# What is block size in cryptography? — Symmetric key ciphers are generally divided into stream ciphers and block ciphers. Block ciphers operate on a fixed length string of bits. The length of this bit string is the block size. Both the input (plaintext) and output (ciphertext) are the same length; the output cannot be shorter than the input – this is logically required by the Pigeonhole principle and the fact that the cipher must be invertible – and it is simply undesirable for the output to be longer than the input.

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