ith a quasirandom bit sequence (usually as long as the plain- text) that is known as key material. The key material can often be mapped through some lookup table or function mechanism onto the plaintext, or as more often is the case, it can be combined additively with the plaintext such as using an exclusive OR (XOR) logical operation in order to produce cipher- text. XORing plaintext traffic with good-quality random material makes the ciphertext extremely dif- ficult for an adversary to tackle. An interesting property of the XOR function is that by XORing this ciphertext with the same additive material, the original plaintext is produced. In other words, a stream cipher has the following relationships: Plaintext P Additive key material K Ciphertext C Ciphertext C Additive key material K Plaintext P Even if the same sequence that is generated at the transmitter is generated at the receiver for the production of the additive key material, ensuring that the correctly corresponding bit positions are used for XORing with the plaintext is not a trivial problem. The problem of synchronizing the addi- tive key material sequence at the receiver with the one used at the transmitter has been notorious. It is known as the problem of preservation of cryptographic synchronization and has been one of the issues that have plagued stream ciphers. Refer to the book Wireless Security: Threats, Models, and Solutions by Randall K. Nichols and Panos C. Lekkas (New York: McGraw-Hill, December 2001) for a deeper discussion of the subject and for a description of a technique that solves this problem. other major problem for stream ciphers has been the statistical quality of the key material, meaning the degree of randomness of the key-generation process. It falls under the same category of problems associated with random or random-like bit sequences. Cryptographic Modes Implementing encryption algorithms in communication systems brings along numerous systems engi- neering concerns that we cannot possibly cover in this short overview. Th