aes implementation in c

aes.c

/*
spec: https://csrc.nist.gov/files/pubs/fips/197/final/docs/fips-197.pdf
author: Mattia Papaccioli
github: https://github.com/sbOogway
email: [email protected]

implementation of rijndael aka aes in c
written for learning purposes not meant for production
defects:
  - padding
  - only 128 bit key implementation
  - deciphering missing

Copyright (c) 2025 Mattia Papaccioli
Permission is hereby granted, free of charge, to any person obtaining a copy of
this software and associated documentation files (the "Software"), to deal in
the Software without restriction, including without limitation the rights to
use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
the Software, and to permit persons to whom the Software is furnished to do so,
subject to the following conditions: The above copyright notice and this
permission notice shall be included in all copies or substantial portions of the
Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO
EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
*/

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

// constants: section 2.2
#define Nk 4
#define Nb 4
#define Nr 10

// figure 14
const unsigned char INV_SBOX[] = {
    0x52, 0x09, 0x6a, 0xd5, 0x30, 0x36, 0xa5, 0x38, 0xbf, 0x40, 0xa3, 0x9e,
    0x81, 0xf3, 0xd7, 0xfb, 0x7c, 0xe3, 0x39, 0x82, 0x9b, 0x2f, 0xff, 0x87,
    0x34, 0x8e, 0x43, 0x44, 0xc4, 0xde, 0xe9, 0xcb, 0x54, 0x7b, 0x94, 0x32,
    0xa6, 0xc2, 0x23, 0x3d, 0xee, 0x4c, 0x95, 0x0b, 0x42, 0xfa, 0xc3, 0x4e,
    0x08, 0x2e, 0xa1, 0x66, 0x28, 0xd9, 0x24, 0xb2, 0x76, 0x5b, 0xa2, 0x49,
    0x6d, 0x8b, 0xd1, 0x25, 0x72, 0xf8, 0xf6, 0x64, 0x86, 0x68, 0x98, 0x16,
    0xd4, 0xa4, 0x5c, 0xcc, 0x5d, 0x65, 0xb6, 0x92, 0x6c, 0x70, 0x48, 0x50,
    0xfd, 0xed, 0xb9, 0xda, 0x5e, 0x15, 0x46, 0x57, 0xa7, 0x8d, 0x9d, 0x84,
    0x90, 0xd8, 0xab, 0x00, 0x8c, 0xbc, 0xd3, 0x0a, 0xf7, 0xe4, 0x58, 0x05,
    0xb8, 0xb3, 0x45, 0x06, 0xd0, 0x2c, 0x1e, 0x8f, 0xca, 0x3f, 0x0f, 0x02,
    0xc1, 0xaf, 0xbd, 0x03, 0x01, 0x13, 0x8a, 0x6b, 0x3a, 0x91, 0x11, 0x41,
    0x4f, 0x67, 0xdc, 0xea, 0x97, 0xf2, 0xcf, 0xce, 0xf0, 0xb4, 0xe6, 0x73,
    0x96, 0xac, 0x74, 0x22, 0xe7, 0xad, 0x35, 0x85, 0xe2, 0xf9, 0x37, 0xe8,
    0x1c, 0x75, 0xdf, 0x6e, 0x47, 0xf1, 0x1a, 0x71, 0x1d, 0x29, 0xc5, 0x89,
    0x6f, 0xb7, 0x62, 0x0e, 0xaa, 0x18, 0xbe, 0x1b, 0xfc, 0x56, 0x3e, 0x4b,
    0xc6, 0xd2, 0x79, 0x20, 0x9a, 0xdb, 0xc0, 0xfe, 0x78, 0xcd, 0x5a, 0xf4,
    0x1f, 0xdd, 0xa8, 0x33, 0x88, 0x07, 0xc7, 0x31, 0xb1, 0x12, 0x10, 0x59,
    0x27, 0x80, 0xec, 0x5f, 0x60, 0x51, 0x7f, 0xa9, 0x19, 0xb5, 0x4a, 0x0d,
    0x2d, 0xe5, 0x7a, 0x9f, 0x93, 0xc9, 0x9c, 0xef, 0xa0, 0xe0, 0x3b, 0x4d,
    0xae, 0x2a, 0xf5, 0xb0, 0xc8, 0xeb, 0xbb, 0x3c, 0x83, 0x53, 0x99, 0x61,
    0x17, 0x2b, 0x04, 0x7e, 0xba, 0x77, 0xd6, 0x26, 0xe1, 0x69, 0x14, 0x63,
    0x55, 0x21, 0x0c, 0x7d};

