AES Implementation Code in C language | AES code in C | AES implmentation | AES Program

#include <stdlib.h>

#include <string.h>

#include <stdio.h>

 

#define AES_RPOL      0x011b // reduction polynomial (x^8 + x^4 + x^3 + x + 1)

#define AES_GEN    0x03   // gf(2^8) generator  (x + 1)

#define AES_SBOX_CC   0x63   // S-Box C constant

 

#define KEY_128 (128/8)

#define KEY_192 (192/8)

#define KEY_256 (256/8)

 

#define aes_mul(a, b) ((a)&&(b)?g_aes_ilogt[(g_aes_logt[(a)]+g_aes_logt[(b)])%0xff]:0)

#define aes_inv(a)  ((a)?g_aes_ilogt[0xff-g_aes_logt[(a)]]:0)

 

unsigned char g_aes_logt[256], g_aes_ilogt[256];

unsigned char g_aes_sbox[256], g_aes_isbox[256];

 

typedef struct {

    unsigned char state[4][4];

    int kcol;

    size_t rounds;

    unsigned long keysched[0];

} aes_ctx_t;

 

void aes_init();

aes_ctx_t *aes_alloc_ctx(unsigned char *key, size_t keyLen);

inline unsigned long aes_subword(unsigned long w);

inline unsigned long aes_rotword(unsigned long w);

void aes_keyexpansion(aes_ctx_t *ctx);

 

inline unsigned char aes_mul_manual(unsigned char a, unsigned char b); // use aes_mul instead

 

void aes_subbytes(aes_ctx_t *ctx);

void aes_shiftrows(aes_ctx_t *ctx);

void aes_mixcolumns(aes_ctx_t *ctx);

void aes_addroundkey(aes_ctx_t *ctx, int round);

void aes_encrypt(aes_ctx_t *ctx, unsigned char input[16], unsigned char output[16]);

 

void aes_invsubbytes(aes_ctx_t *ctx);

void aes_invshiftrows(aes_ctx_t *ctx);

void aes_invmixcolumns(aes_ctx_t *ctx);

void aes_decrypt(aes_ctx_t *ctx, unsigned char input[16], unsigned char output[16]);

 

void aes_free_ctx(aes_ctx_t *ctx);

 

void init_aes()

{

    int i;

    unsigned char gen;

 

    // build logarithm table and it's inverse

    gen = 1;

    for(i = 0; i < 0xff; i++) {

        g_aes_logt[gen]  = i;

        g_aes_ilogt[i]   = gen;

        gen = aes_mul_manual(gen, AES_GEN);

    }

 

    // build S-Box and it's inverse

    for(i = 0; i <= 0xff; i++) {

        char bi;

        unsigned char inv = aes_inv(i);

 

        g_aes_sbox[i] = 0;

        for(bi = 0; bi < 8; bi++) {

            // based on transformation 5.1

            // could also be done with a loop based on the matrix

            g_aes_sbox[i] |= ((inv & (1<<bi)?1:0)

                        ^ (inv & (1 << ((bi+4) & 7))?1:0)

                        ^ (inv & (1 << ((bi+5) & 7))?1:0)

                        ^ (inv & (1 << ((bi+6) & 7))?1:0)

                        ^ (inv & (1 << ((bi+7) & 7))?1:0)

                        ^ (AES_SBOX_CC & (1 << bi)?1:0)

            ) << bi;

        }

        g_aes_isbox[g_aes_sbox[i]] = i;

    }

    // warning: quickhack

    g_aes_sbox[1] = 0x7c;

    g_aes_isbox[0x7c] = 1;

    g_aes_isbox[0x63] = 0;

}

 

aes_ctx_t *aes_alloc_ctx(unsigned char *key, size_t keyLen)

{

    aes_ctx_t *ctx;

    size_t rounds;

    size_t ks_size;

 

    switch(keyLen) {

        case 16: // 128-bit key

            rounds = 10;

            break;

 

        case 24: // 192-bit key

            rounds = 12;

            break;

 

        case 32: // 256-bit key

            rounds = 14;

            break;

 

        defaut:

            return NULL;

    }

 

    ks_size = 4*(rounds+1)*sizeof(unsigned long);

    ctx = malloc(sizeof(aes_ctx_t)+ks_size);

    if(ctx) {

        ctx->rounds = rounds;

        ctx->kcol = keyLen/4;

        memcpy(ctx->keysched, key, keyLen);

        ctx->keysched[43] = 0;

        aes_keyexpansion(ctx);

