SHA-256 Library in C

Overview

SHA-256 is a widely used cryptographic hash function, forming the backbone of many security protocols. Here is a lightweight, high-performance SHA-256 implementation in C, designed for ease of use and efficiency.

Features

• Lightweight: Minimal dependencies, making it suitable for embedded systems.
• Optimized for Speed: Carefully tuned bitwise operations.
• Portable: Works across multiple platforms with standard C.
• Easy to Integrate: Simple API for hashing strings

How It Works

The program above demonstrates how the SHA-256 library in C works by computing the hash of a string. Here are the main steps:

1. Initializing the Data:

   • The input string "Hello World!" is defined.
   • A character array hex[SHA256_HEX_SIZE] is allocated to store the resulting hash in hexadecimal format.

2. Computing the SHA-256 Hash:

   • The function sha256_hex() is called with three arguments:
     • The input string text.
     • The length of the string, obtained using strlen(text).
     • The hex array, which will store the hash.
   • The SHA-256 algorithm processes the input in 512-bit blocks, performing logical and arithmetic operations such as bit shifts, XORs, and modulo 32 additions.
   • Once all blocks are processed, the function produces a unique 256-bit hash (represented as a 64-character hexadecimal string).
3. Displaying the Result:
   • The program prints the original input string.
   • It then outputs the SHA-256 hash in hexadecimal format.

This minimalistic program demonstrates how to use the SHA-256 library to convert a simple string into a secure hash, which can be used for integrity verification or cryptographic applications.

Code

Main

#include "sha256.h"

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

int main(void){
    /* Input text. */
    const char *text = "Hello World!";

    /* Char array to store the hexadecimal SHA-256 string. */
    /* Must be 65 characters big (or larger). */
    /* The last character will be the null-character. */
    char hex[SHA256_HEX_SIZE];

    /* Compute SHA-256 sum. */
    sha256_hex(text, strlen(text), hex);

    /* Print result. */
    printf("The SHA-256 sum of \"%s\" is:\n\n", text);
    printf("%s\n\n", hex);

    return 0;
}

sha256.h

#ifndef SHA256_H
#define SHA256_H

#include <stddef.h>
#include <stdint.h>

#define SHA256_HEX_SIZE (64 + 1)
#define SHA256_BYTES_SIZE 32

/*
 * Compute the SHA-256 checksum of a memory region given a pointer and
 * the size of that memory region.
 * The output is a hexadecimal string of 65 characters.
 * The last character will be the null-character.
 */
void sha256_hex(const void *src, size_t n_bytes, char *dst_hex65);

void sha256_bytes(const void *src, size_t n_bytes, void *dst_bytes32);

typedef struct sha256 {
    uint32_t state[8];
    uint8_t buffer[64];
    uint64_t n_bits;
    uint8_t buffer_counter;
} sha256;

/* Functions to compute streaming SHA-256 checksums. */
void sha256_init(struct sha256 *sha);
void sha256_append(struct sha256 *sha, const void *data, size_t n_bytes);
void sha256_finalize_hex(struct sha256 *sha, char *dst_hex65);
void sha256_finalize_bytes(struct sha256 *sha, void *dst_bytes32);

#endif

sha256.c

#include "sha256.h"

// Rotate right
static inline uint32_t rotr(uint32_t x, int n){
    return (x >> n) | (x << (32 - n));
}

static inline uint32_t step1(uint32_t e, uint32_t f, uint32_t g){
    return (rotr(e, 6) ^ rotr(e, 11) ^ rotr(e, 25)) + ((e & f) ^ ((~ e) & g));
}

static inline uint32_t step2(uint32_t a, uint32_t b, uint32_t c){
    return (rotr(a, 2) ^ rotr(a, 13) ^ rotr(a, 22)) + ((a & b) ^ (a & c) ^ (b & c));
}

static inline void update_w(uint32_t *w, int i, const uint8_t *buffer){
    int j;
    for (j = 0; j < 16; j++){
        if (i < 16){
            w[j] =
                    ((uint32_t)buffer[0] << 24) |
                    ((uint32_t)buffer[1] << 16) |
                    ((uint32_t)buffer[2] <<  8) |
                    ((uint32_t)buffer[3]);
            buffer += 4;
        }else{
            uint32_t a = w[(j + 1) & 15];
            uint32_t b = w[(j + 14) & 15];
            uint32_t s0 = (rotr(a,  7) ^ rotr(a, 18) ^ (a >>  3));
            uint32_t s1 = (rotr(b, 17) ^ rotr(b, 19) ^ (b >> 10));
            w[j] += w[(j + 9) & 15] + s0 + s1;
        }
    }
}

static void sha256_block(struct sha256 *sha){
    // State of the program
    uint32_t *state = sha->state;

