CBC
Cipher Block Chaining
Cipher Block Chaining (CBC) is a mode of operation for block ciphers, a class of symmetric key encryption algorithms. Invented by IBM in 1976, CBC has been widely adopted due to its enhanced security features compared to the basic Electronic Codebook (ECB) mode. In CBC mode, each block of plaintext is XORed (exclusive OR) with the previous ciphertext block before being encrypted. This method of chaining together blocks ensures that identical blocks of plaintext result in different ciphertext blocks, thus obscuring patterns in the input data.
The encryption process in CBC mode starts with an initialization vector (IV), which is a block of random data. The IV is XORed with the first block of plaintext to generate the first block of ciphertext. Subsequently, each plaintext block is XORed with the previous ciphertext block and then encrypted. This chain of dependencies means that even a small change in the plaintext or in the IV results in a significantly different ciphertext, enhancing security against certain types of attacks. During decryption, the process is reversed. Each block of ciphertext is decrypted, and then XORed with the previous ciphertext block to recover the plaintext. The IV is used directly in the XOR operation for the first block.
CBC mode, while more secure than ECB, is not without vulnerabilities. For instance, it is susceptible to padding oracle attacks if not properly implemented with additional integrity checks. Despite its weaknesses, CBC remains a widely used and important mode of operation in the cryptographic landscape, especially when combined with other security measures to enhance its robustness.
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