1. A cryptographical system for users to encrypt plaintext into ciphertext and decrypt the ciphertext back to the plaintext, comprising: a secret key shared between the users; a set of encryption parameters derivable from said secret key including a portion that are user-selectable through said secret key; a mapping determined by said set of encryption parameters for mapping plaintext into ciphertext, said mapping being of the type with a repertoire having a size that is selectable by the users through said secret key; a set of decryption parameters derivable from said secret key; and an inverse mapping determined by said set of decryption parameters for inverse mapping the ciphertext back to the plaintext; whereby the users can adjust the size of the repertoire of mappings through said secret key to achieve a desirable degree of encryption intensity.