How many bits shift is a TEA encryption?

Question

void encrypt (unsigned long* v, unsigned long* k) {

unsigned long v0=v[0], v1=v[1], sum=0, i;           /* set up */
unsigned long delta=0x9e3779b9;                     /* a key schedule constant */
unsigned long k0=k[0], k1=k[1], k2=k[2], k3=k[3];   /* cache key */
for (i=0; i < 32; i++) {                       /* basic cycle start */
    sum += delta;
    v0 += ((v1<<4) + k0) ^ (v1 + sum) ^ ((v1>>5) + k1);
    v1 += ((v0<<4) + k2) ^ (v0 + sum) ^ ((v0>>5) + k3);
}                                              /* end cycle */
v[0]=v0; v[1]=v1;

}

I saw this code on the wiki of TEA encryption and was wondering is this considered a 4-bit or 5-bit algorithm since there was a shift of 4 bits to the left but a shift of 5 bit to the right.

I am confused on the algorithm of this encryption and would appreciate it if somebody can clarify it for me.

Thank you

fgrieu · Accepted Answer · 2017-10-24T07:08:56.330

TEA is neither considered a 4-bit nor 5-bit algorithm (these are not categorized by their shift count). TEA:

Is a 64-bit block cipher, meaning encrypt modifies 64 bits: the block is v[0] v[1] and the code shown (non-portably) assumes unsigned long is 32-bit.
Has a 128-bit key k[0] k[1] k[2] k[3] ; notice that each key has 3 other equivalent ones (obtained by complementing the high-order bits of even or/and odd numbered words), thus key is effectively 126-bit.
Performs 32 double rounds, with in each double round (focusing on shifts because that's asked) two left shifts by 4 bits and two right shifts by 5 bits (on a 32-bit quantity). That's 128 shifts on a machine with a 32-bit barrel shifter, and 5760 shifts with a 32-bit ALU without barrel shifter.

Addition: the only two common reasons to count the shifts in an algorithm are:

Determine how much computational effort is spent on that, to predict how the CPU influences the performance. Continuing on that track: on 8-bit CPU, 32-bit shifts are quite expensive, and naive code with likely have 23040 shift instructions executed per bloc encrypted, with shifting representing I guess like 3/4 of the effort. In assembly language at least, it is possible to do significantly better by realizing that most bits of v1>>5 can be obtained as ((v1<<4)>>8)>>1, with the shift by 8 efficient on an 8-bit CPU. Also, many 8-bit CPUs have an instruction that swaps the halves of an octet, and might be put to use for the shift by 4.
Determining is there's enough diffusion. In TEA, it is used shift rather than rotations (largely because there is no built-in rotation in the C language, and building one would have made at least the source code more complex), and therefore diffusion across bits of the 32 ranks in 32-bit words is mostly by shifting (exclusively so for right diffusion, and with carry propagation contributing slightly to left diffusion). Full diffusion requires about 8 rounds, and that is significantly slower than say in DES, which diffuses I'd say more than twice faster. That's one of several reasons TEA must have a lot of rounds (64 rounds versus 16 in DES).

How many bits shift is a TEA encryption?

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