<P> Classic cryptosystems invariably generate two identical keys at one end of the communication link and somehow transport one of the keys to the other end of the link . However, it simplifies key management to use Diffie--Hellman key exchange instead . </P> <P> The simplest method to read encrypted data without actually decrypting it is a brute force attack--simply attempting every number, up to the maximum length of the key . Therefore, it is important to use a sufficiently long key length; longer keys take exponentially longer to attack, rendering a brute force attack impractical . Currently, key lengths of 128 bits (for symmetric key algorithms) and 2048 bits (for public - key algorithms) are common . </P> <P> A wireless channel is characterized by its two end users . By transmitting pilot signals, these two users can estimate the channel between them and use the channel information to generate a key which is secret only to them . The common secret key for a group of users can be generated based on the channel of each pair of users . </P> <P> A key can also be generated by exploiting the phase fluctuation in a fiber link . </P>

How to define key generators meeting the needs of cryptography