official implementations of the BLAKE3 cryptographic hash function
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Jack O'Connor c61c663ec5 version 1.2.0 4 weeks ago
blake3_c_rust_bindings Match the C binding's target arch detection with the root crate's 2 months ago
.gitignore include example.c 1 year ago
Makefile.testing Use BLAKE3_USE_NEON=0 instead of BLAKE3_NO_NEON def 2 months ago a small capitalization change 1 month ago
blake3.c fix a comment typo 1 month ago
blake3.h version 1.2.0 4 weeks ago
blake3_avx2.c [MSVC] added possible to compile at Microsoft Visual C compiler. 2 years ago
blake3_avx2_x86-64_unix.S Fix #109 1 year ago
blake3_avx2_x86-64_windows_gnu.S Work around clang bug 36144 by replacing anonymous label numbers. 2 years ago
blake3_avx2_x86-64_windows_msvc.asm more file renaming, use underscores more consistently 2 years ago
blake3_avx512.c Work around GCC bug 85328 by forcing trivially masked stores. 2 years ago
blake3_avx512_x86-64_unix.S Fix #109 1 year ago
blake3_avx512_x86-64_windows_gnu.S Do not require AVX512DQ 2 years ago
blake3_avx512_x86-64_windows_msvc.asm Another movd/movq inconsistency. 9 months ago
blake3_dispatch.c Use BLAKE3_USE_NEON=0 instead of BLAKE3_NO_NEON def 2 months ago
blake3_impl.h Use BLAKE3_USE_NEON=0 instead of BLAKE3_NO_NEON def 2 months ago
blake3_neon.c explicitly #error on big-endian ARM 3 months ago
blake3_portable.c fix a couple of big-endianness mistakes in blake3.c 1 year ago
blake3_sse2.c Write _mm_blend_epi16 emulation without multiplication 1 year ago
blake3_sse2_x86-64_unix.S More movd/movq discrepancies. Fixes #149. (#150) 10 months ago
blake3_sse2_x86-64_windows_gnu.S fix incorrect output / undefined behavior in Windows SSE2 assembly 4 weeks ago
blake3_sse2_x86-64_windows_msvc.asm fix incorrect output / undefined behavior in Windows SSE2 assembly 4 weeks ago
blake3_sse41.c [MSVC] added possible to compile at Microsoft Visual C compiler. 2 years ago
blake3_sse41_x86-64_unix.S Fix #109 1 year ago
blake3_sse41_x86-64_windows_gnu.S save missing clobbered registers on Windows 2 years ago
blake3_sse41_x86-64_windows_msvc.asm revert unwanted changes 10 months ago
example.c handle IO errors in example.c 3 months ago
main.c check length arg (out_len_ll) instead of default value (out_len) 3 months ago stop being a jerk and add the context string to test_vectors.json 1 year ago

The official C implementation of BLAKE3.


An example program that hashes bytes from standard input and prints the result:

#include "blake3.h"
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main() {
  // Initialize the hasher.
  blake3_hasher hasher;

  // Read input bytes from stdin.
  unsigned char buf[65536];
  while (1) {
    ssize_t n = read(STDIN_FILENO, buf, sizeof(buf));
    if (n > 0) {
      blake3_hasher_update(&hasher, buf, n);
    } else if (n == 0) {
      break; // end of file
    } else {
      fprintf(stderr, "read failed: %s\n", strerror(errno));

  // Finalize the hash. BLAKE3_OUT_LEN is the default output length, 32 bytes.
  uint8_t output[BLAKE3_OUT_LEN];
  blake3_hasher_finalize(&hasher, output, BLAKE3_OUT_LEN);

  // Print the hash as hexadecimal.
  for (size_t i = 0; i < BLAKE3_OUT_LEN; i++) {
    printf("%02x", output[i]);
  return 0;

