Question

7. Substantiate or refute the following: As the cipher text sample size becomes smaller, cryptanalysis becomes easier.

Answer 1

As we know about the fact that a block cipher always operates to provide a block of ciphertext of n bits whenever a block of plaintext of size n bits is given to it. To make the algorithm reversible ie to make decryption possible there must have a unique system of mapping between the two set of blocks. Which is also sometimes called nonsingular transformation. In order to better understand this look at the figure given below.

As we can see that this is essentially a substitution cipher, which can be susceptible to the standard statistical analysis techniques which are used for simple cryptanalysis of texts (like frequency analysis).So, as the block size increases then this becomes increasingly less possible or should I say feasible. There is an obvious practical difficulty with this proposal which is the number of transformations needed as n increases. In this mapping is essentially the number of bits and the key will determine the size of the key. Also, the key size for an n- bit general substitution block cipher is calculated as follows:

key=n×2n

Suppose for some case if n = 64, the key size becomes 64 × 2⁶⁴ = 10²¹.