// figure 7
const unsigned char SBOX[] = {
    0x63, 0x7c, 0x77, 0x7b, 0xf2, 0x6b, 0x6f, 0xc5, 0x30, 0x01, 0x67, 0x2b,
    0xfe, 0xd7, 0xab, 0x76, 0xca, 0x82, 0xc9, 0x7d, 0xfa, 0x59, 0x47, 0xf0,
    0xad, 0xd4, 0xa2, 0xaf, 0x9c, 0xa4, 0x72, 0xc0, 0xb7, 0xfd, 0x93, 0x26,
    0x36, 0x3f, 0xf7, 0xcc, 0x34, 0xa5, 0xe5, 0xf1, 0x71, 0xd8, 0x31, 0x15,
    0x04, 0xc7, 0x23, 0xc3, 0x18, 0x96, 0x05, 0x9a, 0x07, 0x12, 0x80, 0xe2,
    0xeb, 0x27, 0xb2, 0x75, 0x09, 0x83, 0x2c, 0x1a, 0x1b, 0x6e, 0x5a, 0xa0,
    0x52, 0x3b, 0xd6, 0xb3, 0x29, 0xe3, 0x2f, 0x84, 0x53, 0xd1, 0x00, 0xed,
    0x20, 0xfc, 0xb1, 0x5b, 0x6a, 0xcb, 0xbe, 0x39, 0x4a, 0x4c, 0x58, 0xcf,
    0xd0, 0xef, 0xaa, 0xfb, 0x43, 0x4d, 0x33, 0x85, 0x45, 0xf9, 0x02, 0x7f,
    0x50, 0x3c, 0x9f, 0xa8, 0x51, 0xa3, 0x40, 0x8f, 0x92, 0x9d, 0x38, 0xf5,
    0xbc, 0xb6, 0xda, 0x21, 0x10, 0xff, 0xf3, 0xd2, 0xcd, 0x0c, 0x13, 0xec,
    0x5f, 0x97, 0x44, 0x17, 0xc4, 0xa7, 0x7e, 0x3d, 0x64, 0x5d, 0x19, 0x73,
    0x60, 0x81, 0x4f, 0xdc, 0x22, 0x2a, 0x90, 0x88, 0x46, 0xee, 0xb8, 0x14,
    0xde, 0x5e, 0x0b, 0xdb, 0xe0, 0x32, 0x3a, 0x0a, 0x49, 0x06, 0x24, 0x5c,
    0xc2, 0xd3, 0xac, 0x62, 0x91, 0x95, 0xe4, 0x79, 0xe7, 0xc8, 0x37, 0x6d,
    0x8d, 0xd5, 0x4e, 0xa9, 0x6c, 0x56, 0xf4, 0xea, 0x65, 0x7a, 0xae, 0x08,
    0xba, 0x78, 0x25, 0x2e, 0x1c, 0xa6, 0xb4, 0xc6, 0xe8, 0xdd, 0x74, 0x1f,
    0x4b, 0xbd, 0x8b, 0x8a, 0x70, 0x3e, 0xb5, 0x66, 0x48, 0x03, 0xf6, 0x0e,
    0x61, 0x35, 0x57, 0xb9, 0x86, 0xc1, 0x1d, 0x9e, 0xe1, 0xf8, 0x98, 0x11,
    0x69, 0xd9, 0x8e, 0x94, 0x9b, 0x1e, 0x87, 0xe9, 0xce, 0x55, 0x28, 0xdf,
    0x8c, 0xa1, 0x89, 0x0d, 0xbf, 0xe6, 0x42, 0x68, 0x41, 0x99, 0x2d, 0x0f,
    0xb0, 0x54, 0xbb, 0x16};

const unsigned char RCON_128[] = {
    0x00, 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80, 0x1b, 0x36,
};

/*
free this shit up
this works only for aes 128
*/
unsigned char *getRcon(int index) {
  unsigned char *result = (unsigned char *)malloc(4);
  // unsigned char * result;
  result[0] = RCON_128[index];
  result[1] = RCON_128[0];
  result[2] = RCON_128[0];
  result[3] = RCON_128[0];
  return result;
}

/*
https://www.geeksforgeeks.org/length-of-array-in-c/

breakdown of the len macro

&arr gives the address of the entire array.
&arr + 1 moves the pointer to the address just after the end of
the array arr.
*(&arr + 1) dereferences this pointer to get the memory location
just after the array.
Subtracting the base address of the array (arr) from this value gives
the total number of elements in the array, as pointer arithmetic works
based on the size of the elements (e.g., int in this case).

