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Problems with AES key schedule

encryption c++ aes

Best Answer mebob, 20 November 2013 - 11:08 AM

I solved my problem by rewriting my function as so:

void Expand(const unsigned char input[16], unsigned char output[16*11])
{
    unsigned int i;
    unsigned int j;
    unsigned char t[4];;
    for(i = 0; i < 16*11; i++) output[i] = input[i];
    for(i = 1; i < 11; i++)
    {
        j = i*16;

        t[0] = sbox[output[j-3]] ^ rcon[i];
        t[1] = sbox[output[j-2]];
        t[2] = sbox[output[j-1]];
        t[3] = sbox[output[j-4]];

        /*output[j] = output[j-16] ^ t[0];
        output[j+1] = output[j-15] ^ t[1];
        output[j+2] = output[j-14] ^ t[2];
        output[j+3] = output[j-13] ^ t[3];*/
        *(unsigned int*) (output + j) = *(unsigned int*)  (output + j - 16) ^ *(unsigned int*) t;

        /*output[j+4] = output[j-12] ^ output[j];
        output[j+5] = output[j-11] ^ output[j+1];
        output[j+6] = output[j-10] ^ output[j+2];
        output[j+7] = output[j-9] ^ output[j+3];*/
        *(unsigned int*) (output + j + 4) = *(unsigned int*) (output + j - 12) ^ *(unsigned int*) (output + j);

        /*output[j+8] = output[j-8] ^ output[j+4];
        output[j+9] = output[j-7] ^ output[j+5];
        output[j+10] = output[j-6] ^ output[j+6];
        output[j+11] = output[j-5] ^ output[j+7];*/
        *(unsigned int*) (output + j + 8) = *(unsigned int*) (output + j - 8) ^ *(unsigned int*) (output + j + 4);

        /*output[j+12] = output[j-4] ^ output[j+8];
        output[j+13] = output[j-3] ^ output[j+9];
        output[j+14] = output[j-2] ^ output[j+10];
        output[j+15] = output[j-1] ^ output[j+11];*/
        *(unsigned int*) (output + j + 12) = *(unsigned int*) (output + j - 4) ^ *(unsigned int*) (output + j + 8);
    }
}

where each block of commented code is equivalent in effect to the code just below it. I chose to do those operations by casting to unsigned int pointers because I figured that accessing word-sized pieces of data would be faster than accessing the array byte-by-byte.

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#1 mebob

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Posted 14 November 2013 - 06:17 PM

Hi, I'm currently writing an implementation of the AES encryption algorithm, however I'm running into some issues with the key expansion portion of it. I have this code:

 

KeySchedule.cpp

#include "KeySchedule.h"
#include "AESConstants.h"
#include <iostream>
using namespace std;

void Expand(const unsigned char input[16], unsigned char output[16*11])
{
    unsigned int i;
    unsigned int j;
    unsigned int t;
    for(i = 0; i < 16; i++)
    {
        output[i] = input[i];
    }
    for(i = 1; i < 11; i++)
    {
        j = i*16;
        cout << std::dec << i << ", " << j << endl;
        t = *(unsigned int*)(output + j - sizeof(unsigned int));
        
        //Key schedule core

        cout << std::hex << "0x" << t << " ";
        t = (t >> 8) | (t << 24);
        cout << "0x" << t << " ";
        ((unsigned char*) &t)[0] = sbox[((unsigned char*) &t)[0]];
        ((unsigned char*) &t)[1] = sbox[((unsigned char*) &t)[1]];
        ((unsigned char*) &t)[2] = sbox[((unsigned char*) &t)[2]];
        ((unsigned char*) &t)[3] = sbox[((unsigned char*) &t)[3]];
        ((unsigned char*) &t)[0] ^= rcon[i];

        cout << "0x" << t << " ";
        
        //First column of current round key
        t ^= *(unsigned int*)(output + j - 4*sizeof(unsigned int));
        *(unsigned int*)(output + j) = t;

        cout << "0x" << t << " ";
        
