mirror of
https://github.com/BLAKE3-team/BLAKE3
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168 lines
6.1 KiB
Markdown
168 lines
6.1 KiB
Markdown
This is the C implementation of BLAKE3. The public API consists of one
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struct and five functions in [`blake3.h`](blake3.h):
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- **`typedef struct {...} blake3_hasher`** An incremental BLAKE3 hashing
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state, which can accept any number of updates.
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- **`blake3_hasher_init(...)`** Initialize a `blake3_hasher` in the
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default hashing mode.
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- **`blake3_hasher_init_keyed(...)`** Initialize a `blake3_hasher` in
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the keyed hashing mode, which accepts a 256-bit key.
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- **`blake3_hasher_init_derive_key(...)`** Initialize a `blake3_hasher`
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in the key derivation mode, which accepts a context string of any
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length. In this mode, the key material is given as input after
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initialization. The context string should be hardcoded, globally
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unique, and application-specific. A good default format for such
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strings is `"[application] [commit timestamp] [purpose]"`, e.g.,
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`"example.com 2019-12-25 16:18:03 session tokens v1"`.
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- **`blake3_hasher_update(...)`** Add input to the hasher. This can be
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called any number of times.
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- **`blake3_hasher_finalize(...)`** Finalize the hasher and emit an
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output of any length. This does not modify the hasher itself. It is
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possible to finalize again after adding more input.
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## Example
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Here's an example program that hashes bytes from standard input and
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prints the result:
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```c
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#include "blake3.h"
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#include <stdio.h>
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int main() {
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// Initialize the hasher.
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blake3_hasher hasher;
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blake3_hasher_init(&hasher);
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// Read input bytes from stdin.
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unsigned char buf[65536];
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size_t n;
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while ((n = fread(buf, 1, 65536, stdin)) > 0) {
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blake3_hasher_update(&hasher, buf, n);
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}
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// Finalize the hash. BLAKE3_OUT_LEN is the default output length, 32 bytes.
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uint8_t output[BLAKE3_OUT_LEN];
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blake3_hasher_finalize(&hasher, output, BLAKE3_OUT_LEN);
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// Print the hash as hexadecimal.
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for (size_t i = 0; i < BLAKE3_OUT_LEN; i++) {
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printf("%02x", output[i]);
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}
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printf("\n");
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return 0;
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}
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```
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If you save the example code above as `example.c`, and you're on x86\_64
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with a Unix-like OS, you can compile a working binary like this:
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```bash
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gcc -O3 -o example example.c blake3.c blake3_dispatch.c blake3_portable.c \
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blake3_sse41_x86-64_unix.S blake3_avx2_x86-64_unix.S blake3_avx512_x86-64_unix.S
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```
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## Building
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The Makefile included in this implementation is for testing. It's
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expected that callers will have their own build systems. This section
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describes the compilation steps that build systems (or folks compiling
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by hand) should take. Note that these steps may change in future
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versions.
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### x86
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Dynamic dispatch is enabled by default on x86. The implementation will
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query the CPU at runtime to detect SIMD support, and it will use the
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widest instruction set available. By default, `blake3_dispatch.c`
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expects to be linked with code for four different instruction sets:
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portable C, SSE4.1, AVX2, and AVX-512.
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For each of the x86 SIMD instruction sets, two versions are available,
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one in assembly (with three flavors: Unix, Windows MSVC, and Windows
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GNU) and one using C intrinsics. The assembly versions are generally
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preferred: they perform better, they perform more consistently across
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different compilers, and they build more quickly. On the other hand, the
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assembly versions are x86\_64-only, and you need to select the right
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flavor for your target platform.
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Here's an example of building a shared library on x86\_64 Linux using
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the assembly implementations:
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```bash
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gcc -shared -O3 -o libblake3.so blake3.c blake3_dispatch.c blake3_portable.c \
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blake3_sse41_x86-64_unix.S blake3_avx2_x86-64_unix.S blake3_avx512_x86-64_unix.S
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```
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When building the intrinsics-based implementations, you need to build
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each implementation separately, with the corresponding instruction set
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explicitly enabled in the compiler. Here's the same shared library using
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the intrinsics-based implementations:
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```bash
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gcc -c -fPIC -O3 -msse4.1 blake3_sse41.c -o blake3_sse41.o
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gcc -c -fPIC -O3 -mavx2 blake3_avx2.c -o blake3_avx2.o
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gcc -c -fPIC -O3 -mavx512f -mavx512vl blake3_avx512.c -o blake3_avx512.o
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gcc -shared -O3 -o libblake3.so blake3.c blake3_dispatch.c blake3_portable.c \
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blake3_avx2.o blake3_avx512.o blake3_sse41.o
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```
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Note above that building `blake3_avx512.c` requires both `-mavx512f` and
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`-mavx512vl` under GCC and Clang, as shown above. Under MSVC, the single
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`/arch:AVX512` flag is sufficient.
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If you want to omit SIMD code on x86, you need to explicitly disable
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each instruction set. Here's an example of building a shared library on
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x86 with only portable code:
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```bash
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gcc -shared -O3 -o libblake3.so -DBLAKE3_NO_SSE41 -DBLAKE3_NO_AVX2 -DBLAKE3_NO_AVX512 \
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blake3.c blake3_dispatch.c blake3_portable.c
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```
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### ARM NEON
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The NEON implementation is not enabled by default on ARM, since not all
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ARM targets support it. To enable it, set `BLAKE3_USE_NEON=1`. Here's an
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example of building a shared library on ARM Linux with NEON support:
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```bash
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gcc -shared -O3 -o libblake3.so -DBLAKE3_USE_NEON blake3.c blake3_dispatch.c \
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blake3_portable.c blake3_neon.c
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```
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Note that on some targets (ARMv7 in particular), extra flags may be
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required to activate NEON support in the compiler. If you see an error
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like...
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```
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/usr/lib/gcc/armv7l-unknown-linux-gnueabihf/9.2.0/include/arm_neon.h:635:1: error: inlining failed
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in call to always_inline ‘vaddq_u32’: target specific option mismatch
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```
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...then you may need to add something like `-mfpu=neon-vfpv4
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-mfloat-abi=hard`.
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### Other Platforms
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The portable implementation should work on most other architectures. For
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example:
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```bash
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gcc -shared -O3 -o libblake3.so blake3.c blake3_dispatch.c blake3_portable.c
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```
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## Differences from the Rust Implementation
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The single-threaded Rust and C implementations use the same algorithms,
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and their performance is the same if you use the assembly
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implementations or if you compile the intrinsics-based implementations
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with Clang. (Both Clang and rustc are LLVM-based.)
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The C implementation does not currently support multi-threading. OpenMP
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support or similar might be added in the future.
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Both the C and Rust implementations support output of any length, but
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only the Rust implementation provides an incremental (and seekable)
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output reader. This might also be added in the future.
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