Every modern symmetric encryption algorithm finds ways to work in two basic operations: diffusion and substitution. That is, the content of a cipher text both replaces the bits and bytes of the plain text with different bits and bytes (substitution), and moves those replaced bits and bytes to different locations in the cipher text (diffusion).

Unlike old-fashioned hand-calculated ciphers, modern algorithms inevitably operate primarily at a bit level. Modern algorithms usually do involve some transforms at the nibble, byte, word, or block level; but these larger transforms usually break down and rearrange the bits within these blocks in other parts of the algorithm.

Most modern algorithms produce cipher texts of exactly the same size as the original plain text. Maybe a bit or two here and there is added for identification or error correction, but overall, bits of plain text have a one-to-one relationship with bits of cipher text.

A good algorithm is one that is entirely key-dependent, whether 1's in the plain text are represented by 0's or by 1's in the cipher text. In addition, with such algorithms one cannot quite tell where in the cipher text to find a particular diffused plain text bit (except by knowing the key, and thereby figuring out just where bits have been pushed).