    }

 

    return ctx;

}

 

inline unsigned long aes_subword(unsigned long w)

{

    return g_aes_sbox[w & 0x000000ff] |

        (g_aes_sbox[(w & 0x0000ff00) >> 8] << 8) |

        (g_aes_sbox[(w & 0x00ff0000) >> 16] << 16) |

        (g_aes_sbox[(w & 0xff000000) >> 24] << 24);

}

 

inline unsigned long aes_rotword(unsigned long w)

{

    // May seem a bit different from the spec

    // It was changed because unsigned long is represented with little-endian convention on x86

    // Should not depend on architecture, but this is only a POC

    return ((w & 0x000000ff) << 24) |

        ((w & 0x0000ff00) >> 8) |

        ((w & 0x00ff0000) >> 8) |

        ((w & 0xff000000) >> 8);

}

 

void aes_keyexpansion(aes_ctx_t *ctx)

{

    unsigned long temp;

    unsigned long rcon;

    register int i;

 

    rcon = 0x00000001;

    for(i = ctx->kcol; i < (4*(ctx->rounds+1)); i++) {

        temp = ctx->keysched[i-1];

        if(!(i%ctx->kcol)) {

            temp = aes_subword(aes_rotword(temp)) ^ rcon;

            rcon = aes_mul(rcon, 2);

        } else if(ctx->kcol > 6 && i%ctx->kcol == 4)

            temp = aes_subword(temp);

        ctx->keysched[i] = ctx->keysched[i-ctx->kcol] ^ temp;

    }

}

 

inline unsigned char aes_mul_manual(unsigned char a, unsigned char b)

{

    register unsigned short ac;

    register unsigned char ret;

 

    ac = a;

    ret = 0;

    while(b) {

        if(b & 0x01)

            ret ^= ac;

        ac <<= 1;

        b >>= 1;

        if(ac & 0x0100)

            ac ^= AES_RPOL;

    }

 

    return ret;

}

 

void aes_subbytes(aes_ctx_t *ctx)

{

    int i;

 

    for(i = 0; i < 16; i++) {

        int x, y;

 

        x = i & 0x03;

        y = i >> 2;

        ctx->state[x][y] = g_aes_sbox[ctx->state[x][y]];

    }

}

 

void aes_shiftrows(aes_ctx_t *ctx)

{

    unsigned char nstate[4][4];

    int i;

 

    for(i = 0; i < 16; i++) {

        int x, y;

 

        x = i & 0x03;

        y = i >> 2;

        nstate[x][y] = ctx->state[x][(y+x) & 0x03];

    }

 

    memcpy(ctx->state, nstate, sizeof(ctx->state));

}

 

void aes_mixcolumns(aes_ctx_t *ctx)

{

    unsigned char nstate[4][4];

    int i;

     

    for(i = 0; i < 4; i++) {

        nstate[0][i] = aes_mul(0x02, ctx->state[0][i]) ^

                aes_mul(0x03, ctx->state[1][i]) ^

                ctx->state[2][i] ^

                ctx->state[3][i];

        nstate[1][i] = ctx->state[0][i] ^

                aes_mul(0x02, ctx->state[1][i]) ^

                aes_mul(0x03, ctx->state[2][i]) ^

                ctx->state[3][i];

        nstate[2][i] = ctx->state[0][i] ^

                ctx->state[1][i] ^

                aes_mul(0x02, ctx->state[2][i]) ^

                aes_mul(0x03, ctx->state[3][i]);

        nstate[3][i] = aes_mul(0x03, ctx->state[0][i]) ^

                ctx->state[1][i] ^

                ctx->state[2][i] ^

                aes_mul(0x02, ctx->state[3][i]);

    }

 

    memcpy(ctx->state, nstate, sizeof(ctx->state));

}

 

void aes_addroundkey(aes_ctx_t *ctx, int round)

{

    int i;

 

    for(i = 0; i < 16; i++) {

        int x, y;

 

        x = i & 0x03;

        y = i >> 2;

        ctx->state[x][y] = ctx->state[x][y] ^

            ((ctx->keysched[round*4+y] & (0xff << (x*8))) >> (x*8));

    }

}

 

void aes_encrypt(aes_ctx_t *ctx, unsigned char input[16], unsigned char output[16])

{

    int i;

 

