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; means for deriving a set of user-selectable encryption parameters from said secret key, each of said user-selectable encryption parameters having a range of values; means responsive to said set of user-selectable encryption parameters for generating a mapping among a repertoire thereof for mapping plaintext into ciphertext; means for deriving a set of decryption parameters from said secret key; and means responsive to said set of decryption parameters for generating an inverse mapping associated with said mapping for inverse mapping the ciphertext back to the plaintext; and wherein said repertoire of mappings having a size dependent on the range of values of each of said user-selectable encryption parameters, whereby others not privy to said secret key are confronted with a computationally infeasible task of exhaustive research among a repertoire of mappings of indeterminable size; means responsive to a block size parameter for partitioning the plaintext block-by-block into plaintext blocks or vectors each having a size according to said block size parameter; and wherein: said mapping maps a plaintext vector into a corresponding ciphertext vector; and said set of user-selectable encryption parameters includes said block size parameter and a mapping matrix; said mapping is such that each ciphertext vector is a vector sum that includes a first vector sum component formed by a product of said mapping matrix and the corresponding plaintext vector; said set of decryption parameters includes an inverse mapping matrix associated with said mapping matrix; and said inverse mapping is such that each plaintext vector is a product of said inverse mapping matrix and a resultant vector produced by the corresponding ciphertext vector minus the vector sum except for its first vector sum component.