Mersenne Primes-Based Symmetric-Key Masquerade Block CipherA strong cryptographic system, consisting of very simple encrypting/decrypting algorithms, has been described. The system presented is a sort of a flexible symetric-key block cipher in which plaintext blocks, ciphertext blocks and keys have a form of files. Flexibility denotes here that the size and the format of a plaintext file and a cryptogram file may be arbitrary. The size of the plaintext and cryptogram file may vary from 2 to 3245619 bytes. The system can have a very huge keyspace (key size from 16 to 25964952 bits, while AES offers maximum 256 bits) and works in a masquerade mode. As regards the masquerade mode of operation, it should be emphasized that in this mode the cipher provides cryptograms entirely statistically independent on plaintexts, and the only method of breaking the cipher is the exhaustive search of the key space.