What is a block cipher and how does it differ from a stream cipher?

A block cipher encrypts fixed-size blocks of plaintext (e.g., 128 bits at a time) using a key, while a stream cipher encrypts bit by bit.

The core trade-off is when the cipher commits: a stream cipher acts on each bit as it arrives (ideal for low-latency streams), while a block cipher waits for a whole block, which lets it apply much heavier internal mixing and gives it the structure needed to build integrity checks like CBC-MAC.

Block cipher characteristics:

• Processes data in fixed-size blocks (e.g., 64 or 128 bits)
• The same key is used for every block
• A block cipher is a keyed permutation — for a given key, it's a bijective mapping from plaintext blocks to ciphertext blocks
• Often called the "workhorse" of cryptography — encrypts 90%+ of all data

Key parameters:

vs. Stream ciphers:

• Block ciphers need padding if plaintext isn't a multiple of block size
• Block ciphers can provide integrity (e.g., CBC-MAC)
• Stream ciphers are typically faster for streaming data

Go deeper:

• Block cipher (Wikipedia) — block size, key, rounds, and the security models.