AES (Advanced Encryption Standard)

The Advanced Encryption Standard (AES) is a symmetric-key encryption algorithm used to protect data from unauthorized access. It was first announced by the National Institute of Standards and Technology (NIST) in November 2001 as the successor to the Data Encryption Standard (DES).

AES uses a block cipher, which means that it encrypts data in fixed-length blocks. The standard specifies three key sizes: 128 bits, 192 bits, and 256 bits. AES is a widely used encryption algorithm, and it is considered to be very secure.

The AES algorithm consists of several stages that operate on a block of data. These stages are the key expansion, the initial round, and the main rounds. Let's take a closer look at each of these stages.

Key Expansion:

The first step of AES is to expand the key. This is done by taking the original key and applying a key schedule algorithm to produce a set of round keys. These round keys are used in the main rounds of the algorithm.

The Key schedule algorithm uses a combination of a number of simple bitwise operations such as XOR, Substitution and Shift operations to derive the round keys from the original key.

Initial Round:

The second stage of AES is the initial round. In this stage, the input data is XORed with the first round key. The result is then passed through a series of substitutions and permutations. These operations are designed to obscure any patterns that may exist in the data.

The Substitution operation is called the S-box operation, which replaces each byte of the input data with another byte according to a predefined lookup table.

The Permutation operation is called the ShiftRows operation, which shifts the rows of the data by a certain number of bytes.

Main Rounds:

The main rounds of AES are the heart of the algorithm. They consist of a series of repeated transformations that mix the input data with the round keys. The number of main rounds depends on the key size.

In each main round, the data is passed through four stages: SubBytes, ShiftRows, MixColumns, and AddRoundKey.

The SubBytes stage is similar to the initial round's S-box operation, it applies a lookup table to each byte of the input data.

The ShiftRows stage is similar to the initial round's ShiftRows operation but operates on each row separately.

The MixColumns stage is unique to AES and applies a matrix multiplication to each column of the data. This operation is designed to ensure that each bit of the input data affects multiple bits in the output.

The AddRoundKey stage XORs the data with the round key for that round.

The main rounds are repeated for a specific number of times depending on the key size. For example, if the key size is 128 bits, there are 10 main rounds. If the key size is 192 bits, there are 12 main rounds. And if the key size is 256 bits, there are 14 main rounds.

AES in Action: Let's take an example to illustrate how AES works.

Suppose we want to encrypt the message "HELLO" using the key "SECRETKEY". We will use a 128-bit key, which means that there will be 10 main rounds.

The first step is to expand the key using the key schedule algorithm. The key schedule algorithm produces a set of 11 round keys (one for each main round and one for the initial round).

The next step is to XOR the input data ("HELLO") with the first round key. The result is then passed through the SubBytes and ShiftRows stages. The result after the SubBytes and ShiftRows stages is:rCopy code 8e 4d 3e a3 47 b3 71 7e 1d 1e 2c 64 7c

Next, the result is passed through the MixColumns stage. The result after the MixColumns stage is:rCopy code 04 e0 48 28 66 cb f8 06 81 19 d3 26 e5 9a 7a 4c

Finally, the result is XORed with the second round key. This is the end of the first main round. The output of the first main round is:Copy code a4 68 6f 9f 35 2b c0 6d 6a ae 8e 4f f4 27 ab e5

This process is repeated for a total of 10 main rounds (since we are using a 128-bit key). The final output of the encryption process is the ciphertext:Copy code 39 ff 02 86 b5 5e 00 c5 97 08 7b 0a 5d a1 c3 17

To decrypt the ciphertext, we simply perform the same steps in reverse order. We start by expanding the key and producing the set of round keys. Then we XOR the ciphertext with the last round key and pass the result through the inverse of the MixColumns, ShiftRows, and SubBytes stages in reverse order. We repeat this process for each main round, using the corresponding round key. Finally, we XOR the result with the first round key to obtain the plaintext.

Security of AES:

AES is a widely used encryption algorithm, and it is considered to be very secure. The security of AES is based on the fact that it is very difficult to reverse the encryption process without knowing the key. The key size is large enough that it would take an impractical amount of time to try all possible keys using brute-force methods.

The security of AES is also based on the fact that it is resistant to various attacks, such as differential cryptanalysis and linear cryptanalysis. These attacks are methods of analyzing encryption algorithms to try to find weaknesses that can be exploited to reveal the key or the plaintext.

Conclusion:

AES is a widely used symmetric-key encryption algorithm that is considered to be very secure. It operates on fixed-length blocks of data and uses a set of round keys to mix the input data with the key. The algorithm consists of three stages: key expansion, initial round, and main rounds. The main rounds use a combination of SubBytes, ShiftRows, MixColumns, and AddRoundKey operations to obscure any patterns that may exist in the data. The security of AES is based on the fact that it is very difficult to reverse the encryption process without knowing the key, and it is resistant to various attacks.