The code above is included in this directory as example.c. If you're on x86_64 with a Unix-like OS, you can compile a working binary like this:

gcc -O3 -o example example.c blake3.c blake3_dispatch.c blake3_portable.c \
    blake3_sse2_x86-64_unix.S blake3_sse41_x86-64_unix.S blake3_avx2_x86-64_unix.S \


The Struct

typedef struct {
  // private fields
} blake3_hasher;

An incremental BLAKE3 hashing state, which can accept any number of updates. This implementation doesn't allocate any heap memory, but sizeof(blake3_hasher) itself is relatively large, currently 1912 bytes on x86-64. This size can be reduced by restricting the maximum input length, as described in Section 5.4 of the BLAKE3 spec, but this implementation doesn't currently support that strategy.

Common API Functions

void blake3_hasher_init(
  blake3_hasher *self);

Initialize a blake3_hasher in the default hashing mode.

void blake3_hasher_update(
  blake3_hasher *self,
  const void *input,
  size_t input_len);

Add input to the hasher. This can be called any number of times.

void blake3_hasher_finalize(
  const blake3_hasher *self,
  uint8_t *out,
  size_t out_len);

Finalize the hasher and return an output of any length, given in bytes. This doesn't modify the hasher itself, and it's possible to finalize again after adding more input. The constant BLAKE3_OUT_LEN provides the default output length, 32 bytes, which is recommended for most callers.

Outputs shorter than the default length of 32 bytes (256 bits) provide less security. An N-bit BLAKE3 output is intended to provide N bits of first and second preimage resistance and N/2 bits of collision resistance, for any N up to 256. Longer outputs don't provide any additional security.

Shorter BLAKE3 outputs are prefixes of longer ones. Explicitly requesting a short output is equivalent to truncating the default-length output. (Note that this is different between BLAKE2 and BLAKE3.)

Less Common API Functions

void blake3_hasher_init_keyed(
  blake3_hasher *self,
  const uint8_t key[BLAKE3_KEY_LEN]);

Initialize a blake3_hasher in the keyed hashing mode. The key must be exactly 32 bytes.

void blake3_hasher_init_derive_key(
  blake3_hasher *self,
  const char *context);

Initialize a blake3_hasher in the key derivation mode. The context string is given as an initialization parameter, and afterwards input key material should be given with blake3_hasher_update. The context string is a null-terminated C string which should be hardcoded, globally unique, and application-specific. The context string should not include any dynamic input like salts, nonces, or identifiers read from a database at runtime. A good default format for the context string is "[application] [commit timestamp] [purpose]", e.g., " 2019-12-25 16:18:03 session tokens v1".

This function is intended for application code written in C. For language bindings, see blake3_hasher_init_derive_key_raw below.

void blake3_hasher_init_derive_key_raw(
  blake3_hasher *self,
  const void *context,
  size_t context_len);

As blake3_hasher_init_derive_key above, except that the context string is given as a pointer to an array of arbitrary bytes with a provided length. This is intended for writing language bindings, where C string conversion would add unnecessary overhead and new error cases. Unicode strings should be encoded as UTF-8.

Application code in C should prefer blake3_hasher_init_derive_key, which takes the context as a C string. If you need to use arbitrary bytes as a context string in application code, consider whether you're violating the requirement that context strings should be hardcoded.

void blake3_hasher_finalize_seek(
  const blake3_hasher *self,
  uint64_t seek,
  uint8_t *out,
  size_t out_len);

The same as blake3_hasher_finalize, but with an additional seek parameter for the starting byte position in the output stream. To efficiently stream a large output without allocating memory, call this function in a loop, incrementing seek by the output length each time.


This implementation is just C and assembly files. It doesn't include a public-facing build system. (The Makefile in this directory is only for testing.) Instead, the intention is that you can include these files in whatever build system you're already using. This section describes the commands your build system should execute, or which you can execute by hand. Note that these steps may change in future versions.


