1. A computer implemented method of generating an encryption dictionary, comprising: arranging a plurality of plaintext symbols in lexicographical order; defining respective first and second subsets of the lexicographically arranged symbols; defining a first set of random tokens within a first token space for use with the first subset of symbols; defining a second set of random tokens within a second token space for use with the second subset of symbols; adding a first random constant value to each token in the first set of random tokens; and adding a second random constant value to each token in the second set of random tokens; wherein the second random constant value is greater than the first random constant value.