1. A method of generating a linear transformation matrix A for use in a symmetric-key cipher, the method including: generating a binary &lsqb;n,k,d&rsqb; error-correcting code, represented by a generator matrix G&egr;Z 2 k×n in a standard form G&equals;(I k &par;B), with B &par;Z 2 k×(n−k), where k<n<2k, and d is the minimum distance of the binary error-correcting code; extending matrix B with 2k−n columns such that a resulting matrix C is non-singular, and deriving matrix A from matrix C.