The Cipher Block Chaining MAC (CBC-MAC) method builds a message authentication code from a block-cipher by operating it in CBC mode over the message to authenticate, and outputting the final block as the authentication code.

CBC-MAC is not secure for variable-length messages, or if the same key is used to encrypt a message in CBC mode and then CBC-MAC the message, and is vulnerable to attacks if the initialization vector can be varied.

See the wikipedia page for more information.

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