Let $\mathcal{K}$ be the set of all keys of a substitution cipher on 26-letter alphabet? Compute $\lfloor \log_2(|\mathcal{K}|) \rfloor$.
Eve is given intercepts three encrypted long English texts with a substitution cipher.
- In ciphertext $c_1$ only one key was used for all characters in a plaintext.
- In ciphertext $c_2$ all characters in a plaintext were encrypted with a new random key.
- In ciphertext $c_3$ two keys was used, where what key is chosen for what characters in plaintext is determined at random.
Eve uses and index of coincidence IC to distinguish between $c_1, c_2$ and $c_3$. Answer the following questions with numbers rounded to 2 decimal places:
- What IC will Eve expect for $c_1$?
- What IC will Eve expect for $c_2$?
- What IC will Eve expect for $c_3$?
Is computationally feasible to do an exhaustive search on the key space in order to decrypt $c_1$?
Answer with yes or no.
Submit the answer as a sequence like 31 0.01 0.02 0.02 yes.