A symmetric block cipher processes data one block at a time. For DES and 3DES, each block is 64 bits long, while AES uses 128-bit blocks. When encrypting longer plaintext, the data must be divided into these fixed-length blocks, padding the last block if necessary. To accommodate various encryption needs, the National Institute of Standards and Technology (NIST) has defined five modes of operation as documented in Special Publication 800-38A. These modes, suitable for any symmetric block cipher including AES and triple DES, aim to provide security for a wide range of encryption applications. Below are brief descriptions of the most common modes.
Electronic Codebook (ECB) Mode
The Electronic Codebook mode is the simplest form of operation where each block of plaintext is encrypted separately using the same key. This method is termed "codebook" because for any given key, there exists a unique ciphertext for every possible plaintext block of the same size. However, ECB mode has a significant weakness: repeated plaintext blocks produce the same ciphertext. For highly structured or lengthy messages, this can expose patterns that make the system vulnerable to cryptanalysis. Cryptanalysts might exploit known structures, such as predefined fields, leading to potential exposure or manipulation of the message content.
Cipher Block Chaining (CBC) Mode
Cipher Block Chaining (CBC) mode addresses ECB's weaknesses by linking the encryption of each plaintext block to the preceding ciphertext block through an XOR operation. This chaining means that the encryption of similar plaintext blocks will yield different ciphertexts, even if the plaintext does not change. Decryption works by applying an XOR to the output of the decryption function with the preceding ciphertext block, effectively reversing the chaining process to reveal the original plaintext. An initialization vector (IV) is employed to encrypt the first block, and for maximum security, the IV should be kept as secure as the encryption key to prevent adversaries from altering the plaintext.
Cipher Feedback (CFB) Mode
The Cipher Feedback mode transforms a block cipher into a stream cipher by enabling encryption of data in smaller increments, such as 8-bit characters, without padding the plaintext to a full block size. Each plaintext character is encrypted and transmitted immediately, offering an advantage in real-time data transmission. In CFB mode, an initialization vector is used to create a shift register. As plaintext is processed, its units are XORed with segments of the output from the encryption function to produce ciphertext, thereby chaining together the sequence of plaintext inputs. Decryption uses the same approach, with the ciphertext being XORed to retrieve the original plaintext.
Counter (CTR) Mode
Counter mode operates by encrypting a counter value and XORing it with the plaintext to produce the ciphertext. Each block uses a unique counter value to ensure different encryption outputs even for identical plaintext blocks. This mode allows multiple blocks to be encrypted or decrypted simultaneously, enhancing throughput. Several advantages include efficient hardware and software implementations, the possibility of preprocessing future outputs, random access capability, and security levels comparable to or exceeding other modes. CTR mode only requires the encryption algorithm, simplifying its implementation when compared to ECB and CBC, which need both encryption and decryption algorithms.
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