However, similar information-theoretic considerations are occasionally useful, for example, to determine a recommended key change interval for a particular cryptosystem. Cryptanalysts also employ a variety of statistical and information-theoretic tests to help guide the analysis in the most promising directions. One of the fundamental axioms of cryptography is that the enemy is in full possession of the details of the general cryptographic system, and lacks only the specific key data employed in the encryption. Repeated use of a finite amount of key provides redundancy that can eventually facilitate cryptanalytic progress. Thus, especially in modern communication systems where vast amounts of information are transferred, both parties must have not only a sound cryptosystem but also enough key material to cover the traffic.
Key management refers to the distribution, authentication, and handling of keys.
This should give an inkling of the extent of the key management problem. Chaotic equations and fractals produce an apparent randomness from relatively compact generators. Perhaps the simplest example is a linear congruential sequence, one of the most popular types of random number generators, where there is no obvious dependence between seeds and outputs. Unfortunately the graph of any such sequence will, in a high enough dimension, show up as a regular lattice. Mathematically this lattice corresponds to structure which is notoriously easy for cryptanalysts to exploit.