Recipe2.30.Calculating CRC-64 Cyclic Redundancy Checks

Recipe 2.30. Calculating CRC-64 Cyclic Redundancy Checks

Credit: Gian Paolo Ciceri

Problem

You need to ensure the integrity of some data by computing the data's cyclic redundancy check (CRC), and you need to do so according to the CRC-64 specifications of the ISO-3309 standard.

Solution

The Python Standard Library does not include any implementation of CRC-64 (only one of CRC-32 in function zlib.crc32), so we need to program it ourselves. Fortunately, Python can perform bitwise operations (masking, shifting, bitwise-and, bitwise-or, xor, etc.) just as well as, say, C (and, in fact, with just about the same syntax), so it's easy to transliterate a typical reference implementation of CRC-64 into a Python function as follows:

# prepare two auxiliary tables tables (using a function, for speed), # then remove the function, since it's not needed any more: CRCTableh = [0] * 256 CRCTablel = [0] * 256 def _inittables(CRCTableh, CRCTablel, POLY64REVh, BIT_TOGGLE):     for i in xrange(256):         partl = i         parth = 0L         for j in xrange(8):             rflag = partl & 1L             partl >>= 1L             if parth & 1:                 partl ^= BIT_TOGGLE             parth >>= 1L             if rflag:                 parth ^= POLY64REVh         CRCTableh[i] = parth         CRCTablel[i] = partl # first 32 bits of generator polynomial for CRC64 (the 32 lower bits are # assumed to be zero) and bit-toggle mask used in _inittables POLY64REVh = 0xd8000000L BIT_TOGGLE = 1L << 31L # run the function to prepare the tables _inittables(CRCTableh, CRCTablel, POLY64REVh, BIT_TOGGLE) # remove all names we don't need any more, including the function del _inittables, POLY64REVh, BIT_TOGGLE # this module exposes the following two functions: crc64, crc64digest def crc64(bytes, (crch, crcl)=(0,0)):     for byte in bytes:         shr = (crch & 0xFF) << 24         temp1h = crch >> 8L         temp1l = (crcl >> 8L) | shr         tableindex = (crcl ^ ord(byte)) & 0xFF         crch = temp1h ^ CRCTableh[tableindex]         crcl = temp1l ^ CRCTablel[tableindex]     return crch, crcl def crc64digest(aString):     return "%08X%08X" % (crc64(bytes)) if _ _name_ _ == '_ _main_ _':     # a little test/demo, for when this module runs as main-script     assert crc64("IHATEMATH") == (3822890454, 2600578513)     assert crc64digest("IHATEMATH") == "E3DCADD69B01ADD1"     print 'crc64: dumb test successful'

Discussion

Cyclic redundancy checks (CRCs) are a popular way to ensure that data (in particular, a file) has not been accidentally damaged. CRCs can readily detect accidental damage, but they are not intended to withstand inimical assault the way other cryptographically strong checksums are. CRCs can be computed much faster than other kinds of checksums, making them useful in those cases where the only damage we need to guard against is accidental damage, rather than deliberate adversarial tampering.

Mathematically speaking, a CRC is computed as a polynomial over the bits of the data we're checksumming. In practice, as this recipe shows, most of the computation can be done once and for all and summarized in tables that, when properly indexed, give the contribution of each byte of input data to the result. So, after initialization (which we do with an auxiliary function because computation in Python is much faster when using a function's local variables than when using globals), actual CRC computation is quite fast. Both the computation of the tables and their use for CRC computation require a lot of bitwise operations, but, fortunately, Python's just as good at such operations as other languages such as C. (In fact, Python's syntax for the various bitwise operands is just about the same as C's.)

The algorithm to compute the standard CRC-64 checksum is described in the ISO-3309 standard, and this recipe does nothing more than implement that algorithm. The generator polynomial is x64 + x4 + x3 + x + 1. (The "See Also" section within this recipe provides a reference for obtaining information about the computation.)

We represent the 64-bit result as a pair of Python ints, holding the low and high 32-bit halves of the result. To allow the CRC to be computed incrementally, in those cases where the data comes in a little at a time, we let the caller of function crc64 optionally feed in the "initial value" for the (crch, crcl) pair, presumably obtained by calling crc64 on previous parts of the data. To compute the CRC in one gulp, the caller just needs to pass in the data (a string of bytes), since in this case, we initialize the result to (0, 0) by default.