEA + nibble-checksum-gated mode23 XOR path; brute-force fallback reserved for offline decrypt
• Frequencies: 433.92 MHz

Decoder Steps

1. WaitEdge (State0) — detect the preamble rate (250 µs → Pattern 1, 125 µs → Pattern 2).
2. CountPattern250/125 (State1/3) — count preamble pulses past the threshold, then a long pulse enters the Manchester decode state.
3. DecodeManchester250/125 (State2/4) — Manchester-decode to 80 bits (key1 + validation), detect end-of-packet, then finalize_frame runs the cheap mode23 XOR gate and emits only on a successful field-bearing decrypt.

Encoder

Supported (mode23 path). Increments the counter, builds key1/validation via encode_mode23 (inverse XOR stage + nibble checksum), then emits an 80-pair preamble, a sync, 64 key1 bits + 16 validation bits MSB-first, and an end burst.

Validation

Decodes REAL IMPORTS captures (IMPORTS/GROUPE PSA, serial 0x99EB25). Unit tests also cover the TEA round trip, the mode23 encode→decode round trip, the nibble checksum vs. the reference, and the emitted Manchester frame length.