Note: Please note that these methods only works when the array is declared
in the same scope. These methods will fail if we try them on an array which
is passed as a pointer. This happens due to Array Decay. So, it is recommended
to keep a variable that tracks the size of the array.
*/
#define len(e) (*(&e + 1) - e)

#define print(e) (printf("%s\n", e))

#define NL (printf("\n"))

// section 5.2
#define RotWord(word)                                                          \
  {                                                                            \
    int word_len = len(word);                                                  \
    unsigned char tmp = *(word);                                               \
    for (int i = 0; i < word_len - 1; i++) {                                   \
      *(word + i) = *(word + i + 1);                                           \
    }                                                                          \
    *(word + word_len - 1) = tmp;                                              \
    tmp = '\0';                                                                \
  }

// section 5.2
#define SubWord(word)                                                          \
  {                                                                            \
    assert(len(word) == 4 /*word must be 4 bytes long*/);                      \
    applySbox(SBOX, word, len(word));                                          \
  }

/*
rotate tha shit

// */
// void rotWord(unsigned char *word, int len) {
//   unsigned char tmp = *(word);
//   for (int i = 0; i < len - 1; i++) {
//     *(word + i) = *(word + i + 1);
//   }
//   *(word + len - 1) = tmp;
//   tmp = '\0';
// }

/*
result is stored in w1
*/
void xor
    (unsigned char *w1, unsigned char *w2, int start, int len) {
      for (int i = 0; i < len; i++) {
        w1[i] ^= w2[start + i];
      }
    }

    int translate_index(unsigned char byte) {
  return (byte >> 0x4) * 0x10 + 0xf & byte;
}

void applySbox(const unsigned char *sbox, unsigned char *word, int word_len) {
  for (int i = 0; i < word_len; i++) {
    word[i] = sbox[translate_index(word[i])];
  }
}

void printx(unsigned char *w, int l) {
  for (int i = 0; i < l; i += 4) {
    printf("%02d - %#04x %#04x %#04x %#04x\n", i / 4, w[i], w[i + 1], w[i + 2],
           w[i + 3]);
  }
}

void printv(unsigned char *w) { printf("%s\n", w); }

// figure 11
void KeyExpansion(unsigned char *key, unsigned char *word, int key_length) {
  unsigned char tmp1[] = {0x0, 0x0, 0x0, 0x0};
  int i = 0;

  while (i < Nk * 4) {
    word[i] = key[i];
    word[i + 1] = key[i + 1];
    word[i + 2] = key[i + 2];
    word[i + 3] = key[i + 3];
    i += 4;
  }

  for (int i = 0; i < Nk * 4; i++) {
    word[i] = key[i];
  }

  i = Nk * 4;
  unsigned char *rcon;
  for (; i < key_length; i += 4) {
    *(tmp1) = word[i - 4];
    *(tmp1 + 1) = word[i - 3];
    *(tmp1 + 2) = word[i - 2];
    *(tmp1 + 3) = word[i - 1];

    if ((i / 4) % Nk == 0) {
      RotWord(tmp1);
      SubWord(tmp1);
      rcon = getRcon(i / 4 / Nk);
      xor(tmp1, rcon, 0, 4);
      free(rcon);
    } else if (Nk > 6 && (i / 4) % Nk == 4) {
      SubWord(tmp1);
    }
    word[i] = word[i - 4 - 12] ^ tmp1[0];
    word[i + 1] = word[i - 3 - 12] ^ tmp1[1];
    word[i + 2] = word[i - 2 - 12] ^ tmp1[2];
    word[i + 3] = word[i - 1 - 12] ^ tmp1[3];
  }
  // free(rcon);
}

void SubBytes(unsigned char *buffer, int l) {
  for (int i = 0; i < l; i++) {
    buffer[i] = SBOX[translate_index(buffer[i])];
  }
}

void swap(unsigned char *b, int x1, int x2) {
  unsigned char tmp = b[x1];
  b[x1] = b[x2];
  b[x2] = tmp;
}

