1. An encryption/decryption unit for encrypting a plaintext into a ciphertext and/or decrypting a ciphertext into a plaintext, comprising: first encryption/decryption means for performing an encryption or decryption process; first substitution means for performing data substitution of an output from said first encryption/decryption means according to a predetermined permutation table; second encryption/decryption means for performing an encryption or decryption process for an output from said first substitution means; second substitution means for performing data substitution of an output from said second encryption/decryption means according to a predetermined permutation table; and third encryption/decryption means for performing an encryption or decryption process for an output from said second substitution means.