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@mysticaltech
Forked from Lauszus/crc.py
Created Oct 11, 2021
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Generic CRC-8, CRC-16 and CRC-32 calculations in Python
#!/usr/bin/env python
# Inspired by: https://www.youtube.com/watch?v=izG7qT0EpBw
# The CRC values are verified using: https://crccalc.com/
def reflect_data(x, width):
# See: https://stackoverflow.com/a/20918545
if width == 8:
x = ((x & 0x55) << 1) | ((x & 0xAA) >> 1)
x = ((x & 0x33) << 2) | ((x & 0xCC) >> 2)
x = ((x & 0x0F) << 4) | ((x & 0xF0) >> 4)
elif width == 16:
x = ((x & 0x5555) << 1) | ((x & 0xAAAA) >> 1)
x = ((x & 0x3333) << 2) | ((x & 0xCCCC) >> 2)
x = ((x & 0x0F0F) << 4) | ((x & 0xF0F0) >> 4)
x = ((x & 0x00FF) << 8) | ((x & 0xFF00) >> 8)
elif width == 32:
x = ((x & 0x55555555) << 1) | ((x & 0xAAAAAAAA) >> 1)
x = ((x & 0x33333333) << 2) | ((x & 0xCCCCCCCC) >> 2)
x = ((x & 0x0F0F0F0F) << 4) | ((x & 0xF0F0F0F0) >> 4)
x = ((x & 0x00FF00FF) << 8) | ((x & 0xFF00FF00) >> 8)
x = ((x & 0x0000FFFF) << 16) | ((x & 0xFFFF0000) >> 16)
else:
raise ValueError('Unsupported width')
return x
def crc_poly(data, n, poly, crc=0, ref_in=False, ref_out=False, xor_out=0):
g = 1 << n | poly # Generator polynomial
# Loop over the data
for d in data:
# Reverse the input byte if the flag is true
if ref_in:
d = reflect_data(d, 8)
# XOR the top byte in the CRC with the input byte
crc ^= d << (n - 8)
# Loop over all the bits in the byte
for _ in range(8):
# Start by shifting the CRC, so we can check for the top bit
crc <<= 1
# XOR the CRC if the top bit is 1
if crc & (1 << n):
crc ^= g
# Reverse the output if the flag is true
if ref_out:
crc = reflect_data(crc, n)
# Return the CRC value
return crc ^ xor_out
msg = b'Hi!'
# CRC-8
crc = crc_poly(msg, 8, 0x07)
# print(hex(crc), '{0:08b}'.format(crc))
assert crc == 0x78
# CRC-8/ITU
crc = crc_poly(msg, 8, 0x07, xor_out=0x55)
# print(hex(crc), '{0:08b}'.format(crc))
assert crc == 0x2D
# CRC-8/DARC
crc = crc_poly(msg, 8, 0x39, ref_in=True, ref_out=True)
# print(hex(crc), '{0:08b}'.format(crc))
assert crc == 0x94
# CRC-16/XMODEM
crc = crc_poly(msg, 16, 0x1021)
# print(hex(crc), '{0:016b}'.format(crc))
assert crc == 0x31FD
# CRC-16/MAXIM
crc = crc_poly(msg, 16, 0x8005, ref_in=True, ref_out=True, xor_out=0xFFFF)
# print(hex(crc), '{0:016b}'.format(crc))
assert crc == 0xA191
# CRC-16/USB
crc = crc_poly(msg, 16, 0x8005, crc=0xFFFF, ref_in=True, ref_out=True, xor_out=0xFFFF)
# print(hex(crc), '{0:016b}'.format(crc))
assert crc == 0x61E0
# CRC-32/BZIP2
crc = crc_poly(msg, 32, 0x04C11DB7, crc=0xFFFFFFFF, xor_out=0xFFFFFFFF)
# print(hex(crc), '{0:032b}'.format(crc))
assert crc == 0x9523B4B4
# CRC-32C
crc = crc_poly(msg, 32, 0x1EDC6F41, crc=0xFFFFFFFF, ref_in=True, ref_out=True, xor_out=0xFFFFFFFF)
# print(hex(crc), '{0:032b}'.format(crc))
assert crc == 0x43AC6E72
# CRC-32/XFER
crc = crc_poly(msg, 32, 0x000000AF)
# print(hex(crc), '{0:032b}'.format(crc))
assert crc == 0x2E83E24F
# CRC-32/MPEG-2
crc = crc_poly(msg, 32, 0x04C11DB7, crc=0xFFFFFFFF)
# print(hex(crc), '{0:032b}'.format(crc))
assert crc == 0x6ADC4B4B
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