About substitution-cipher
In this cryptosystem units of the plaintext (generally single letters or pairs of letters of ordinary text) are replaced with other symbols or groups of symbols.
This cryptosystem is not secure. The possible number of keys are 26!, it can be broken down using Frequency Analysis. The cipher-text attack model can be used to crack the cryptosystem.
Encryption:
For each permutation π ∈ K, The Encryption function is defined as 𝑒𝜋(x) = 𝜋(x)
Decryption:
For each permutation π ∈ K, The Decryption function is defined as 𝑑𝜋(y) = 𝜋^−1(𝑥) where 𝜋^−1 is inverse permutation of 𝜋.
