**– The AES or advanced encryption standard** like the data encryption standard is also a specification established by the NIST (national institute of standards and technology, U.S.) for encrypting the electronic data.

– Vincent Rijmen and Joan Daemen are the two Belgian cryptographers who developed the Rijndael cipher that serves as a basis for the advance encryption standard.

– These two cryptographers proposed this algorithm during the AES selection process and the algorithm was eventually selected by NIST.

– The U.S. government was the first one to adopt it and gradually AES gained worldwide popularity.

– The DES (data encryption standard) was followed worldwide before AES came in to the scene.

**– AES describes a symmetric key algorithm which means that both the encryption and decryption can be done by the one and the same key.**

– NIST announced the AES as the U.S. FIPS PUB 197 in the year of 2001.

– After this announcement a five year standardization process followed in which rijndael cipher was selected.

– This algorithm works up on the principle of substitution permutation network.

**– This algorithm is known for giving fast performance in both software and hardware.**

– AES does not use a feistel network such as the DES.

– A variant of rijndael is used in AES with key of size 128, 192 or 256 bits and a block of fixed size (128 bits).

– The state which is a 4×4 column major order matrix (of bytes) is what up on which the AES operates.

– Rijndael cipher is even available in larger block sizes that too with additional state columns.

– The special field where the most of the calculations of AES are carried out is known as the finite field.

– The number of transformation rounds that have to be made for converting the plain text (i.e., the input) in to the cipher text (i.e., the output) is specified by the AES cipher.

**Following are the number of cycles of repetition:**

- 10 cycles for key of size 128 bits.
- 12 cycles for key of size 192 bits.
- 14 cycles for key of size 256 bits.

– Several processing steps are carried out in each round.

– In each of the rounds, there are five similar but different steps out of which one depends up on the encryption key.

– When it is required to convert the cipher text back in to the plain text, the reverse rounds are carried out using the encryption key.

**– Optimization of the cipher is a difficult task.**

– The execution speed of the cipher can be increased if the sub-bytes and shift rows step is combined with step of mix columns and then converting them in to table look-ups sequence.

– This operation will require memory of 4 KB and four 256 entry and 32 bit tables.

– 16 look up tables will then be used in a round involving 12 32 – bit XOR operations.

– If the size of the resulting table is too large, the same operation can be carried out using just one 256 – entry 32 bit table by the means of circular rotation.

– A single round operation can be formed by combining the sub – bytes shift rows with the mix columns step by following a byte – oriented approach.

– Till now, only the side channel attacks have been successful in hampering the security offered by AES.

– For providing protection up to secret level, the AES algorithms key lengths’ design and strength have been stated as sufficient.

– For top secret information, greater key lengths are required.

**– In the year of 2003, the AES got approval for protection of the classified information also.**

– If the products means for protecting the national security systems use AES, NSA has to review and certify before they can be used.

– A 64 bit RC5 key faced the largest successful brute force attack.

– This attack targeted its block cipher encryption.

– Another class of attacks called the cryptographic breaks are quite faster than the brute force attacks.

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