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

#if CHAR_BIT != 8
 #error CHAR_BIT must be 8!
#elif ULONG_MAX != 0xFFFFFFFFuL
 #error 'unsigned long' must be 32 bits!
#endif

/*
 * Tiny Encryption Algorithm
 *
 * D.J. Wheeler & R.M. Needham
 */

#define ITERATIONS 32

static void encode(unsigned long *plain,
                   unsigned long *key,
                   unsigned long *enc)
{
    unsigned long y = plain[0];
    unsigned long z = plain[1];
    unsigned long sum = 0;
    unsigned long delta = 0x9E3779B9;
    int i;

    for (i=0; i < ITERATIONS; ++i)
    {
        sum += delta;
#if 0
        /* The code in the original paper is WRONG WRONG WRONG */
        y += ((z << 4) + key[0]) ^ ((z >> 5) + key[1]) ^ (z + sum);
        z += ((y << 4) + key[2]) ^ ((y >> 5) + key[3]) ^ (y + sum);
#else
        y += (z << 4) + (key[0] ^ z) + (sum ^ (z >> 5)) + key[1];
        z += (y << 4) + (key[2] ^ y) + (sum ^ (y >> 5)) + key[3];
#endif
    }
    enc[0] = y;
    enc[1] = z;
}

static void decode(unsigned long *enc, unsigned long *key, unsigned long *dec)
{
    unsigned long y = enc[0];
    unsigned long z = enc[1];
    unsigned long delta = 0x9E3779B9;
    unsigned long sum = delta * ITERATIONS;
    int i;
   
    for (i=0; i < ITERATIONS; ++i)
    {
#if 0
        /* The code in the original paper is WRONG WRONG WRONG */
        z -= ((y << 4) + key[2]) ^ ((y >> 5) + key[3]) ^ (y + sum);
        y -= ((z << 4) + key[0]) ^ ((z >> 5) + key[1]) ^ (z + sum);
#else
        z -= (y << 4) + (key[2] ^ y) + (sum ^ (y >> 5)) + key[3];
        y -= (z << 4) + (key[0] ^ z) + (sum ^ (z >> 5)) + key[1];
#endif
        sum -= delta;
    }
    dec[0] = y;
    dec[1] = z;
}


int TEA_encode(void *plain, void *crypt, size_t len,
               void *key, size_t keylen)
{
    unsigned long *lplain = plain;
    unsigned long *lcrypt = crypt;
    int npairs = len/8;
    int i;

    /* Paranoia: check key size */
    if (keylen != 16)
      return -1;

    for (i=0; i < npairs; ++i)
      encode(lplain+2*i, key, lcrypt+2*i);

    if (len % 8) {
        /* Partial block left over; copy it unencrypted */
        memcpy(lcrypt+2*i, lplain+2*i, len%8);
    }

    return 0;
}

int TEA_decode(void *crypt, void *plain, size_t len, void *key, size_t nkey)
{
    unsigned long *lcrypt = crypt;
    unsigned long *lplain = plain;
    int npairs = len/8;
    int i;

    /* Paranoia: check key size */
    if (nkey != 16)
      return -1;

    for (i=0; i < npairs; ++i)
      decode(lcrypt+2*i, key, lplain+2*i);

    if (len % 8) {
        /* Partial block left over; copy it undecrypted */
        memcpy(lplain+2*i, lcrypt+2*i, len%8);
    }

    return 0;
}


int main(void)
{
    char key[] = "1234567890123456";
    char test[] = "Having been some days in preparation\n"
                  "A splendid time is guaranteed for all.\n";
    char enc[sizeof test];
    char dec[sizeof test];
    TEA_encode(test, enc, sizeof test, key, 16);
    TEA_decode(enc, dec, sizeof test, key, 16);
    printf("%s\n", test);
    printf("%s\n", dec);
    return 0;
}