Lecture Notes: Block Ciphers What is a Block Cipher? A block cipher is a type of cryptographic algorithm that operates on fixed-size blocks of plaintext. The plaintext is divided into blocks of a specific size, and each block is encrypted independently using a cryptographic key. Key Components of Block Ciphers ● Block Size: The fixed size of the plaintext blocks that the cipher operates on. Common block sizes are 64 bits and 128 bits. ● Key: A secret value used to encrypt and decrypt the plaintext. ● Encryption Algorithm: The mathematical function used to transform plaintext into ciphertext. ● Decryption Algorithm: The inverse function used to transform ciphertext back into plaintext. Modes of Operation Block ciphers are typically used in conjunction with a mode of operation to encrypt and decrypt data. Common modes of operation include: ● Electronic Codebook (ECB): Each block of plaintext is encrypted independently using the same key. ● Cipher Block Chaining (CBC): Each plaintext block is XORed with the previous ciphertext block before encryption. ● Counter Mode (CTR): A counter is used to generate a unique keystream for each block of plaintext. ● Cipher Feedback (CFB): A portion of the previous ciphertext is used to generate a keystream for the current block. ● Output Feedback (OFB): A keystream is generated independently and used to encrypt each block of plaintext. Advantages of Block Ciphers ● Security: Block ciphers can provide strong security when used with appropriate keys and modes of operation. ● Efficiency: Block ciphers are often efficient to implement, especially in hardware. ● Versatility: Block ciphers can be used for various cryptographic applications, including encryption, authentication, and digital signatures. Disadvantages of Block Ciphers ● Susceptibility to Attacks: Block ciphers can be vulnerable to attacks like brute-force attacks and differential cryptanalysis. ● Fixed Block Size: The fixed block size can limit the flexibility of block ciphers in certain applications. Popular Block Ciphers ● Data Encryption Standard (DES): A 64-bit block cipher that has been widely used but is now considered insecure due to its relatively small key size. ● Triple DES (3DES): A variant of DES that uses three keys to provide increased security. ● Advanced Encryption Standard (AES): A 128-bit block cipher that is considered to be highly secure and widely used. Conclusion Block ciphers are a fundamental component of modern cryptography. By understanding the principles of block ciphers and selecting appropriate modes of operation, organizations can protect sensitive data from unauthorized access.