Lecture Notes: Stream Ciphers What is a Stream Cipher? A stream cipher is a type of cryptographic algorithm that encrypts individual bits or characters of plaintext one at a time. It generates a keystream, a sequence of random-looking bits, and combines it with the plaintext using a bitwise operation (usually XOR) to produce the ciphertext. Key Components of Stream Ciphers ● Keystream Generator: Generates a pseudo-random sequence of bits that is used for encryption and decryption. ● Key: A secret value used to initialize the keystream generator. ● Encryption Process: Each plaintext bit is XORed with the corresponding bit of the keystream to produce the ciphertext. Advantages of Stream Ciphers ● Efficiency: Stream ciphers can be very efficient, especially for encrypting large amounts of data. ● Simplicity: The encryption and decryption processes are relatively simple to implement. ● Real-Time Encryption: Stream ciphers can be used for real-time encryption and decryption of data. Disadvantages of Stream Ciphers ● Vulnerability to Resynchronization Attacks: If the keystream is reused for multiple messages, it can be vulnerable to attacks that exploit the repetition. ● Limited Error Propagation: Errors in the keystream can propagate throughout the ciphertext, making it difficult to recover the plaintext. Types of Stream Ciphers ● Linear Feedback Shift Register (LFSR): A hardware-based keystream generator that produces a pseudo-random sequence of bits. ● Block Ciphers in Stream Mode: Block ciphers can be used in stream cipher mode by generating a keystream from a block cipher's output. ● Software-Based Stream Ciphers: Some stream ciphers are implemented entirely in software, such as RC4 and Salsa20. Applications of Stream Ciphers ● Encryption of Data Streams: Stream ciphers are well-suited for encrypting continuous data streams, such as audio, video, or network traffic. ● Wireless Communications: Stream ciphers are often used in wireless communication protocols to protect data from eavesdropping. ● Cryptographic Hardware: Some hardware implementations of stream ciphers can provide high-speed encryption and decryption. Conclusion Stream ciphers are a versatile and efficient type of cryptographic algorithm. They are widely used in various applications, from wireless communications to data encryption. However, it is essential to choose a secure and reliable stream cipher and implement it correctly to protect sensitive data.