// Thanks to http://www.samiam.org/key-schedule.html
#include <stdint.h>

static const uint8_t sbox[256] =   {
  //0     1    2      3     4    5     6     7      8    9     A      B    C     D     E     F
  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 };

static void rotate(unsigned char *in) {
    unsigned char a,c;
    a = in[0];
    for(c=0;c<3;c++) 
        in[c] = in[c + 1];
    in[3] = a;
    return;
}

/* Calculate the rcon used in key expansion */
static unsigned char rcon(unsigned char in) {
    unsigned char c=1;
    if(in == 0)  
        return 0; 
    while(in != 1) {
        unsigned char b;
        b = c & 0x80;
        c <<= 1;
        if(b == 0x80) {
            c ^= 0x1b;
        }
        in--;
    }
    return c;
}

/* This is the core key expansion, which, given a 4-byte value,
 * does some scrambling */
static void schedule_core(unsigned char *in, unsigned char i) {
    char a;
    /* Rotate the input 8 bits to the left */
    rotate(in);
    /* Apply Rijndael's s-box on all 4 bytes */
    for(a = 0; a < 4; a++) 
        in[a] = sbox[in[a]];
    /* On just the first byte, add 2^i to the byte */
    in[0] ^= rcon(i);
}

void expand_key(unsigned char *in) {
    unsigned char t[4];
    /* c is 16 because the first sub-key is the user-supplied key */
    unsigned char c = 16;
    unsigned char i = 1;
    unsigned char a;

    /* We need 11 sets of sixteen bytes each for 128-bit mode */
    while(c < 176) {
        /* Copy the temporary variable over from the last 4-byte
         * block */
        for(a = 0; a < 4; a++) 
            t[a] = in[a + c - 4];
        /* Every four blocks (of four bytes), 
         * do a complex calculation */
        if(c % 16 == 0) {
            schedule_core(t,i);
            i++;
        }
        for(a = 0; a < 4; a++) {
            in[c] = in[c - 16] ^ t[a];
            c++;
        }
    }
}