CRC computation

Scapy provides configurable CRC algorithms in scapy.libs.crc. Each variant is a class that can be called on bytes to obtain an integer checksum.

Computing a checksum

>>> from scapy.libs.crc import CRC_32
>>> CRC_32(b"123456789")
3421780262

Built-in variants include CRC_16, CRC_32, CRC_16_CCITT and CRC_32_AUTOSAR.

Custom parameters

Use from_parameters() or a CRCParam object when the polynomial or reflection settings differ from the built-in classes:

>>> from scapy.libs.crc import CRC, CRCParam
>>> params = CRCParam(poly=0x589, size=16, init_crc=0xffff, xor=0xffff,
...                   reflect_input=False, reflect_output=False)
>>> CRC.from_parameters(params, do_not_register=True)(b"test")

Incremental computation

For large or streamed data, use create_context() and init / update / finish:

>>> crc = CRC_32.create_context()
>>> crc.update(b"12345")
>>> crc.update(b"6789")
>>> crc.finish()
3421780262

Finding embedded CRCs

When a binary format stores a CRC next to the protected data, search() locates candidate (substring, algorithm) pairs. Pass only_registry=True to restrict the search to registered classes:

>>> import struct
>>> from scapy.libs.crc import CRC, CRC_32
>>> data = b"payload"
>>> blob = data + struct.pack("!I", CRC_32(data))
>>> CRC.search(blob, only_registry=True)
[((0, 7), 1110206997, CRC_32)]

API reference

See scapy.libs.crc in the API reference for the full list of classes and methods.