# Mode of Operation

**The mode of operation** refers to **how the AES cipher is used to encrypt or decrypt data that is longer than a single block.**

AES operates on fixed-size blocks of 128 bits, so if the plaintext or ciphertext is longer than 128 bits, a mode of operation is required to break it up into multiple blocks and apply the cipher to each block.

There are several modes of operation that can be used with AES, each with its own strengths and weaknesses.

<mark style="color:red;">**All modes have serious weaknesses when used incorrectly.**</mark>


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