# AES Decryption Algorithm: A Comprehensive Guide for Data Scientists

To implement AES decryption, you can leverage existing cryptographic libraries or APIs provided by programming languages such as Python or Java. These libraries often encapsulate the complex details of the AES decryption algorithm, allowing you to focus on the higher-level aspects of your application.

## What is the AES algorithm?

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## Background

In the 1990s, DES was rendered insecure due to its relatively small 56-bit key size. So, in 1997, the **Advanced Encryption Standard** (**AES**) was proposed in response to a public call for proposals by the National Institute of Standards and Technology (NIST). The table below shows how AES compares to its predecessor.

DES | AES | |
---|---|---|

Developed | 1977 | 2000 |

Cipher Type | Symmetric block cipher | Symmetric block cipher |

Block size | 64 bits | 128 bits |

Key length | 56 bits | 128/192/256 bits |

Security | Rendered insecure | Considered secure |

## What is the AES algorithm?

The **AES algorithm** (also known as the **Rijndael algorithm**) is a symmetrical block cipher algorithm that takes plain text in blocks of 128 bits and converts them to ciphertext using keys of 128, 192, and 256 bits. Since the AES algorithm is considered secure, it is in the worldwide standard.

## AES Decryption Algorithm: A Comprehensive Guide for Data Scientists

In the world of data security, encryption is a fundamental technique used to protect sensitive information. The Advanced Encryption Standard (AES) is a widely adopted symmetric encryption algorithm that ensures secure communication and data storage. In this article, we will dive deep into the AES decryption algorithm, explaining its inner workings and providing practical insights for data scientists and software engineers.

By **Saturn Cloud** | Tuesday, July 18, 2023 | Miscellaneous

## AES Decryption Algorithm: A Comprehensive Guide for Data Scientists

## Introduction

In the world of data security, encryption is a fundamental technique used to protect sensitive information. The Advanced Encryption Standard (AES) is a widely adopted symmetric encryption algorithm that ensures secure communication and data storage. In this article, we will dive deep into the AES decryption algorithm, explaining its inner workings and providing practical insights for data scientists and software engineers.

## Understanding AES Encryption

Before we delve into AES decryption, let’s briefly touch upon AES encryption. AES is a block cipher algorithm that operates on fixed-size blocks of data. It supports three key sizes: 128, 192, and 256 bits. To encrypt data with AES, a secret key is used to transform the plaintext into ciphertext, making it unreadable without the corresponding decryption key.

AES encryption consists of several rounds, with each round applying a set of operations to the data. These operations include substitution, permutation, and mixing of the data. The number of rounds depends on the key size: 10 rounds for 128-bit keys, 12 rounds for 192-bit keys, and 14 rounds for 256-bit keys.

## AES Decryption Process

AES decryption is the inverse process of encryption. It takes the ciphertext and the decryption key as input and produces the original plaintext. The decryption algorithm reverses the operations performed during encryption, allowing the recovery of the original data.

Let’s outline the steps involved in the AES decryption process:

**Key Expansion**: AES decryption starts with expanding the encryption key into a set of round keys. This step is essential to generate the correct round keys used in the reverse order compared to encryption.**Initial Round**: In the initial round, the ciphertext block is subjected to a reverse key addition, where the round key is XORed with the ciphertext.**Rounds**: AES decryption consists of multiple rounds, just like encryption. Each round involves four main operations: inverse substitution, inverse permutation, inverse mixing, and reverse key addition. These operations are performed in the reverse order compared to encryption.**Final Round**: The final round includes inverse substitution, inverse permutation, and reverse key addition. However, there is no inverse mixing operation in the final round.**Output**: After the final round, the decrypted plaintext is obtained, which is an exact replica of the original data before encryption.

## Implementing AES Decryption

To implement AES decryption, you can leverage existing cryptographic libraries or APIs provided by programming languages such as Python or Java. These libraries often encapsulate the complex details of the AES decryption algorithm, allowing you to focus on the higher-level aspects of your application.