/* this is very inefficient? */
void ShiftRows(unsigned char *buffer) {
  swap(buffer, 0x01, 0x05);
  swap(buffer, 0x05, 0x09);
  swap(buffer, 0x09, 0x0d);

  swap(buffer, 0x02, 0x0a);
  swap(buffer, 0x0e, 0x06);

  swap(buffer, 0x03, 0x0f);
  swap(buffer, 0x07, 0x0f);
  swap(buffer, 0x0f, 0x0b);
}

#define POLY 0x011B // x^8 + x^4 + x^3 + x + 1

unsigned char gf_mult(unsigned char a, unsigned char b) {
  unsigned char result = 0;
  unsigned char temp = 0;

  while (b) {
    if (b & 1) {
      result ^= a; // Add (XOR) a to result
    }
    b >>= 1;         // Divide b by 2
    temp = a & 0x80; // Check if the highest bit of a is set
    a <<= 1;         // Multiply a by x (shift left)
    if (temp) {
      a ^= POLY; // If the highest bit was set, reduce modulo the polynomial
    }
  }
  return result;
}

void MixColumns(unsigned char *buffer) {
  char tmp[] = {0x00, 0x00, 0x00, 0x00};
  for (int i = 0; i < 4; i++) {
    tmp[0] = buffer[i * 4];
    tmp[1] = buffer[i * 4 + 1];
    tmp[2] = buffer[i * 4 + 2];
    tmp[3] = buffer[i * 4 + 3];

    buffer[i * 4] =
        gf_mult(0x02, tmp[0]) ^ gf_mult(0x03, tmp[1]) ^ tmp[2] ^ tmp[3];
    buffer[i * 4 + 1] =
        gf_mult(0x02, tmp[1]) ^ gf_mult(0x03, tmp[2]) ^ tmp[3] ^ tmp[0];
    buffer[i * 4 + 2] =
        gf_mult(0x02, tmp[2]) ^ gf_mult(0x03, tmp[3]) ^ tmp[0] ^ tmp[1];
    buffer[i * 4 + 3] =
        gf_mult(0x02, tmp[3]) ^ gf_mult(0x03, tmp[0]) ^ tmp[1] ^ tmp[2];
  }
}

/*
key is expanded in here
*/
void cipher(unsigned char *in, unsigned char *out, unsigned char *key) {
  int len = Nb * (Nr + 1) * 4;
  unsigned char *state;
  unsigned char *expandedKeys; // = (unsigned char *)malloc(len);
  KeyExpansion(key, expandedKeys, len);

  state = in;

  xor(state, expandedKeys, 0, 16);
  // printx(state, 16);
  // NL;

  for (int i = 1; i < Nr; i++) {
    // printf("round -> %d\n", i);
    SubBytes(state, 16);
    // printx(state, 16);
    ShiftRows(state);
    // NL;
    // printx(state, 16);
    // NL;

    MixColumns(state);
    // printx(state, 16);
    // NL;

    xor(state, expandedKeys, i * 16, 16);

    // printx(state, 16);
    // NL;
    // exit(10);
  }

  SubBytes(state, 16);
  ShiftRows(state);
  xor(state, expandedKeys, 160, 16);
  // printx(state, 16);

  // printf("dbg\n");
  memcpy(out, state, 16);
  // out = state;
  // printx(state, 16);
  // NL;

  // free(expandedKeys);
}

int main() {
  // unsigned char key[] = {0x2b, 0x7e, 0x15, 0x16, 0x28, 0xae, 0xd2, 0xa6,
  //                        0xab, 0xf7, 0x15, 0x88, 0x09, 0xcf, 0x4f, 0x3c};
  unsigned char key[] = "aaaaaaaaaaaaaaaa";
  unsigned char input[] = "aaaaaaaaaaaaaaaa";

  // unsigned char input[] = {
  //     0x32, 0x43, 0xf6, 0xa8, 0x88, 0x5a, 0x30, 0x8d,
  //     0x31, 0x31, 0x98, 0xa2, 0xe0, 0x37, 0x07, 0x34,
  // };

  unsigned char PLAINTEXT[] = {
      0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77,
      0x88, 0x99, 0xaa, 0xbb, 0xcc, 0xdd, 0xee, 0xff,
  };
  unsigned char KEY[] = {
      0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
      0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
  };

  unsigned char *out = (unsigned char *)malloc(16);
  cipher(input, out, key);

  // printf("dbg\n");

  printx(out, 16);
  // printv(out);

  printf("%s\n", out);
  free(out);
  return EXIT_SUCCESS;
}