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The Advanced Encryption Standard
Because of advances in the speed of computers, a cipher having only a 56-bit key is no longer secure. One response to this has been the use of Triple-DES. The standard method of using Triple-DES is to encrypt a 64-bit block with DES using one key, decrypt it with another, and then encrypt it again either with the first key or with a third key. This gives a key length of either 112 bits or 168 bits; in addition, if all three 56-bit keys are the same, the decryption in the middle will undo one of the encryptions, allowing interoperability with equipment using ordinary DES.
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