In Python, you can use the pycryptodome library, which provides a straightforward interface for AES decryption:

`from Crypto.Cipher import AES def aes_decrypt(ciphertext, key): cipher = AES.new(key, AES.MODE_ECB) plaintext = cipher.decrypt(ciphertext) return plaintext `

In the above example, ciphertext represents the encrypted data, and key is the decryption key. The AES.new() function initializes the AES cipher with the specified key, and cipher.decrypt() performs the decryption operation, returning the plaintext.

## Best Practices for AES Decryption

To ensure the security and effectiveness of AES decryption, consider the following best practices:

**Key Management**: Properly manage encryption keys by using secure key storage mechanisms and regularly rotating keys. Avoid hardcoding keys in source code or storing them in plain text files.**Secure Transmission**: When transmitting ciphertext or decryption keys over a network, utilize secure communication protocols such as HTTPS or SSH to prevent eavesdropping or data interception.**Use Secure Libraries**: Always use trusted and well-vetted cryptographic libraries to implement AES decryption. These libraries undergo rigorous security testing and maintenance, reducing the risk of vulnerabilities.**Keep Software Up to Date**: Regularly update your software dependencies, including cryptographic libraries, to incorporate security patches and bug fixes. Outdated software may contain known vulnerabilities that attackers can exploit.**Secure the Environment**: Protect the systems and environments where AES decryption operations take place. Employ strong access controls, network segmentation, and intrusion detection systems to mitigate the risk of unauthorized access.

## Conclusion

In this article, we explored the AES decryption algorithm, an essential component of data security. We discussed the steps involved in AES decryption, the implementation of AES decryption in Python, and best practices for secure AES decryption.

As a data scientist or software engineer, understanding AES decryption allows you to work with encrypted data and develop secure applications that safeguard sensitive information. By following the best practices outlined in this article, you can ensure the integrity and confidentiality of data through robust AES decryption techniques.

Remember, AES encryption and decryption play a crucial role in securing data, and staying informed about cryptographic algorithms is vital in today’s digital landscape.

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## Steps in the AES Encryption Process

The encryption process uses a set of specially derived keys called round keys. These are applied, along with other operations, on an array of data that holds exactly one block of data?the data to be encrypted. This array we call the state array.

You take the following aes steps of encryption for a 128-bit block:

- Derive the set of round keys from the cipher key.
- Initialize the state array with the block data (plaintext).
- Add the initial round key to the starting state array.
- Perform nine rounds of state manipulation.
- Perform the tenth and final round of state manipulation.
- Copy the final state array out as the encrypted data (ciphertext).

The reason that the rounds have been listed as “nine followed by a final tenth round” is because the tenth round involves a slightly different manipulation from the others.

The block to be encrypted is just a sequence of 128 bits. AES works with byte quantities so we first convert the 128 bits into 16 bytes. We say “convert,” but, in reality, it is almost certainly stored this way already. Operations in RSN/AES are performed on a two-dimensional byte array of four rows and four columns. At the start of the encryption, the 16 bytes of data, numbered D0 ? D15, are loaded into the array as shown in Table A.5.

Each round of the encryption process requires a series of steps to alter the state array. These steps involve four types of operations called:

The details of these operations are described shortly, but first we need to look in more detail at the generation of the Round Keys, so called because there is a different one for each round in the process.

The cipher key used for encryption is 128 bits long. Where this key comes from is not important here; refer to Chapter 10 on key hierarchy and how the temporal encryption keys are produced. The cipher key is already the result of many hashing and cryptographic transformations and, by the time it arrives at the AES block encryption, it is far removed from the secret master key held by the authentication server. Now, finally, it is used to generate a set of eleven 128-bit round keys that will be combined with the data during encryption. Although there are ten rounds, eleven keys are needed because one extra key is added to the initial state array before the rounds start. The best way to view these keys is an array of eleven 16-byte values, each made up of four 32-bit words, as shown in Table A.6.

To start with, the first round key Rkey0 is simply initialized to the value of the cipher key (that is the secret key delivered through the key hierarchy). Each of the remaining ten keys is derived from this as follows.

Table A.6. Round Key Array