Feistel Cipher test implementation inspired by the "Feistel Cipher - Computerphile" Youtube video

feistel_cipher_test.c

/**
 * build:
 *   gcc feistel_cipher_test.c -o feistel_cipher_test
 *
 * run:
 *   ./feistel_cipher_test
 *
 * Feistel Cipher test implementation inspired by the "Feistel Cipher - Computerphile"
 * Youtube video: https://youtu.be/FGhj3CGxl8I
 * 
 * This code was a created after watching Dr Mike Pound explain the principles
 * underlying the Feistel Cipher. It follows the steps as demostrated in the
 * video for the fun and education.
 *
 * The function used is a the HMAC-sha256 stream cypher which is explained by 
 * Dr Mike Pound here: https://youtu.be/wlSG3pEiQdc
 * 
 * In this example we will use a fixed size for all buffers and keys so we can
 * stay close the examples given in the video. 
 *
 * None of this code is meant for real-world encryption or production and is 
 * only created to better understand the 'trick' Dr Mike demonstrates.
 * 
 * - Using the sha256 implementation by Ilya O. Levin, see license below.
 * - Using the hmac-sha256 implementation by Adrian Perez, see license below.
 *  
 *   Copyright (c) 2020, Klaas Speller, http://github.com/spllr
 * 
 *   Permission to use, copy, modify, and distribute this software for any
 *   purpose with or without fee is hereby granted, provided that the above
 *   copyright notice and this permission notice appear in all copies.
 *
 *   THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 *   WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 *   MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 *   ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 *   WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 *   ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 *   OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 *
 */

#include <limits.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

#define CHUNK_SIZE 32

typedef uint8_t key_t[CHUNK_SIZE];
typedef uint8_t buf_t[CHUNK_SIZE + 1];


// MARK: - The Feistel Cipher Stuff

void F(buf_t dest, buf_t value, key_t key);
void xor(buf_t dest, buf_t lh, buf_t rh);
void feistel_cipher_apply(buf_t left_out, buf_t right_out, buf_t left_in, buf_t right_in, key_t keys[], size_t num_keys);


// MARK: - Utils

void key_generator_init();
void key_generate(uint8_t dest[]);
void print_hex(uint8_t buf[], size_t buf_size);
void print_pass_info(size_t pass_index, key_t key, buf_t left, buf_t right);
void print_begin_state(buf_t left, buf_t right);
void print_end_state(buf_t left, buf_t right);
void strip_newline(buf_t buf);
void hmac_sha256 (uint8_t out[CHUNK_SIZE],
             const uint8_t *data, size_t data_len,
             const uint8_t *key, size_t key_len);


// MARK: - Main

int main (int argc, char const *argv[])
{
  key_generator_init();
  
  buf_t left_in = { 0 }, right_in = { 0 }, left_out, right_out;
  key_t keys[2];

  // Generate the keys
  
  key_generate(keys[0]);
  key_generate(keys[1]);
  
  
  // Get the left value
  
  printf("left value (max 31 chars): ");
  fgets((char *)left_in, CHUNK_SIZE, stdin);
  strip_newline(left_in);


  // Get the right value  
  
  printf("right value (max 31 chars): ");
  fgets((char *)right_in, CHUNK_SIZE, stdin);
  strip_newline(right_in);
  
  
  // Encrypt
  
  printf("\n- encrypt");
  
  feistel_cipher_apply(left_out, right_out, left_in, right_in, keys, 2);
  

  // Decrypt
  
  printf("\n\n- decrypt");
  
  key_t reversed_keys[2];

  memcpy(reversed_keys[0], keys[1], CHUNK_SIZE);
  memcpy(reversed_keys[1], keys[0], CHUNK_SIZE);
  
  feistel_cipher_apply(left_in, right_in, left_out, right_out, reversed_keys, 2);
  
  return 0;
}


// MARK: - The Feistel Cipher Stuff Implementation

void feistel_cipher_apply(buf_t left_out, buf_t right_out, buf_t left_in, buf_t right_in, key_t keys[], size_t num_keys)
{
  buf_t out;
  buf_t left;
  buf_t right;
  buf_t next_right;
  
  memcpy(left, left_in, CHUNK_SIZE);
  memcpy(right, right_in, CHUNK_SIZE);
  
  print_begin_state(left, right);
  
  for (size_t i = 0; i < num_keys; i++)
  {
    F(out, right, keys[i]);
    xor(next_right, left, out);

    memcpy(left, right, CHUNK_SIZE);
    memcpy(right, next_right, CHUNK_SIZE);
    
    print_pass_info(i, keys[i], left, right);
  }
  
  memcpy(left_out, right, CHUNK_SIZE);
  memcpy(right_out, left, CHUNK_SIZE);
  
  print_end_state(left_out, right_out);  
}

void F(buf_t dest, buf_t value, key_t key)
{
  hmac_sha256(dest, value, CHUNK_SIZE, key, CHUNK_SIZE);
}

void xor(buf_t dest, buf_t lh, buf_t rh)
{
  for (size_t i = 0; i < CHUNK_SIZE; i++)
  {
    dest[i] = lh[i] ^ rh[i];
  }
}


