Checksums

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Checksums detect accidental changes, such as a corrupted file or a truncated download. They are fast, but they do not protect against deliberate changes: anyone can compute the checksum of modified data. Use a cryptographic hash for that.

CRC-32

Crc32 computes the CRC-32 of ISO 3309 (CRC-32/ISO-HDLC): the CRC of zlib, gzip, ZIP, PNG and Ethernet.

var crc = Crc32.Compute("123456789"u8);
Assert.Equal(0xCBF43926u, crc);

// The CRC of data received in several parts
var running = Crc32.Compute("1234"u8);
running = Crc32.Append(running, "56789"u8);
Assert.Equal(crc, running);

Compute(data) returns the CRC-32 of the data, 0 for empty data.
Append(crc, data) returns the CRC-32 of the data that crc covers followed by data: Append(Compute(a), b) is Compute(a + b). Start from 0 when there is no preceding data. Use it for data that arrives in several parts, such as the buffers read from a stream.

Adler-32

Adler32 computes the Adler-32 checksum of RFC 1950: the checksum of zlib streams.

var checksum = Adler32.Compute("Wikipedia"u8);
Assert.Equal(0x11E60398u, checksum);

// The checksum of data received in several parts
var running = Adler32.Compute("Wiki"u8);
running = Adler32.Append(running, "pedia"u8);
Assert.Equal(checksum, running);

Compute and Append work like the ones of Crc32, except that the checksum of empty data is 1: start from 1 when there is no preceding data.

Performance

Both types use the instructions of the processor when they are available: the CRC32 instructions of Arm64 processors and the carry-less multiplication of x64 processors for Crc32, the vector instructions (AdvSimd, AVX2, SSSE3) for Adler32. Every path computes the same value. Pass large buffers rather than a few bytes at a time to benefit from them.