77 lines
1.9 KiB
Python
77 lines
1.9 KiB
Python
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"""
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CRC-ITU Implementation for KKS Badge Protocol
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Uses CRC-16/X-25 (reflected CRC-CCITT):
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Polynomial: 0x8408 (reflected 0x1021)
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Initial value: 0xFFFF
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Final XOR: 0xFFFF
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"""
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from typing import List
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# ---------------------------------------------------------------------------
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# Pre-computed CRC lookup table (256 entries, reflected polynomial 0x8408)
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# ---------------------------------------------------------------------------
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_CRC_TABLE: List[int] = []
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def _generate_crc_table() -> List[int]:
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"""Generate the CRC-16/X-25 lookup table for reflected polynomial 0x8408."""
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table: List[int] = []
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for i in range(256):
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crc = i
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for _ in range(8):
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if crc & 1:
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crc = (crc >> 1) ^ 0x8408
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else:
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crc >>= 1
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table.append(crc)
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return table
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_CRC_TABLE = _generate_crc_table()
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def crc_itu(data: bytes) -> int:
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"""
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Compute the CRC-ITU checksum for the given data.
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Uses the CRC-16/X-25 algorithm (reflected CRC-CCITT with final XOR).
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For a KKS protocol packet this should be the bytes from (and including)
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the packet-length field through the serial-number field.
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Parameters
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----------
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data : bytes
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The data to compute the CRC over.
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Returns
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-------
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int
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16-bit CRC value.
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"""
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crc: int = 0xFFFF
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for byte in data:
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crc = (crc >> 8) ^ _CRC_TABLE[(crc ^ byte) & 0xFF]
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return crc ^ 0xFFFF
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def verify_crc(data: bytes, expected_crc: int) -> bool:
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"""
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Verify that *data* produces the *expected_crc*.
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Parameters
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----------
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data : bytes
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The data slice to check (same range used when computing the CRC).
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expected_crc : int
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The 16-bit CRC value to compare against.
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Returns
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-------
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bool
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``True`` if the computed CRC matches *expected_crc*.
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"""
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return crc_itu(data) == (expected_crc & 0xFFFF)
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