Advanced Encryption Standard is an encryption scheme established by U.S National Institute of Standards and Technology. AES is a symmetric key algorithm that produces the same key for both encryption and decryption. To note, protocols like SSL/TLS or IPsec use these algorithms to generate keys (in a secure fashion) that will be used to encrypt data. AES as an algorithm is an advancement over DES (1977). AES currently offers three different key sizes (blocks of encrypted data) - 128, 192 or 256 bits. AES is considered to be one of the most efficient algorithms since it is very fast and consumes less memory. AES is also a federal government standard in United States as approved by Secretary of Commerce.
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It is usually considered that longer the key size of an algorithm, the more difficult it is to break it. This is not necessarily true and there have been attacks that have been proven to be easier to carry out on AES 256 than AES 128. This should not however raise off any alarms as the attacks that were suggested are more theoretical than practical. There have been no practical attacks discovered for any flavor of AES. This does men that there were attacks found but they are no cause of concern at the moment since they are either very difficult to carry out or need very specific privileges or information that is not easily available.