Example

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

Using the C++ API:

#include "llvm/Support/BLAKE3.h"
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main() {
  // Initialize the hasher.
  llvm::BLAKE3 hasher;

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

  // Finalize the hash. Default output length is 32 bytes.
  auto output = hasher.final();

  // Print the hash as hexadecimal.
  for (uint8_t byte : output) {
    printf("%02x", byte);
  }
  printf("\n");
  return 0;
}

Using the C API:

#include "llvm-c/blake3.h"
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main() {
  // Initialize the hasher.
  llvm_blake3_hasher hasher;
  llvm_blake3_hasher_init(&hasher);

  // Read input bytes from stdin.
  unsigned char buf[65536];
  while (1) {
    ssize_t n = read(STDIN_FILENO, buf, sizeof(buf));
    if (n > 0) {
      llvm_blake3_hasher_update(&hasher, buf, n);
    } else if (n == 0) {
      break; // end of file
    } else {
      fprintf(stderr, "read failed: %s\n", strerror(errno));
      exit(1);
    }
  }

  // Finalize the hash. LLVM_BLAKE3_OUT_LEN is the default output length, 32 bytes.
  uint8_t output[LLVM_BLAKE3_OUT_LEN];
  llvm_blake3_hasher_finalize(&hasher, output, LLVM_BLAKE3_OUT_LEN);

  // Print the hash as hexadecimal.
  for (size_t i = 0; i < LLVM_BLAKE3_OUT_LEN; i++) {
    printf("%02x", output[i]);
  }
  printf("\n");
  return 0;
}

API

The Class/Struct

class BLAKE3 {
  // API
private:
  llvm_blake3_hasher Hasher;
};

typedef struct {
  // private fields
} llvm_blake3_hasher;

An incremental BLAKE3 hashing state, which can accept any number of updates. This implementation doesn‘t allocate any heap memory, but sizeof(llvm_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

BLAKE3::BLAKE3();

void BLAKE3::init();

void llvm_blake3_hasher_init(
  llvm_blake3_hasher *self);

Initialize a llvm_blake3_hasher in the default hashing mode.

void BLAKE3::update(ArrayRef<uint8_t> Data);

void BLAKE3::update(StringRef Str);

void llvm_blake3_hasher_update(
  llvm_blake3_hasher *self,
  const void *input,
  size_t input_len);

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

template <size_t NumBytes = LLVM_BLAKE3_OUT_LEN>
using BLAKE3Result = std::array<uint8_t, NumBytes>;

template <size_t NumBytes = LLVM_BLAKE3_OUT_LEN>
void BLAKE3::final(BLAKE3Result<NumBytes> &Result);

template <size_t NumBytes = LLVM_BLAKE3_OUT_LEN>
BLAKE3Result<NumBytes> BLAKE3::final();

void llvm_blake3_hasher_finalize(
  const llvm_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 LLVM_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 llvm_blake3_hasher_init_keyed(
  llvm_blake3_hasher *self,
  const uint8_t key[LLVM_BLAKE3_KEY_LEN]);

Initialize a llvm_blake3_hasher in the keyed hashing mode. The key must be exactly 32 bytes.

void llvm_blake3_hasher_init_derive_key(
  llvm_blake3_hasher *self,
  const char *context);

Initialize a llvm_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 llvm_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., "example.com 2019-12-25 16:18:03 session tokens v1".

This function is intended for application code written in C. For language bindings, see llvm_blake3_hasher_init_derive_key_raw below.

void llvm_blake3_hasher_init_derive_key_raw(
  llvm_blake3_hasher *self,
  const void *context,
  size_t context_len);

As llvm_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 llvm_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 llvm_blake3_hasher_finalize_seek(
  const llvm_blake3_hasher *self,
  uint64_t seek,
  uint8_t *out,
  size_t out_len);

The same as llvm_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.

void llvm_blake3_hasher_reset(
  llvm_blake3_hasher *self);

Reset the hasher to its initial state, prior to any calls to llvm_blake3_hasher_update. Currently this is no different from calling llvm_blake3_hasher_init or similar again. However, if this implementation gains multithreading support in the future, and if llvm_blake3_hasher holds (optional) threading resources, this function will reuse those resources.

Building

This implementation is just C and assembly files.

x86

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.

ARM NEON

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.

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

Other Platforms

The portable implementation should work on most other architectures.

Multithreading

The implementation doesn't currently support multithreading.