How to calculate CRC32-C checksum with zlib

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I am using the C zlib API because it has the crc32_combine function to concatenate checksums together, whereas the Boost one does not.

However, I need to implement the CRC32-C (Castagnoli) checksum, with polynomial 0x1EDC6F41, instead of the standard CRC32 checksum.

With Boost I can apparently use:

#include <boost/crc.hpp>

using crc_32c_type = boost::crc_optimal<32, 0x1EDC6F41, 0xFFFFFFFF, 0xFFFFFFFF, true, true>;
crc_32c_type result;
result.process_bytes(reinterpret_cast<const char*>(&buffer), len);
return result.checksum();

Which can use the 0x1EDC6F41 polynomial.

Is there a similar way for me to do this with zlib?

zlib
crc
asked on Stack Overflow May 15, 2018 by Anton

2 Answers

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zlib is open source. You can simply take the source and modify for your own needs. You can change the line: odd[0] = 0xedb88320UL; to the reflection of the Castagnoli polynomial.

answered on Stack Overflow May 15, 2018 by Mark Adler
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For Objective C

Took me a while to find one that worked.

//------------------------------------------------------------------------------------
//                    crc32c
//           Calculate crc32c (Castagnoli) Checksum
//------------------------------------------------------------------------------------
+ (uint32_t) crc32c:(NSData *)data {
    /* CRC-32C (iSCSI) polynomial in reversed bit order. */


    int k;
    const unsigned char *buf = [data bytes];
    unsigned long len = [data length];
    uint32_t crc = 0xFFFFFFFF;
    while (len--) {
        crc ^= *buf++;
        for (k = 0; k < 8; k++)
            //CRC-32C polynomial 0x1EDC6F41 in reversed bit order.
            crc = crc & 1 ? (crc >> 1) ^ 0x82f63b78 : crc >> 1;
    }
    return ~crc;
}
answered on Stack Overflow Nov 3, 2020 by birdman

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