  Cryptanalysis increases the level of confidence in cryptographic algorithms.
We analyze the security of a symmetric cryptographic algorithm - quantum
permutation pad (QPP) [8]. We found the instances of ciphertext the same as
plaintext even after the action of QPP with the probability 1/N when the entire
set of permutation matrices of dimension N is used and with the probability
1/N^m when an incomplete set of m permutation matrices of dimension N are used.
We visually show such instances in a cipher image created by QPP of 256
permutation matrices of different dimensions. For any practical usage of QPP,
we recommend a set of 256 permutation matrices of a dimension more or equal to
2048.
