A method for performing a cryptographic function on text to generate converted text comprises producing a random key stream having a first block size in a first frequency domain; converting the random key stream having a first block size in the first frequency domain to a random key stream in a second frequency domain; converting the random key stream having the first block size in the second frequency domain into smaller block sizes, thereby producing smaller block-sized random key stream of the second frequency domain; and converting the text using the smaller block-sized random key stream of the second frequency domain to produce the converted text. The frequency in the first frequency domain is preferably lower than the frequency in the second frequency domain.