Dynamic dispatch is enabled by default on x86. The implementation will query the CPU at runtime to detect SIMD support, and it will use the widest instruction set available. By default, blake3_dispatch.c expects to be linked with code for five different instruction sets: portable C, SSE2, SSE4.1, AVX2, and AVX-512.

For each of the x86 SIMD instruction sets, four versions are available: three flavors of assembly (Unix, Windows MSVC, and Windows GNU) and one version using C intrinsics. The assembly versions are generally preferred. They perform better, they perform more consistently across different compilers, and they build more quickly. On the other hand, the assembly versions are x86_64-only, and you need to select the right flavor for your target platform.

Here's an example of building a shared library on x86_64 Linux using the assembly implementations:

gcc -shared -O3 -o blake3.c blake3_dispatch.c blake3_portable.c \
    blake3_sse2_x86-64_unix.S blake3_sse41_x86-64_unix.S blake3_avx2_x86-64_unix.S \

When building the intrinsics-based implementations, you need to build each implementation separately, with the corresponding instruction set explicitly enabled in the compiler. Here's the same shared library using the intrinsics-based implementations:

gcc -c -fPIC -O3 -msse2 blake3_sse2.c -o blake3_sse2.o
gcc -c -fPIC -O3 -msse4.1 blake3_sse41.c -o blake3_sse41.o
gcc -c -fPIC -O3 -mavx2 blake3_avx2.c -o blake3_avx2.o
gcc -c -fPIC -O3 -mavx512f -mavx512vl blake3_avx512.c -o blake3_avx512.o
gcc -shared -O3 -o blake3.c blake3_dispatch.c blake3_portable.c \
    blake3_avx2.o blake3_avx512.o blake3_sse41.o blake3_sse2.o

Note above that building blake3_avx512.c requires both -mavx512f and -mavx512vl under GCC and Clang. Under MSVC, the single /arch:AVX512 flag is sufficient. The MSVC equivalent of -mavx2 is /arch:AVX2. MSVC enables SSE2 and SSE4.1 by defaut, and it doesn't have a corresponding flag.

If you want to omit SIMD code entirely, you need to explicitly disable each instruction set. Here's an example of building a shared library on x86 with only portable code:

gcc -shared -O3 -o -DBLAKE3_NO_SSE2 -DBLAKE3_NO_SSE41 -DBLAKE3_NO_AVX2 \
    -DBLAKE3_NO_AVX512 blake3.c blake3_dispatch.c blake3_portable.c


The NEON implementation is enabled by default on AArch64, but not on other ARM targets, since not all of them support it. To enable it, set BLAKE3_USE_NEON=1. Here's an example of building a shared library on ARM Linux with NEON support:

gcc -shared -O3 -o -DBLAKE3_USE_NEON=1 blake3.c blake3_dispatch.c \
    blake3_portable.c blake3_neon.c

To explicitiy disable using NEON instructions on AArch64, set BLAKE3_USE_NEON=0.

gcc -shared -O3 -o -DBLAKE3_USE_NEON=0 blake3.c blake3_dispatch.c \

Note that on some targets (ARMv7 in particular), extra flags may be required to activate NEON support in the compiler. If you see an error like...

/usr/lib/gcc/armv7l-unknown-linux-gnueabihf/9.2.0/include/arm_neon.h:635:1: error: inlining failed
in call to always_inline ‘vaddq_u32’: target specific option mismatch

...then you may need to add something like -mfpu=neon-vfpv4 -mfloat-abi=hard.

Other Platforms

The portable implementation should work on most other architectures. For example:

gcc -shared -O3 -o blake3.c blake3_dispatch.c blake3_portable.c


Unlike the Rust implementation, the C implementation doesn't currently support multithreading. A future version of this library could add support by taking an optional dependency on OpenMP or similar. Alternatively, we could expose a lower-level API to allow callers to implement concurrency themselves. The former would be more convenient and less error-prone, but the latter would give callers the maximum possible amount of control. The best choice here depends on the specific use case, so if you have a use case for multithreaded hashing in C, please file a GitHub issue and let us know.