        //Second column of current round key
        t ^= *(unsigned int*)(output + j - 3*sizeof(unsigned int));
        *(unsigned int*)(output + j + 1*sizeof(unsigned int)) = t;

        cout << "0x" << t << " ";
        
        //Third column of current round key
        t ^= *((unsigned int*)output + j - 2*sizeof(unsigned int));
        *(unsigned int*)(output + j + 2*sizeof(unsigned int)) = t;

        cout << "0x" << t << " ";

        //Fourth column of current round key
        t ^= *(unsigned int*)(output + j - 1*sizeof(unsigned int));
        *(unsigned int*)(output + j + 3*sizeof(unsigned int)) = t;

        cout << "0x" << t << endl;
    }
}

AESConstants.h

unsigned char rcon[11] =
{
    0x8D, 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80
};

unsigned char sbox[256] =
{
   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
};

main.cpp

#include <iostream>
#include "KeySchedule.h"
using namespace std;

int main()
{
    unsigned char testvector[16] = {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0};
    unsigned char expanded[176];
    Expand(testvector, expanded);

    cout << sizeof(unsigned int) << endl;
    cout << std::hex;
    for(int i = 0; i < 176; i++)
    {
        cout << (unsigned int)expanded[i] << " ";
    }

}

The expanded key outputted by Expand() is correct for the first 8 characters, but then diverges from there...and I'm utterly confused.

 

The source for the "test vector" key: http://www.samiam.or...y-schedule.html

 

I'm probably just glancing over something obvious  :)  Thank you in advance for any help


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#2 mebob

mebob

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Posted 20 November 2013 - 11:08 AM   Best Answer

I solved my problem by rewriting my function as so:

void Expand(const unsigned char input[16], unsigned char output[16*11])
{
    unsigned int i;
    unsigned int j;
    unsigned char t[4];;
    for(i = 0; i < 16*11; i++) output[i] = input[i];
    for(i = 1; i < 11; i++)
    {
        j = i*16;

        t[0] = sbox[output[j-3]] ^ rcon[i];
        t[1] = sbox[output[j-2]];
        t[2] = sbox[output[j-1]];
        t[3] = sbox[output[j-4]];

        /*output[j] = output[j-16] ^ t[0];
        output[j+1] = output[j-15] ^ t[1];
        output[j+2] = output[j-14] ^ t[2];
        output[j+3] = output[j-13] ^ t[3];*/
        *(unsigned int*) (output + j) = *(unsigned int*)  (output + j - 16) ^ *(unsigned int*) t;

        /*output[j+4] = output[j-12] ^ output[j];
        output[j+5] = output[j-11] ^ output[j+1];
        output[j+6] = output[j-10] ^ output[j+2];
        output[j+7] = output[j-9] ^ output[j+3];*/
        *(unsigned int*) (output + j + 4) = *(unsigned int*) (output + j - 12) ^ *(unsigned int*) (output + j);

        /*output[j+8] = output[j-8] ^ output[j+4];
        output[j+9] = output[j-7] ^ output[j+5];
        output[j+10] = output[j-6] ^ output[j+6];
        output[j+11] = output[j-5] ^ output[j+7];*/
        *(unsigned int*) (output + j + 8) = *(unsigned int*) (output + j - 8) ^ *(unsigned int*) (output + j + 4);

        /*output[j+12] = output[j-4] ^ output[j+8];
        output[j+13] = output[j-3] ^ output[j+9];
        output[j+14] = output[j-2] ^ output[j+10];
        output[j+15] = output[j-1] ^ output[j+11];*/
        *(unsigned int*) (output + j + 12) = *(unsigned int*) (output + j - 4) ^ *(unsigned int*) (output + j + 8);
    }
}

where each block of commented code is equivalent in effect to the code just below it. I chose to do those operations by casting to unsigned int pointers because I figured that accessing word-sized pieces of data would be faster than accessing the array byte-by-byte.


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