Question

A- Briefly explain what is the purpose of the SW operation in S-DES i.e. How would S-DES be weakened if the SW operation was ommitted.

B- Briefly explain why in DES the key is 56-bits instead of 64-bits?

Answer 1

S-DES
(Simplified - Data Encryption Standard)

It is a block cipher.
It takes plain text as input, encrypts it using a key, and gives the result.

Steps for performing encryption:

STEP 1: Initial Permutation
STEP 2: A function labeled f_k
STEP 3: The Switch Function(SW)
STEP 4: A function labeled f_k
STEP 5: Initial Permutation Inverse

Part A

Function f_k of Step 2 only changes the 4 leftmost bits of the input. Now, in Step 3, switch function (SW) interchanges the left and right 4 bits so that in step 4 function f_k operates on a different 4 bits.

If this Step is not performed function f_k of Step 4 will be performed again on the same 4 leftmost bits which will certainly weaken the encryption and Encryption will be half done.

Part B

Now, We know that S-DES works on 64-bit input blocks. So the initial key has 64 bits.
But, before actually starting DES, every 8th bit of the original 64 bit key is discarded.


So Discarding Position will be 8, 16, 24, 32, 40, 48, 56 and 64.
Eight of those discarded keys are used only as parity checks.

After Discarding we are left with effective limited 56-bit key, which is then used in encryption.

please comment for any further clarification.