    // copy input to state

    for(i = 0; i < 16; i++)

        ctx->state[i & 0x03][i >> 2] = input[i];

 

    aes_addroundkey(ctx, 0);

 

    for(i = 1; i < ctx->rounds; i++) {

        aes_subbytes(ctx);

        aes_shiftrows(ctx);

        aes_mixcolumns(ctx);

        aes_addroundkey(ctx, i);

    }

 

    aes_subbytes(ctx);

    aes_shiftrows(ctx);

    aes_addroundkey(ctx, ctx->rounds);

 

    // copy state to output

    for(i = 0; i < 16; i++)

        output[i] = ctx->state[i & 0x03][i >> 2];

}

 

void aes_invshiftrows(aes_ctx_t *ctx)

{

    unsigned char nstate[4][4];

    int i;

 

    for(i = 0; i < 16; i++) {

        int x, y;

 

        x = i & 0x03;

        y = i >> 2;

        nstate[x][(y+x) & 0x03] = ctx->state[x][y];

    }

 

    memcpy(ctx->state, nstate, sizeof(ctx->state));

}

 

void aes_invsubbytes(aes_ctx_t *ctx)

{

    int i;

 

    for(i = 0; i < 16; i++) {

        int x, y;

 

        x = i & 0x03;

        y = i >> 2;

        ctx->state[x][y] = g_aes_isbox[ctx->state[x][y]];

    }

}

 

void aes_invmixcolumns(aes_ctx_t *ctx)

{

    unsigned char nstate[4][4];

    int i;

     

    for(i = 0; i < 4; i++) {

        nstate[0][i] = aes_mul(0x0e, ctx->state[0][i]) ^

                aes_mul(0x0b, ctx->state[1][i]) ^

                aes_mul(0x0d, ctx->state[2][i]) ^

                aes_mul(0x09, ctx->state[3][i]);

        nstate[1][i] = aes_mul(0x09, ctx->state[0][i]) ^

                aes_mul(0x0e, ctx->state[1][i]) ^

                aes_mul(0x0b, ctx->state[2][i]) ^

                aes_mul(0x0d, ctx->state[3][i]);

        nstate[2][i] = aes_mul(0x0d, ctx->state[0][i]) ^

                aes_mul(0x09, ctx->state[1][i]) ^

                aes_mul(0x0e, ctx->state[2][i]) ^

                aes_mul(0x0b, ctx->state[3][i]);

        nstate[3][i] = aes_mul(0x0b, ctx->state[0][i]) ^

                aes_mul(0x0d, ctx->state[1][i]) ^

                aes_mul(0x09, ctx->state[2][i]) ^

                aes_mul(0x0e, ctx->state[3][i]);

    }

 

    memcpy(ctx->state, nstate, sizeof(ctx->state));

}

 

void aes_decrypt(aes_ctx_t *ctx, unsigned char input[16], unsigned char output[16])

{

    int i, j;

 

    // copy input to state

    for(i = 0; i < 16; i++)

        ctx->state[i & 0x03][i >> 2] = input[i];

 

    aes_addroundkey(ctx, ctx->rounds);

    for(i = ctx->rounds-1; i >= 1; i--) {

        aes_invshiftrows(ctx);

        aes_invsubbytes(ctx);

        aes_addroundkey(ctx, i);

        aes_invmixcolumns(ctx);

    }

 

    aes_invshiftrows(ctx);

    aes_invsubbytes(ctx);

    aes_addroundkey(ctx, 0);

 

    // copy state to output

    for(i = 0; i < 16; i++)

        output[i] = ctx->state[i & 0x03][i >> 2];

}

 

void aes_free_ctx(aes_ctx_t *ctx)

{

    free(ctx);

}

 

int main(int argc, char *argv[])

{

    unsigned char key[KEY_128] = "uber strong key!";

    unsigned char ptext[16] = "Attack at dawn!";

    unsigned char ctext[16];

    unsigned char decptext[16];

    aes_ctx_t *ctx;

 

    init_aes();

    ctx = aes_alloc_ctx(key, sizeof(key));

    if(!ctx) {

        perror("aes_alloc_ctx");

        return EXIT_FAILURE;

    }

    aes_encrypt(ctx, ptext, ctext);

    aes_decrypt(ctx, ctext, decptext);

    puts(decptext);

 

    aes_free_ctx(ctx);

    return EXIT_SUCCESS;

}




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