A SHA256 implementatoin written in C.

sha256.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdint.h>

// K[0] to K[63] are initialized as the first 32 bits of the fractional
// parts of the cube roots of the first 64 primes
const uint32_t k[64] = {
   0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5,
   0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3, 0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174,
   0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc, 0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da,
   0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7, 0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967,
   0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13, 0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85,
   0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3, 0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070,
   0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5, 0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3,
   0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208, 0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2,
};

typedef struct {
	uint32_t h[8];
} Hash;

void hash_init(Hash *hash) {
	// h0 to h7 are initialized as the first 32 bits of the
	// fractional part of the square roots of the first 8 primes
	hash->h[0] = 0x6a09e667;
	hash->h[1] = 0xbb67ae85;
	hash->h[2] = 0x3c6ef372;
	hash->h[3] = 0xa54ff53a;
	hash->h[4] = 0x510e527f;
	hash->h[5] = 0x9b05688c;
	hash->h[6] = 0x1f83d9ab;
	hash->h[7] = 0x5be0cd19;	
}

int asprintf(char **strp, const char *fmt, ...);
char *hash_hexdigest(Hash *hash) {
	char *str;
	// *Output* What we computed
	asprintf(&str, "%08x%08x%08x%08x%08x%08x%08x%08x",
		hash->h[0], hash->h[1], hash->h[2], hash->h[3], hash->h[4], hash->h[5], hash->h[6], hash->h[7]);
	return str;
}

// Rightrotate n by m bits
uint32_t rightrotate(uint32_t n, uint32_t m) {
	return (n >> m) | (n << (32 - m));
}

uint32_t pack32(int *idx, uint8_t chunk[]) {
	uint32_t val = 0;
	val = ((chunk[*idx] << 24) | (chunk[(*idx) + 1] << 16) | (chunk[(*idx) + 2] << 8) | chunk[(*idx) + 3]);
	*idx += 4;
	return val;
}

Hash hash_sha256(uint8_t bytes[], uint64_t size) {
	Hash hash;
	hash_init(&hash);

	int total_size = ((size / 512) + 1) * 512; // In bits
	int chunk_count = total_size / 512; // Each chunk is 512 bits (64 bytes)
	int total_bytes = total_size / 8;
	uint8_t *padded_bytes = calloc(total_bytes, sizeof(uint8_t));
	int size_bytes = size / 8; // Size in bytes not bits
	memcpy(padded_bytes, bytes, size_bytes);
	padded_bytes[size_bytes] = 0x80; // Append a single 1
	padded_bytes[total_bytes - 8] = (size >> 56) & 0xFF;
	padded_bytes[total_bytes - 7] = (size >> 48) & 0xFF;
	padded_bytes[total_bytes - 6] = (size >> 40) & 0xFF;
	padded_bytes[total_bytes - 5] = (size >> 32) & 0xFF;
	padded_bytes[total_bytes - 4] = (size >> 24) & 0xFF;
	padded_bytes[total_bytes - 3] = (size >> 16) & 0xFF;
	padded_bytes[total_bytes - 2] = (size >> 8) & 0xFF;
	padded_bytes[total_bytes - 1] = size & 0xFF;

	for (int idx = 0; idx < chunk_count; idx++) {
		uint32_t w[64]; // Message schedule

		// Copy the 512 bit chunk into the first 16 elements of the message block
		int offset = 0;
		uint8_t *chunk = malloc(64 * sizeof(uint8_t));
		memcpy(chunk, padded_bytes + (idx * 64), 64);
		for (int i = 0; i < 16; i++)
			w[i] = pack32(&offset, chunk);
		free(chunk);

		// Extend the first 16 elements to the 48 others
		for (int i = 16; i < 64; i++) {
			uint32_t s0 = rightrotate(w[i - 15], 7) ^ rightrotate(w[i - 15], 18) ^ (w[i - 15] >> 3);
			uint32_t s1 = rightrotate(w[i - 2], 17) ^ rightrotate(w[i -  2], 19) ^ (w[i -  2] >> 10);
			w[i] = w[i - 16] + s0 + w[i - 7] + s1;
		}

		// Main compression loop:
		uint32_t a = hash.h[0];
		uint32_t b = hash.h[1];
		uint32_t c = hash.h[2];
		uint32_t d = hash.h[3];
		uint32_t e = hash.h[4];
		uint32_t f = hash.h[5];
		uint32_t g = hash.h[6];
		uint32_t h = hash.h[7];

		for (int i = 0; i < 64; i++) {
			uint32_t s0 = rightrotate(a, 2) ^ rightrotate(a, 13) ^ rightrotate(a, 22);
			uint32_t maj = (a & b) ^ (a & c) ^ (b & c);
			uint32_t t2 = s0 + maj;
			uint32_t s1 = rightrotate(e, 6) ^ rightrotate(e, 11) ^ rightrotate(e, 25);
			uint32_t ch = (e & f) ^ (~e & g);
			uint32_t t1 = h + s1 + ch + k[i] + w[i];
			h = g;
			g = f;
			f = e;
			e = d + t1;
			d = c;
			c = b;
			b = a;
			a = t1 + t2;
		}

		hash.h[0] += a;
		hash.h[1] += b;
		hash.h[2] += c;
		hash.h[3] += d;
		hash.h[4] += e;
		hash.h[5] += f;
		hash.h[6] += g;
		hash.h[7] += h;
	}

	free(padded_bytes);
	return hash;
}

int main() {
	// "Hello world" utf8 encoeded bytes
	uint8_t hello_world[11] = {0x68, 0x65, 0x6C, 0x6C, 0x6F, 0x20, 0x77, 0x6F, 0x72, 0x6C, 0x64};
	char *hello_world_hash = "b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9";
	Hash h = hash_sha256(hello_world, 11 * 8); // Size is provided in bits
	char *hashdigest = hash_hexdigest(&h);
	if (strcmp(hashdigest, hello_world_hash) == 0)
		printf("Working SHA256 implementation!\n");
	free(hashdigest);
}