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#!/usr/bin/env python3
import sys
"""
PSA Checksum Calculator
This script calculates the 2-byte checksum used in PSA multicast protocol messages.
The checksum algorithm is based on reverse-engineered firmware behavior.
How it works:
-------------
- Initialize a 16-bit value `var_e` to 0x0000
- For each byte in the payload:
- Add the byte's index to its value (i.e., byte + index)
- XOR the result into `var_e`
- The result is a 2-byte checksum (big endian: high byte first, low byte second)
This checksum is appended to the payload to form a valid message.
"""
def compute_psa_checksum(data: bytes) -> bytes:
var_e = 0x0000
for i in range(len(data)):
var_e ^= (data[i] + i) & 0xFFFF
return var_e.to_bytes(2, 'big')
def add_checksum(hexstring: str) -> str:
try:
data = bytes.fromhex(hexstring.strip())
except ValueError:
return "Invalid hex input!"
checksum = compute_psa_checksum(data)
return (data + checksum).hex()
if __name__ == "__main__":
if len(sys.argv) != 2:
print("Usage: ./hex_checksum.py <hexstring_without_checksum>")
sys.exit(1)
result = add_checksum(sys.argv[1])
print(result)
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