A totally random keystream is called a one-time pad and is the only known encryption scheme that is mathematically proven to protect against certain types of attacks. One-time pads are not commonly used because the keystream must be perfectly random and the same length as the data that will be protected, and it can never be reused.
Figure 5-2. Keyed stream cipher operation
Attackers are not limited to attacking the underlying cipher. They can choose to exploit any weak point in a cryptographic system. One famous Western intelligence effort, code-named VENONA, broke Soviet messages encrypted with one-time pads that were reused. The National Security Agency has made some information on the project public at http://www.nsa.gov/docs/venona. It is easy to understand the temptation to reuse the onetime pads. Huge volumes of keying material are necessary to protect even a small amount of data, and those keying pads must be securely distributed, which in practice proves to be a major challenge.
Stream ciphers are a compromise between security and practicality. The perfect randomness (and perfect security) of a one-time pad is attractive, but the practical difficulties and cost incurred in generating and distributing the keying material is worthwhile only for short messages that require the utmost security. Stream ciphers use a less random keystream but one that is random enough for most applications.
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