Block cipher modes of operation define how block ciphers handle plaintext and ciphertext, particularly when dealing with data larger than the cipher’s block size. They enhance the security and versatility of block ciphers, ensuring confidentiality, authenticity, or both. Below are common modes of operation:
- Electronic Codebook (ECB):
- Each block of plaintext is encrypted independently.
- Simple but vulnerable to pattern attacks due to identical plaintext blocks producing identical ciphertext.
- Cipher Block Chaining (CBC):
- Each plaintext block is XORed with the previous ciphertext block before encryption.
- Requires an initialization vector (IV) to encrypt the first block securely.
- Cipher Feedback (CFB):
- Converts a block cipher into a self-synchronizing stream cipher.
- Plaintext is XORed with the output of an encrypted IV or previous ciphertext block.
- Output Feedback (OFB):
- Similar to CFB but uses the encryption output instead of ciphertext for feedback.
- Ensures no error propagation during encryption.
- Counter (CTR):
- Uses a counter value encrypted with the key to produce a unique keystream.
- Enables parallel processing and is efficient for high-speed applications.