// MARK: - Util Implementation

void key_generator_init()
{
  srand(time(NULL));
}

void key_generate(uint8_t dest[])
{
  for (size_t i = 0; i < CHUNK_SIZE; i++)
  {
    dest[i] = rand() % UINT8_MAX;
  }
}

void print_pass_info(size_t pass_index, key_t key, buf_t left, buf_t right)
{
  printf("\n\npass %zu: \n key=", pass_index + 1);
  print_hex(key, CHUNK_SIZE);    
  printf("\n left=");
  print_hex(left, CHUNK_SIZE);
  printf("\n right=");
  print_hex(right, CHUNK_SIZE);
}


void print_begin_state(buf_t left, buf_t right)
{
  printf("\n left=");
  print_hex(left, CHUNK_SIZE);
  printf("\n right=");
  print_hex(right, CHUNK_SIZE);  
}

void print_end_state(buf_t left, buf_t right)
{
  printf("\n\nresult:");
  printf("\n left=");
  print_hex(left, CHUNK_SIZE);
  printf("\n right=");
  print_hex(right, CHUNK_SIZE);
  printf("\n output: %s %s\n", left, right);
}

void print_hex(uint8_t buffer[], size_t buf_size)
{
  size_t str_size = buf_size * 2 + 1;
  char hex_str[str_size];
  hex_str[str_size - 1] = '\0';
  
  for (size_t i = 0; i < buf_size; i++)
  {
    sprintf((hex_str + i), "%X", buffer[i]);
  }
  
  printf("0x%s", hex_str);
}

void strip_newline(buf_t buf)
{
  buf[strnlen((char *)buf, CHUNK_SIZE) - 1] = '\0';
}


/*
 *   SHA-256 implementation, Mark 2
 *
 *   Copyright (c) 2010,2014 Ilya O. Levin, http://www.literatecode.com
 *
 *   Permission to use, copy, modify, and distribute this software for any
 *   purpose with or without fee is hereby granted, provided that the above
 *   copyright notice and this permission notice appear in all copies.
 *
 *   THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 *   WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 *   MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 *   ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 *   WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 *   ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 *   OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 *
 */ 

// MARK:- HMAC and SHA256 interface

#define HMAC_SHA256_DIGEST_SIZE CHUNK_SIZE
#define SHA256_DIGEST_SIZE CHUNK_SIZE

typedef struct {
	uint8_t  buf[64];
	uint32_t hash[8];
	uint32_t bits[2];
	uint32_t len;
} sha256_context;

void sha256_init(sha256_context *ctx);
void sha256_hash(sha256_context *ctx, const void *data, size_t len);
void sha256_done(sha256_context *ctx, uint8_t *hash);
 
#define FN_ inline static

static 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
};

#ifdef MINIMIZE_STACK_IMPACT
static uint32_t W[64];
#endif

/* -------------------------------------------------------------------------- */
FN_ uint8_t _shb(uint32_t x, uint32_t n)
{
	return ( (x >> (n & 31)) & 0xff );
} /* _shb */

/* -------------------------------------------------------------------------- */
FN_ uint32_t _shw(uint32_t x, uint32_t n)
{
	return ( (x << (n & 31)) & 0xffffffff );
} /* _shw */

/* -------------------------------------------------------------------------- */
FN_ uint32_t _r(uint32_t x, uint8_t n)
{
	return ( (x >> n) | _shw(x, 32 - n) );
} /* _r */

/* -------------------------------------------------------------------------- */
FN_ uint32_t _Ch(uint32_t x, uint32_t y, uint32_t z)
{
	return ( (x & y) ^ ((~x) & z) );
} /* _Ch */

/* -------------------------------------------------------------------------- */
FN_ uint32_t _Ma(uint32_t x, uint32_t y, uint32_t z)
{
	return ( (x & y) ^ (x & z) ^ (y & z) );
} /* _Ma */

/* -------------------------------------------------------------------------- */
FN_ uint32_t _S0(uint32_t x)
{
	return ( _r(x, 2) ^ _r(x, 13) ^ _r(x, 22) );
} /* _S0 */

