sha-256.c

#include "sha-256.h"

#define TOTAL_LEN_LEN 8

/*
 * Comments from pseudo-code at https://en.wikipedia.org/wiki/SHA-2 are reproduced here.
 * When useful for clarification, portions of the pseudo-code are reproduced here too.
 */

/*
 * @brief Rotate a 32-bit value by a number of bits to the right.
 * @param value The value to be rotated.
 * @param count The number of bits to rotate by.
 * @return The rotated value.
 */
static inline uint32_t right_rot(uint32_t value, unsigned int count)
{
	/*
	 * Defined behaviour in standard C for all count where 0 < count < 32, which is what we need here.
	 */
	return value >> count | value << (32 - count);
}

/*
 * @brief Update a hash value under calculation with a new chunk of data.
 * @param h Pointer to the first hash item, of a total of eight.
 * @param p Pointer to the chunk data, which has a standard length.
 *
 * @note This is the SHA-256 work horse.
 */
static inline void consume_chunk(uint32_t *h, const uint8_t *p)
{
	unsigned i, j;
	uint32_t ah[8];

	/* Initialize working variables to current hash value: */
	for (i = 0; i < 8; i++)
		ah[i] = h[i];

	/*
	 * The w-array is really w[64], but since we only need 16 of them at a time, we save stack by
	 * calculating 16 at a time.
	 *
	 * This optimization was not there initially and the rest of the comments about w[64] are kept in their
	 * initial state.
	 */

	/*
	 * create a 64-entry message schedule array w[0..63] of 32-bit words (The initial values in w[0..63]
	 * don't matter, so many implementations zero them here) copy chunk into first 16 words w[0..15] of the
	 * message schedule array
	 */
	uint32_t w[16];

	/* Compression function main loop: */
	for (i = 0; i < 4; i++) {
		for (j = 0; j < 16; j++) {
			if (i == 0) {
				w[j] =
				    (uint32_t)p[0] << 24 | (uint32_t)p[1] << 16 | (uint32_t)p[2] << 8 | (uint32_t)p[3];
				p += 4;
			} else {
				/* Extend the first 16 words into the remaining 48 words w[16..63] of the
				 * message schedule array: */
				const uint32_t s0 = right_rot(w[(j + 1) & 0xf], 7) ^ right_rot(w[(j + 1) & 0xf], 18) ^
						    (w[(j + 1) & 0xf] >> 3);
				const uint32_t s1 = right_rot(w[(j + 14) & 0xf], 17) ^
						    right_rot(w[(j + 14) & 0xf], 19) ^ (w[(j + 14) & 0xf] >> 10);
				w[j] = w[j] + s0 + w[(j + 9) & 0xf] + s1;
			}
			const uint32_t s1 = right_rot(ah[4], 6) ^ right_rot(ah[4], 11) ^ right_rot(ah[4], 25);
			const uint32_t ch = (ah[4] & ah[5]) ^ (~ah[4] & ah[6]);

			/*
			 * Initialize array of round constants:
			 * (first 32 bits of the fractional parts of the cube roots of the first 64 primes 2..311):
			 */
			static const uint32_t k[] = {
			    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};

			const uint32_t temp1 = ah[7] + s1 + ch + k[i << 4 | j] + w[j];
			const uint32_t s0 = right_rot(ah[0], 2) ^ right_rot(ah[0], 13) ^ right_rot(ah[0], 22);
			const uint32_t maj = (ah[0] & ah[1]) ^ (ah[0] & ah[2]) ^ (ah[1] & ah[2]);
			const uint32_t temp2 = s0 + maj;

			ah[7] = ah[6];
			ah[6] = ah[5];
			ah[5] = ah[4];
			ah[4] = ah[3] + temp1;
			ah[3] = ah[2];
			ah[2] = ah[1];
			ah[1] = ah[0];
			ah[0] = temp1 + temp2;
		}
	}

	/* Add the compressed chunk to the current hash value: */
	for (i = 0; i < 8; i++)
		h[i] += ah[i];
}

