Which cipher type processes data in fixed-size blocks?

Which cipher type processes data in fixed-size blocks?

• A. Stream cipher
• B. Permutation cipher
• C. Block cipher
• D. Substitution cipher

Answer: (C)

Block ciphers process data in fixed-size blocks. They take input in uniform chunks (like 64 or 128 bits) and apply a key-dependent transformation to each block. This block-based approach lets you secure long messages by chaining blocks together using various modes of operation, ensuring consistency and strong security properties across the entire data stream. In contrast, a stream cipher generates a keystream and combines it with the plaintext as the data arrives, typically bit-by-bit or byte-by-byte rather than in fixed blocks. Substitution ciphers replace individual symbols with others, not inherently working on fixed-sized blocks. Permutation (transposition) ciphers rearrange the order of symbols, which also isn’t defined by fixed block processing. So the one that inherently uses fixed-size blocks is the block cipher.

Block ciphers process data in fixed-size blocks. They take input in uniform chunks (like 64 or 128 bits) and apply a key-dependent transformation to each block. This block-based approach lets you secure long messages by chaining blocks together using various modes of operation, ensuring consistency and strong security properties across the entire data stream.

In contrast, a stream cipher generates a keystream and combines it with the plaintext as the data arrives, typically bit-by-bit or byte-by-byte rather than in fixed blocks. Substitution ciphers replace individual symbols with others, not inherently working on fixed-sized blocks. Permutation (transposition) ciphers rearrange the order of symbols, which also isn’t defined by fixed block processing. So the one that inherently uses fixed-size blocks is the block cipher.