/* -------------------------------------------------------------------------- */
FN_ uint32_t _S1(uint32_t x)
{
	return ( _r(x, 6) ^ _r(x, 11) ^ _r(x, 25) );
} /* _S1 */

/* -------------------------------------------------------------------------- */
FN_ uint32_t _G0(uint32_t x)
{
	return ( _r(x, 7) ^ _r(x, 18) ^ (x >> 3) );
} /* _G0 */

/* -------------------------------------------------------------------------- */
FN_ uint32_t _G1(uint32_t x)
{
	return ( _r(x, 17) ^ _r(x, 19) ^ (x >> 10) );
} /* _G1 */

/* -------------------------------------------------------------------------- */
FN_ uint32_t _word(uint8_t *c)
{
	return ( _shw(c[0], 24) | _shw(c[1], 16) | _shw(c[2], 8) | (c[3]) );
} /* _word */

/* -------------------------------------------------------------------------- */
FN_ void  _addbits(sha256_context *ctx, uint32_t n)
{
	if ( ctx->bits[0] > (0xffffffff - n) )
		ctx->bits[1] = (ctx->bits[1] + 1) & 0xFFFFFFFF;
	ctx->bits[0] = (ctx->bits[0] + n) & 0xFFFFFFFF;
} /* _addbits */

/* -------------------------------------------------------------------------- */
static void _hash(sha256_context *ctx)
{
	register uint32_t a, b, c, d, e, f, g, h, i;
	uint32_t t[2];
#ifndef MINIMIZE_STACK_IMPACT
	uint32_t W[64];
#endif

	a = ctx->hash[0];
	b = ctx->hash[1];
	c = ctx->hash[2];
	d = ctx->hash[3];
	e = ctx->hash[4];
	f = ctx->hash[5];
	g = ctx->hash[6];
	h = ctx->hash[7];

	for (i = 0; i < 64; i++) {
		if ( i < 16 )
			W[i] = _word(&ctx->buf[_shw(i, 2)]);
		else
			W[i] = _G1(W[i - 2]) + W[i - 7] + _G0(W[i - 15]) + W[i - 16];

		t[0] = h + _S1(e) + _Ch(e, f, g) + K[i] + W[i];
		t[1] = _S0(a) + _Ma(a, b, c);
		h = g;
		g = f;
		f = e;
		e = d + t[0];
		d = c;
		c = b;
		b = a;
		a = t[0] + t[1];
	}

	ctx->hash[0] += a;
	ctx->hash[1] += b;
	ctx->hash[2] += c;
	ctx->hash[3] += d;
	ctx->hash[4] += e;
	ctx->hash[5] += f;
	ctx->hash[6] += g;
	ctx->hash[7] += h;
} /* _hash */

/* -------------------------------------------------------------------------- */
void sha256_init(sha256_context *ctx)
{
	if ( ctx != NULL ) {
		ctx->bits[0]  = ctx->bits[1] = 0;
		ctx->len      = 0;
		ctx->hash[0] = 0x6a09e667;
		ctx->hash[1] = 0xbb67ae85;
		ctx->hash[2] = 0x3c6ef372;
		ctx->hash[3] = 0xa54ff53a;
		ctx->hash[4] = 0x510e527f;
		ctx->hash[5] = 0x9b05688c;
		ctx->hash[6] = 0x1f83d9ab;
		ctx->hash[7] = 0x5be0cd19;
	}
} /* sha256_init */

/* -------------------------------------------------------------------------- */
void sha256_hash(sha256_context *ctx, const void *data, size_t len)
{
	register size_t i;
	const uint8_t *bytes = (const uint8_t *)data;

	if ( (ctx != NULL) && (bytes != NULL) )
		for (i = 0; i < len; i++) {
			ctx->buf[ctx->len] = bytes[i];
			ctx->len++;
			if (ctx->len == sizeof(ctx->buf) ) {
				_hash(ctx);
				_addbits(ctx, sizeof(ctx->buf) * 8);
				ctx->len = 0;
			}
		}
} /* sha256_hash */

/* -------------------------------------------------------------------------- */
void sha256_done(sha256_context *ctx, uint8_t *hash)
{
	register uint32_t i, j;

	if ( ctx != NULL ) {
		j = ctx->len % sizeof(ctx->buf);
		ctx->buf[j] = 0x80;
		for (i = j + 1; i < sizeof(ctx->buf); i++)
			ctx->buf[i] = 0x00;

		if ( ctx->len > 55 ) {
			_hash(ctx);
			for (j = 0; j < sizeof(ctx->buf); j++)
				ctx->buf[j] = 0x00;
		}

