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CRC64 with ARM RAN Library

Dan Touitou

Hello, 

I am trying to used crc64 function in crc_common.hpp to compute crc64. However is is nor clear to me what should be  the const poly64_t constants[] parameter. According to the function description: 

/**
 * Computes a CRC64 in big- or little-endian mode using the specified shifts
 * and polynomials. This can be used for smaller polynomials by shifting
 * them to a degree 64 polynomial.
 *
 * @tparam     BarretShift     the shift used when computing @c ls1_divp.
 * @param[in]  size            number of bytes of the given buffer
 * @param[in]  input           points to the input byte sequence
 * @param[out] crc24           the computed CRC on 24 bits
 * @param[in]  constants       the constants specific to each polynomial:
                               constants[0] = padding
                               constants[1] = (1<<128) / P - (1<<64)
                               constants[2:11] = [ (1<<(64*k)) mod P,
                                 for k in [1,1,2,3,4,5,6,7,8,9] ]
 */

What  "constants[1] = (1<<128) / P - (1<<64)" stands for?  The result doesnt seems to be a 64th bit polynomial. 

Thanks


  • +1 Arm Employee Badge

    Hi there,

    “constants” is an array of poly64_t. Each poly64_t is an unsigned integer representation of the coefficients of a GF(2) polynomial (that is, a polynomial with coefficients 0 or 1). For example, 1<<64 can represent x^64. P is the generator polynomial of the CRC being calculated. The modulus and division operators here are polynomial modulus and division, so:

    (1<<128) / P - (1<<64)

    is the poly64_t representation of the polynomial

    x^128 / (some degree 64 polynomial) - x^64

    Please note that crc64 isn’t intended to be a user-callable function so it does not include full documentation on how to generate all the parameters. Instead it is a common implementation that underlies all our user-callable CRC routines.

    Regards,

    Nick

  • 0 in reply to Nick Dingle

    Thanks Nick.  Assuming that I would like to evaluate crc64 for the following polynomial : 0xad93d23594c935a9  what should I fill into the constants?  

    Thanks

  • 0 Arm Employee Badge in reply to Dan Touitou

    Hi there,

    Could I clarify, please: is the most-significant bit of your polynomial assumed to be 1, and therefore not stored in that representation? Or is it a degree-63 polynomial with the most-significant bit explictly stored?

    Many thanks,

    Nick

  • 0 in reply to Nick Dingle

    Hi Nick, this is crc-64-jones polynomial. Definition is from https://pycrc.org/models.html : 

    Width 64
    Poly 0xad93d23594c935a9
    Reflect In True
    XOR In 0xffffffffffffffff
    Reflect Out True
    XOR Out 0x0000000000000000

    Thanks a lot! 

    dan

  • +1 Arm Employee Badge in reply to Dan Touitou

    Hi there,

    Unfortunately our crc64 function is not suitable for evaluating arbitrary 64-bit polynomials. Because we are focused on 5G applications we have only implemented the CRCs listed in the appropriate 5G standard (Technical Specification 38.212) and these have a maximum length of 24 bits. The code is not designed to support longer CRCs.

    It would be interesting to know the application domain for the crc-64-jones polynomial, however. Is it used in a different telecommunications specification, for example?

    Apologies,

    Nick