/*
 * Public functions. See header file for documentation.
 */

void sha_256_init(struct Sha_256 *sha_256, uint8_t hash[SIZE_OF_SHA_256_HASH])
{
	sha_256->hash = hash;
	sha_256->chunk_pos = sha_256->chunk;
	sha_256->space_left = SIZE_OF_SHA_256_CHUNK;
	sha_256->total_len = 0;
	/*
	 * Initialize hash values (first 32 bits of the fractional parts of the square roots of the first 8 primes
	 * 2..19):
	 */
	sha_256->h[0] = 0x6a09e667;
	sha_256->h[1] = 0xbb67ae85;
	sha_256->h[2] = 0x3c6ef372;
	sha_256->h[3] = 0xa54ff53a;
	sha_256->h[4] = 0x510e527f;
	sha_256->h[5] = 0x9b05688c;
	sha_256->h[6] = 0x1f83d9ab;
	sha_256->h[7] = 0x5be0cd19;
}

void sha_256_write(struct Sha_256 *sha_256, const void *data, size_t len)
{
	sha_256->total_len += len;

	const uint8_t *p = data;

	while (len > 0) {
		/*
		 * If the input chunks have sizes that are multiples of the calculation chunk size, no copies are
		 * necessary. We operate directly on the input data instead.
		 */
		if (sha_256->space_left == SIZE_OF_SHA_256_CHUNK && len >= SIZE_OF_SHA_256_CHUNK) {
			consume_chunk(sha_256->h, p);
			len -= SIZE_OF_SHA_256_CHUNK;
			p += SIZE_OF_SHA_256_CHUNK;
			continue;
		}
		/* General case, no particular optimization. */
		const size_t consumed_len = len < sha_256->space_left ? len : sha_256->space_left;
		memcpy(sha_256->chunk_pos, p, consumed_len);
		sha_256->space_left -= consumed_len;
		len -= consumed_len;
		p += consumed_len;
		if (sha_256->space_left == 0) {
			consume_chunk(sha_256->h, sha_256->chunk);
			sha_256->chunk_pos = sha_256->chunk;
			sha_256->space_left = SIZE_OF_SHA_256_CHUNK;
		} else {
			sha_256->chunk_pos += consumed_len;
		}
	}
}

uint8_t *sha_256_close(struct Sha_256 *sha_256)
{
	uint8_t *pos = sha_256->chunk_pos;
	size_t space_left = sha_256->space_left;
	uint32_t *const h = sha_256->h;

	/*
	 * The current chunk cannot be full. Otherwise, it would already have been consumed. I.e. there is space left for
	 * at least one byte. The next step in the calculation is to add a single one-bit to the data.
	 */
	*pos++ = 0x80;
	--space_left;

	/*
	 * Now, the last step is to add the total data length at the end of the last chunk, and zero padding before
	 * that. But we do not necessarily have enough space left. If not, we pad the current chunk with zeroes, and add
	 * an extra chunk at the end.
	 */
	if (space_left < TOTAL_LEN_LEN) {
		memset(pos, 0x00, space_left);
		consume_chunk(h, sha_256->chunk);
		pos = sha_256->chunk;
		space_left = SIZE_OF_SHA_256_CHUNK;
	}
	const size_t left = space_left - TOTAL_LEN_LEN;
	memset(pos, 0x00, left);
	pos += left;
	size_t len = sha_256->total_len;
	pos[7] = (uint8_t)(len << 3);
	len >>= 5;
	int i;
	for (i = 6; i >= 0; --i) {
		pos[i] = (uint8_t)len;
		len >>= 8;
	}
	consume_chunk(h, sha_256->chunk);
	/* Produce the final hash value (big-endian): */
	int j;
	uint8_t *const hash = sha_256->hash;
	for (i = 0, j = 0; i < 8; i++) {
		hash[j++] = (uint8_t)(h[i] >> 24);
		hash[j++] = (uint8_t)(h[i] >> 16);
		hash[j++] = (uint8_t)(h[i] >> 8);
		hash[j++] = (uint8_t)h[i];
	}
	return sha_256->hash;
}

void calc_sha_256(uint8_t hash[SIZE_OF_SHA_256_HASH], const void *input, size_t len)
{
	struct Sha_256 sha_256;
	sha_256_init(&sha_256, hash);
	sha_256_write(&sha_256, input, len);
	(void)sha_256_close(&sha_256);
}