Abstract
This chapter provides a comprehensive examination of stream ciphers, a fundamental class of symmetric-key algorithms utilized in modern cryptography. It begins by establishing the structural distinctions between block ciphers and stream ciphers, defining the latter as sequence ciphers that process plaintext iteratively bit by bit. The text explores the theoretical foundations of these systems by revisiting the Vernam cipher and the concept of perfect secrecy, specifically addressing the logistical challenges of key generation, distribution, and storage associated with key lengths equal to the plaintext. To demonstrate how practical stream ciphers resolve these limitations while maintaining robust security, the chapter details their standard operational modes. Furthermore, it provides an in-depth analysis of the precise encryption and decryption mechanisms of widely deployed algorithms, specifically the RC4 cipher and the ZUC cipher.