A typical, although not quite universal, feature of sub-algorithms in symmetric encryption algorithms is the use of "S-boxes" (the "S" stands for "substitution"). S-boxes are in fact just functions. Rather than simply operating on individual bits (as XOR does), an S-box takes N bits of input and produces N bits of output. At their heart, S-boxes must be one-to-one functions because reversibility is required for decryption (but see the "Avalanche effects" panels for some complicating details). Each bit is still transformed to a new value, but its transformation depends on the bits around it.

Actually, once we start to look at S-boxes, the notion of tracing a specific bit as it travels through an algorithm breaks down somewhat. It is not so much that bit-one of an S-box input corresponds to bit-one of its output; rather, the whole output block corresponds (a one-to-one relation) with the whole input block. But whether each individual bit is substituted, moved, both, or neither is irrelevant. As long as the correspondence is one-to-one, we can reverse the operation during decryption.