		_addbits(ctx, ctx->len * 8);
		ctx->buf[63] = _shb(ctx->bits[0],  0);
		ctx->buf[62] = _shb(ctx->bits[0],  8);
		ctx->buf[61] = _shb(ctx->bits[0], 16);
		ctx->buf[60] = _shb(ctx->bits[0], 24);
		ctx->buf[59] = _shb(ctx->bits[1],  0);
		ctx->buf[58] = _shb(ctx->bits[1],  8);
		ctx->buf[57] = _shb(ctx->bits[1], 16);
		ctx->buf[56] = _shb(ctx->bits[1], 24);
		_hash(ctx);

		if ( hash != NULL )
			for (i = 0, j = 24; i < 4; i++, j -= 8) {
				hash[i     ] = _shb(ctx->hash[0], j);
				hash[i +  4] = _shb(ctx->hash[1], j);
				hash[i +  8] = _shb(ctx->hash[2], j);
				hash[i + 12] = _shb(ctx->hash[3], j);
				hash[i + 16] = _shb(ctx->hash[4], j);
				hash[i + 20] = _shb(ctx->hash[5], j);
				hash[i + 24] = _shb(ctx->hash[6], j);
				hash[i + 28] = _shb(ctx->hash[7], j);
			}
	}
} /* sha256_done */

/* -------------------------------------------------------------------------- */
void sha256(const void *data, size_t len, uint8_t *hash)
{
	sha256_context ctx;

	sha256_init(&ctx);
	sha256_hash(&ctx, data, len);
	sha256_done(&ctx, hash);
} /* sha256 */


/* ========================================================================== */


/*
 *   hmac-sha256.c
 *   Copyright (C) 2017 Adrian Perez <aperez@igalia.com>
 *
 *   Distributed under terms of the MIT license.
 */
 

typedef sha256_context sha256_t;
#define sha256_final(ctx, hash) sha256_done(ctx, hash)
#define sha256_update(ctx, data, len) sha256_hash(ctx, data, len)

#define B 64
#define L (SHA256_DIGEST_SIZE)
#define K (SHA256_DIGEST_SIZE * 2)

#define I_PAD 0x36
#define O_PAD 0x5C

void hmac_sha256 (uint8_t out[HMAC_SHA256_DIGEST_SIZE],
             const uint8_t *data, size_t data_len,
             const uint8_t *key, size_t key_len)
{
    // api_check_return (out);
//     api_check_return (data);
//     api_check_return (key);
//     api_check_return (key_len <= B);

    sha256_t ss;
    uint8_t kh[SHA256_DIGEST_SIZE];

    /*
     * If the key length is bigger than the buffer size B, apply the hash
     * function to it first and use the result instead.
     */
    if (key_len > B) {
        sha256_init (&ss);
        sha256_update (&ss, key, key_len);
        sha256_final (&ss, kh);
        key_len = SHA256_DIGEST_SIZE;
        key = kh;
    }

    /*
     * (1) append zeros to the end of K to create a B byte string
     *     (e.g., if K is of length 20 bytes and B=64, then K will be
     *     appended with 44 zero bytes 0x00)
     * (2) XOR (bitwise exclusive-OR) the B byte string computed in step
     *     (1) with ipad
     */
    uint8_t kx[B];
    for (size_t i = 0; i < key_len; i++) kx[i] = I_PAD ^ key[i];
    for (size_t i = key_len; i < B; i++) kx[i] = I_PAD ^ 0;

    /*
     * (3) append the stream of data 'text' to the B byte string resulting
     *     from step (2)
     * (4) apply H to the stream generated in step (3)
     */
    sha256_init (&ss);
    sha256_update (&ss, kx, B);
    sha256_update (&ss, data, data_len);
    sha256_final (&ss, out);

    /*
     * (5) XOR (bitwise exclusive-OR) the B byte string computed in
     *     step (1) with opad
     *
     * NOTE: The "kx" variable is reused.
     */
    for (size_t i = 0; i < key_len; i++) kx[i] = O_PAD ^ key[i];
    for (size_t i = key_len; i < B; i++) kx[i] = O_PAD ^ 0;

    /*
     * (6) append the H result from step (4) to the B byte string
     *     resulting from step (5)
     * (7) apply H to the stream generated in step (6) and output
     *     the result
     */
    sha256_init (&ss);
    sha256_update (&ss, kx, B);
    sha256_update (&ss, out, SHA256_DIGEST_SIZE);
    sha256_final (&ss, out);
}