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Advanced Encryption Standard (AES)

Glossary Contents

The Advanced Encryption Standard (AES) is a specification for encrypting electronic data. Established by the U.S. National Institute of Standards and Technology (NIST) in 2001, AES has become a widely used algorithm in symmetric key cryptography. This glossary entry will delve into the intricacies of AES, its history, its structure, and its application in data privacy.

AES is a block cipher, which means it encrypts data in fixed-size blocks, in this case, 128 bits. It's based on a design principle known as a substitution-permutation network and is fast in both software and hardware. Unlike its predecessor, the Data Encryption Standard (DES), AES does not use a Feistel network. This glossary entry will provide a detailed understanding of these concepts and more.

History of AES

The journey of AES began in 1997 when NIST announced that it was looking for a successor to the ageing Data Encryption Standard (DES), which was becoming increasingly vulnerable to brute-force attacks. This prompted a call for candidates for a new Advanced Encryption Standard.

The selection process was fully open to public scrutiny and comment, ensuring a thorough, transparent analysis. Fifteen competing designs were submitted, and in 2000, Rijndael was selected as the proposed algorithm for AES.

The Rijndael Block Cipher

Rijndael, created by two Belgian cryptographers, Vincent Rijmen and Joan Daemen, was chosen for its combination of security, performance, efficiency, ease of implementation and flexibility. It was designed to handle additional block sizes and key lengths beyond those initially required for AES.

After a period of public consultation, NIST announced in October 2000 that it intended to propose Rijndael as the AES. The standard was subsequently approved in 2001.

Structure of AES

AES operates on a 4×4 column-major order matrix of bytes, termed the state. The key size used for an AES cipher specifies the number of repetitions of transformation rounds that convert the input, called the plaintext, into the final output, called the ciphertext. The number of rounds depends on the key size: 10 rounds for a 128-bit key, 12 rounds for a 192-bit key, and 14 rounds for a 256-bit key.

Each round consists of several processing steps, including one that depends on the encryption key. A set of reverse rounds is applied to transform ciphertext back into the original plaintext using the same encryption key.

Key Expansions in AES

Key expansions are the first step in the AES encryption process. The key is expanded into an array of forty-four 32-bit words, and the process consists of several steps as defined in the AES specification.

Each word is populated with data from the previous word, the word four positions back, and a round constant. This process is repeated until the entire key schedule has been filled.

Initial Round of AES Encryption

The initial round of AES encryption consists of a single operation: AddRoundKey. The 128 bits of the block are treated as sixteen bytes, and these bytes are mapped into the 4×4 array. Each byte of the state is combined with a block of the round key using bitwise xor.

This process is repeated for the number of rounds specified by the key size, with the final round omitting the MixColumns step.

Applications of AES

AES is widely used across many industries and applications to secure data. It is used in software and hardware to encrypt data in transit and at rest. It is used in secure file transfer protocols like FTPS, HTTPS, and SSH. It is also used in IPsec VPNs to secure network traffic and Wi-Fi networks to secure wireless traffic.

Moreover, AES is used in the encryption of sensitive data, such as credit card numbers and personal health information, in databases and file systems. It is also used in digital rights management (DRM) to prevent unauthorised use or reproduction of copyrighted material.

Government Use of AES

AES is used extensively by the U.S. government for protecting sensitive information. In fact, AES was developed as a result of a government competition specifically to create a new standard encryption method that would be widely available to the public.

Today, AES encryption is mandatory in government systems to protect classified information. It is also used in commercial systems that may handle sensitive government data.

Commercial Use of AES

In the commercial sector, AES is used in a wide range of applications, from securing financial transactions to protecting data stored on personal electronic devices. It is used in mobile and desktop operating systems, in secure email services, and in various types of software that require data protection.

Many commercial virtual private network (VPN) services use AES encryption to protect data in transit. It is also used in cloud storage services to protect data at rest.

Strengths and Weaknesses of AES

One of AES's main strengths is its resistance to all known attacks. This is due to its high number of rounds, large block size, and use of complex operations in the encryption process. As of today, the only successful attacks against AES have been side-channel attacks, which do not attack the underlying cipher but rather its implementation.

Another strength of AES is its flexibility. It can be used in a wide range of applications and is available in various software and hardware implementations. It is also scalable, with key sizes that can be increased for additional security.


AES has a strong security track record. It has been analysed extensively by the cryptographic community over the past two decades, and no practical attack has been discovered. This is a testament to the strength of its design and the thoroughness of the selection process.

Furthermore, AES is efficient in both software and hardware implementations. This is an important consideration for applications that require high-speed encryption, such as secure web browsing or VPNs.


Despite its strengths, AES has a few weaknesses. The most significant is its vulnerability to side-channel attacks. These attacks exploit information leaked during the encryption process, such as timing information or power consumption, rather than attacking the cipher itself.

Another potential weakness of AES is its key size. While a 128-bit key is currently considered secure, advances in computing power could make it vulnerable in the future. However, AES also supports 192-bit and 256-bit keys, which are currently considered secure against brute-force attacks.


The Advanced Encryption Standard (AES) is a robust and versatile encryption algorithm that has become a cornerstone of modern cryptography. Its combination of security, performance, and flexibility make it an excellent choice for protecting sensitive data in a wide range of applications.

Despite its vulnerabilities to side-channel attacks and potential future threats to its key size, AES remains the standard for data encryption and is likely to remain so for the foreseeable future. Its widespread adoption and continued analysis by the cryptographic community attest to its reliability and resilience.

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