Fractal Crypt-Hash (FCH)

Fractal Crypt-Hash (FCH) is an experimental cryptographic hash function based on a fractal (self-similar) recursive structure rather than traditional round-based compression.

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Security Notice (Read First)

FCH is a research / experimental hash design and reference implementation. It has not undergone broad public cryptanalysis or independent security review.

Do not use FCH in production or for security-critical purposes, including (but not limited to):

• authentication, signatures, tokens, or MACs
• password hashing / key derivation
• integrity checks where adversaries exist
• any scenario where a broken hash causes harm

This repository is intended for learning, experimentation, benchmarking, and discussion.

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Overview

Unlike conventional hash functions that rely on repeated rounds, FCH achieves diffusion through recursive fractal decomposition.

A small change in the input propagates:

• locally at leaf nodes,
• recursively through variable n-way splits,
• and globally at the root via recompression.

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Features

• Fractal recursive hash structure
• Variable n-way (2–6) splitting derived from the entire node input
• Order-dependent tree recombination
• Separate root, internal-node, leaf, and child domains
• Full-width leaf states with XOR / ADD / ROTATE / S-box mixing
• Recompression at each internal node
• Deterministic, non-keyed hash function

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Supported Variants

Variant	Output Size	Internal State
FCH-256	256 bits	4 × uint64
FCH-512	512 bits	8 × uint64

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Usage

uint8_t out256[32];
uint8_t out512[64];

int ok256 = fch_hash_256_checked(data, len, out256);
int ok512 = fch_hash_512_checked(data, len, out512);

The checked functions return 1 on success and 0 on invalid input, allocation failure, or an unsupported input length. The original void functions remain available as compatibility wrappers.

Bounded-memory streaming API

FCH provides a streaming API that writes incoming chunks to an anonymous temporary file. Finalization reads that data through fixed-size buffers, so RAM usage does not grow with the total input size. The digest is identical to the one-shot API. This mode requires temporary-file support and can be slower.

#include "fch_stream.h"

fch256_ctx ctx;
fch256_init(&ctx);
fch256_update(&ctx, chunk1, chunk1_len);
fch256_update(&ctx, chunk2, chunk2_len);
int ok = fch256_final_checked(&ctx, out256);
fch256_free(&ctx);

After finalization, further updates and repeated finalization are rejected. The CLI uses the same bounded-memory path for files and standard input.

CLI

Build the CLI:

cd build
make all

Hash a file:

./fch -256 path/to/file
./fch -512 path/to/file

Hash stdin:

cat path/to/file | ./fch -256

Testing

The implementation includes:

• Statistical avalanche tests (length/bit diffusion)
• Determinism tests (same input → same output)
• Boundary condition tests
• Structural invariant tests (split coverage)
• Portability and domain-separation tests
• Split-configuration sensitivity tests

The reference implementation includes extensive tests for determinism, boundary conditions, structural invariants, and statistical diffusion behavior.

Test programs:

• tests/test_avalanche.c
• tests/test_consistency.c
• tests/test_boundaries.c
• tests/test_invariants.c
• tests/test_portability.c
• tests/test_vectors.c
• tests/test_split_sensitivity.c

Build/run:

cd build
make check

The CI workflow runs the suite with GCC and Clang and also runs AddressSanitizer and UndefinedBehaviorSanitizer.

If make is unavailable (Windows), you can compile directly with gcc:

gcc -Wall -Wextra -O2 -Iinclude tests/test_consistency.c src/*.c -o build/test_consistency.exe
gcc -Wall -Wextra -O2 -Iinclude tests/test_boundaries.c  src/*.c -o build/test_boundaries.exe
gcc -Wall -Wextra -O2 -Iinclude tests/test_invariants.c  src/*.c -o build/test_invariants.exe
gcc -Wall -Wextra -O2 -Iinclude tests/test_avalanche.c   src/*.c -o build/test_avalanche.exe
gcc -Wall -Wextra -O2 -Iinclude tests/test_vectors.c     src/*.c -o build/test_vectors.exe

gcc -Wall -Wextra -O2 -Iinclude tools/fch.c              src/*.c -o build/fch.exe

build\\fch.exe -256 README.md