TEA

The Tiny Encryption Algorithm, or TEA, is a Feistel cipher invented by David Wheeler. It is intended for
use in applications where code size is at a premium, or where it is necessary for someone to remember the
algorithm and code it on an arbitrary machine at a later time. 

The round function is based on a shift and add operation, with the carry bit providing the nonlinearity. The
golden ratio $\frac{\sqrt{5}-1}{2}$ is added in every second round to prevent chosen plaintext attacks
(can you see why?). The official specification is given in C and is: 

 
void code(long* v, long* k) 
{ 
    unsigned long y=v[0],z=v[1],sum=0,      /* set up */
                  delta=0x9e3779b9, n=32 ;  /* key schedule constant*/

    while (n-->0) 
    {                                       /* basic cycle start */
        sum += delta;
        y += (z<<4)+k[0] ^ z+sum ^ (z>>5)+k[1];
        z += (y<<4)+k[2] ^ y+sum ^ (y>>5)+k[3]; /* end cycle */
    }
    v[0]=y; v[1]=z;

}

Since its round function is relatively weak, with nonlinearity coming only from the carry propagation, TEA
has 64 rounds. However, its simplicity means that it runs more quickly in software than many other
algorithms with fewer, more complex, rounds.