CRC-32C (Castagnoli) for C++ and .NET

This is a hardware-accelerated implementation of CRC-32C (Castagnoli, polynomial 0x1EDC6F41 / 0x82F63B78) for Windows (C++ and .NET), opensourced under zlib license and BSD license. Intel's CRC32 instruction is used if available. Otherwise this library uses super fast software fallback.

.NET Framework doesn't provide any CRC algorithms and existing opensource libraries contain suboptimal algorithms. Creating really fast CRC algorithm is hard. This library encapsulates all that complexity for you and offers you a neat, simple API, so that every application can now afford fast CRC algorithm.

Note on CRC variants

Don't confuse CRC-32C (Castagnoli) with the older CRC-32 (also called CRC-32-IEEE). As you might know, CRC algorithm is defined by its so-called polynomial, essentially a 32-bit number. CRC-32C's polynomial can be seen in the box to the right.

Advantages of CRC-32C

The polynomial in CRC-32C was shown to have better error detection properties, which is the reason for its adoption in newer standards (iSCSI, SCTP, ext4).

Aside from higher reliability, CRC-32C now has the advantage of dedicated instruction on newer Intel processors. That's why it is being chosen for high-performance applications, for example Snappy compression algorithm.

Tutorial for C++

C++ NuGet package contains source code that is compiled together with your project. Your project will therefore have no DLL dependencies and there will be no C++ runtime issues. It however means that Debug build of your project will contain slower Debug build of CRC-32C.

<PackageReference Include="Crc32C" Version="1.2.1" />

Alternatively, you can download plain 7-Zip archive of the DLLs and associated LIBs and headers. Sources are available from GitHub and Bitbucket. C++ code and binaries are distributed under zlib license.

After downloading the library, your first step is to include the header:

#include "crc32c.h"

After that, you will get access to the following function:

    Computes CRC-32C (Castagnoli) checksum. Uses Intel's CRC32
    instruction if it is available. Otherwise it uses a very
    fast software fallback.
extern "C" uint32_t crc32c_append(
    uint32_t crc,               // Initial CRC value. Typically it's 0.
                                // You can supply non-trivial initial
                                // value here. Initial value can be used
                                // to chain CRC from multiple buffers.
    const uint8_t *input,       // Data to be put through the CRC
                                // algorithm.
    size_t length);             // Length of the data in the input
                                // buffer.

You can call the function like this:

uint32_t crc = crc32c_append(0, input, 10000);

As the comments in above declaration suggest, you can compute crc from multiple chained buffers:

uint32_t intermediate = crc32c_append(0, input, 3000);
uint32_t crc = crc32c_append(intermediate, input + 3000, 7000);

And that's it. You can add any pre/post-processing of the CRC value if your application requires it.

Tutorial for .NET

.NET DLL is AnyCPU, but it automatically forwards all calls to one of the two native DLLs depending on whether the current process is 32-bit or 64-bit. The two native DLLs are embedded as resources and unpacked into temporary location before first use.

<PackageReference Include="Crc32C.NET" Version="1.0.5" />

Alternatively, you can download plain 7-Zip archive of the DLLs. Sources are available from GitHub and Bitbucket. .NET code is distributed under BSD license. Underlying C++ code is distributed under zlib license.

After downloading the library, your can use the CRC-32C namespace:

using Crc32C;

You will get access to class Crc32CAlgorithm that implements HashAlgorithm with all its features:

/// <summary>
/// Implementation of CRC-32C (Castagnoli) with polynomial 0x1EDC6F41.
/// It can detect errors more reliably than the older CRC-32-IEEE.
/// This class will use CRC32 instruction on recent Intel processors
/// if it is available. Otherwise it will transparently fall back to
/// a very fast software implementation. Besides standard
/// HashAlgorithm methods, this class supports several convenient
/// static methods returning the CRC as UInt32.
/// </summary>
public class Crc32CAlgorithm : HashAlgorithm

And here are the handy static methods mentioned in the class documentation. They map directly to the underlying native library.

public static uint Compute(byte[] input);
public static uint Compute(byte[] input, int offset, int length);
public static uint Append(uint initial, byte[] input);
public static uint Append(uint initial, byte[] input, int offset,
                          int length);

You can then compute CRC-32C of a buffer with the following C# code:

uint crc = Crc32CAlgorithm.Compute(array);

You can also submit smaller section of a buffer:

uint crc = Crc32CAlgorithm.Compute(array, 0, 10000);

If you would like to chain multiple buffers, you can do it like this:

uint intermediate = Crc32CAlgorithm.Compute(array, 0, 3000);
uint crc = Crc32CAlgorithm.Append(intermediate, array, 3000, 7000);

You can of course call Append as many times as you wish. You can add pre/post-processing of the CRC value depending on requirements of your application.


Recent Intel processors have specialized instruction for calculating CRC-32C: the CRC32 instruction. Fast software fallback is used on all other hardware, i.e. AMD processors and some older Intel processors.

Hardware performance (Core i7 3.4GHz):

Software performance (Core i7 3.4GHz with hardware acceleration disabled):

The library is optimized for larger buffers of several dozens of kilobytes. Above results have been measured on buffers ranging between 0KB and 64KB with 32KB average.

PCLMULQDQ instruction

Recent AMD processors contain PCLMULQDQ instruction, which could be used to accelerage CRC-32C among other things. This library doesn't use the instruction, because there was no code at the time that could be easily incorporated.

I've nevertheless performed benchmarks and PCLMULQDQ-based code for CRC-32-IEEE performed at 2.8 GB/s or 40% faster than my software implementation for CRC-32C.

Intel has submitted PCLMULQDQ CRC code to zlib under permissive BSD license. Unfortunately, Intel's code is full of strange looking constants that have been generated from CRC-32-IEEE polynomial. In order to use Intel's code in my project, I would have to regenerate all these constants for CRC-32C polynomial.

I managed to do that for all constants but one. The constant-generating code is in the Hg repo in “constants” project. The one elusive constant is 0x9db42487. Jim Kukunas of Intel, who submitted the code to zlib, has described it as “magic” constant that produces FFFFFF00000…0000 when folded 4 times. My attempts to simulate this folding process yielded all kinds of strange constants but no FFFFFF00000…0000.

Anyway, even if I managed to generate this last constant, it still wouldn't be over. Firstly, it might turn out that the “magic” constant cannot be generated for CRC-32C polynomial. Secondly, CRC has fancy bit-flipped variants and I am not sure, maybe the zlib code is one such strange variant. Removing the flipping would be tricky without devoting time to fully understand Intel's algorithm. Maybe I will get back to it someday.


This implementation of CRC-32C was written by Robert Važan. At its core is a Windows port of Mark Adler's CRC-32C. You can submit issues on GitHub (C++, .NET) or Bitbucket (C++, .NET), including requests for documentation. Pull requests are also welcome via GitHub (C++, .NET) or Bitbucket (C++, .NET).

How to implement your own CRC algorithm:

Known bugs and issues: