BLAKE3/c/README.md

The official C implementation of BLAKE3.

Example

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(void) {
  // Initialize the hasher.
  blake3_hasher hasher;
  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) {
      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. 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]);
  }
  printf("\n");
  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 \
    blake3_avx512_x86-64_unix.S

API

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. See the Security Notes below.

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., "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 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.

---

void blake3_hasher_reset(
  blake3_hasher *self);

Reset the hasher to its initial state, prior to any calls to blake3_hasher_update. Currently this is no different from calling blake3_hasher_init or similar again. However, if this implementation gains multithreading support in the future, and if blake3_hasher holds (optional) threading resources, this function will reuse those resources. Until then, this is mainly for feature compatibility with the Rust implementation.

Security Notes

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.

Avoid relying on the secrecy of the output offset, that is, the seek argument of blake3_hasher_finalize_seek. Block-Cipher-Based Tree Hashing by Aldo Gunsing shows that an attacker who knows both the message and the key (if any) can easily determine the offset of an extended output. For comparison, AES-CTR has a similar property: if you know the key, you can decrypt a block from an unknown position in the output stream to recover its block index. Callers with strong secret keys aren't affected in practice, but secret offsets are a design smell in any case.

Building

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.

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.

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

gcc -shared -O3 -o libblake3.so 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 \
    blake3_avx512_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 libblake3.so 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 default, 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 libblake3.so -DBLAKE3_NO_SSE2 -DBLAKE3_NO_SSE41 -DBLAKE3_NO_AVX2 \
    -DBLAKE3_NO_AVX512 blake3.c blake3_dispatch.c blake3_portable.c

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. Here's an example of building a shared library on ARM Linux with NEON support:

gcc -shared -O3 -o libblake3.so -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 libblake3.so -DBLAKE3_USE_NEON=0 blake3.c blake3_dispatch.c \
    blake3_portable.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 libblake3.so blake3.c blake3_dispatch.c blake3_portable.c

Multithreading

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.