ciphertextblocks

Ciphertext blocks are the outputs produced by a block cipher when encrypting data. A block cipher operates on fixed-size blocks of data, commonly 64 or 128 bits. When a message is encrypted with a block cipher, the plaintext is divided into blocks of this fixed size, and each block is transformed by the cipher into a corresponding ciphertext block. The concatenation of these ciphertext blocks constitutes the encrypted message. In typical usage the ciphertext block size matches the cipher’s block size, with additional data such as an initialization vector or padding overhead as required by the mode of operation.

Block ciphers are used in various modes of operation to handle multi-block messages. In electronic codebook (ECB) mode, blocks are encrypted independently, giving a direct one-to-one mapping between plaintext blocks and ciphertext blocks. This can reveal patterns if the same plaintext block repeats. In cipher block chaining (CBC), cipher feedback (CFB), and output feedback (OFB) modes, blocks are interdependent, so each ciphertext block is influenced by previous blocks and, in CBC and CFB, by an initialization vector (IV). Decryption in these modes uses the previous ciphertext block or the IV. In counter (CTR) mode, a nonce and counter generate a keystream block that is XORed with each plaintext block to produce ciphertext blocks. Padding is typically required to fill the final block when the message length is not a multiple of the block size, with common schemes such as PKCS#7.

Security considerations for ciphertext blocks include secure key management, appropriate mode selection, and integrity protection. ECB is generally discouraged due to pattern leakage, and modern practice favors authenticated encryption or encryption with a separate MAC to prevent tampering and padding oracle attacks. The block size of the cipher influences exposure to block-repetition risks in long messages, with 128-bit blocks being standard in modern schemes like AES.