    // Declare the K constant
    static const uint32_t k[8 * 8] = {
        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,
    };

    uint32_t a = state[0];
    uint32_t b = state[1];
    uint32_t c = state[2];
    uint32_t d = state[3];
    uint32_t e = state[4];
    uint32_t f = state[5];
    uint32_t g = state[6];
    uint32_t h = state[7];

    uint32_t w[16];

    int i, j;
    for (i = 0; i < 64; i += 16){
        update_w(w, i, sha->buffer);

        for (j = 0; j < 16; j += 4){
            uint32_t temp;
            temp = h + step1(e, f, g) + k[i + j + 0] + w[j + 0];
            h = temp + d;
            d = temp + step2(a, b, c);
            temp = g + step1(h, e, f) + k[i + j + 1] + w[j + 1];
            g = temp + c;
            c = temp + step2(d, a, b);
            temp = f + step1(g, h, e) + k[i + j + 2] + w[j + 2];
            f = temp + b;
            b = temp + step2(c, d, a);
            temp = e + step1(f, g, h) + k[i + j + 3] + w[j + 3];
            e = temp + a;
            a = temp + step2(b, c, d);
        }
    }

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

void sha256_init(struct sha256 *sha){
    sha->state[0] = 0x6a09e667;
    sha->state[1] = 0xbb67ae85;
    sha->state[2] = 0x3c6ef372;
    sha->state[3] = 0xa54ff53a;
    sha->state[4] = 0x510e527f;
    sha->state[5] = 0x9b05688c;
    sha->state[6] = 0x1f83d9ab;
    sha->state[7] = 0x5be0cd19;
    sha->n_bits = 0;
    sha->buffer_counter = 0;
}

void sha256_append_byte(struct sha256 *sha, uint8_t byte){
    sha->buffer[sha->buffer_counter++] = byte;
    sha->n_bits += 8;

    if (sha->buffer_counter == 64){
        sha->buffer_counter = 0;
        sha256_block(sha);
    }
}

void sha256_append(struct sha256 *sha, const void *src, size_t n_bytes){
    const uint8_t *bytes = (const uint8_t*)src;
    size_t i;

    for (i = 0; i < n_bytes; i++){
        sha256_append_byte(sha, bytes[i]);
    }
}

void sha256_finalize(struct sha256 *sha){
    int i;
    uint64_t n_bits = sha->n_bits;

    sha256_append_byte(sha, 0x80);

    while (sha->buffer_counter != 56){
        sha256_append_byte(sha, 0);
    }

    for (i = 7; i >= 0; i--){
        uint8_t byte = (n_bits >> 8 * i) & 0xff;
        sha256_append_byte(sha, byte);
    }
}

void sha256_finalize_hex(struct sha256 *sha, char *dst_hex65){
    int i, j;
    sha256_finalize(sha);

    for (i = 0; i < 8; i++){
        for (j = 7; j >= 0; j--){
            uint8_t nibble = (sha->state[i] >> j * 4) & 0xf;
            *dst_hex65++ = "0123456789abcdef"[nibble];
        }
    }

    *dst_hex65 = '\0';
}

void sha256_finalize_bytes(struct sha256 *sha, void *dst_bytes32){
    uint8_t *ptr = (uint8_t*)dst_bytes32;
    int i, j;
    sha256_finalize(sha);

    for (i = 0; i < 8; i++){
        for (j = 3; j >= 0; j--){
            *ptr++ = (sha->state[i] >> j * 8) & 0xff;
        }
    }
}

void sha256_hex(const void *src, size_t n_bytes, char *dst_hex65){
    struct sha256 sha;
    sha256_init(&sha);
    sha256_append(&sha, src, n_bytes);
    sha256_finalize_hex(&sha, dst_hex65);
}

void sha256_bytes(const void *src, size_t n_bytes, void *dst_bytes32){
    struct sha256 sha;
    sha256_init(&sha);
    sha256_append(&sha, src, n_bytes);
    sha256_finalize_bytes(&sha, dst_bytes32);
}

Download

The detailed example on this page can be downloaded. It has been compiled and run with Qt Creator:

• sha256_c_cpp.zip

  See also

• How to compile C/C++ files with an external library
• How to install Qt Creator on Ubuntu 16.04?
• How to install Qt Creator on Ubuntu 18.04
• How to install Visual Studio Code on Ubuntu 18.04
• How to resize shared memory limit on Ubuntu 14.04
• How to swap variables in C or C++
• How to read XML files with Qt ?
• C++ std::vector

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