Definition

Advanced Encryption Standard (AES)

Contributor(s): Corinne Bernstein, Michael Cobb, GEM100, Borys Pawliw

The Advanced Encryption Standard, or AES, is a symmetric block cipher chosen by the U.S. government to protect classified information and is implemented in software and hardware throughout the world to encrypt sensitive data.

The National Institute of Standards and Technology (NIST) started development of AES in 1997 when it announced the need for a successor algorithm for the Data Encryption Standard (DES), which was starting to become vulnerable to brute-force attacks.

Essential for computer security, cybersecurity and electronic data protection, the newer, advanced encryption algorithm would be unclassified and had to be "capable of protecting sensitive government information well into the next century," according to the NIST announcement of the process for development of an advanced encryption standard algorithm. It was intended to be easy to implement in hardware and software, as well as in restricted environments (for example, in a smart card) and offer decent defenses against various attack techniques.

How AES encryption works

AES comprises three block ciphers: AES-128, AES-192 and AES-256.

AES-128 uses a 128-bit key length to encrypt/decrypt a block of messages, AES-192 uses a 192-bit key length and AES-256 a 256-bit key length to encrypt/decrypt messages. Each cipher encrypts and decrypts data in blocks of 128 bits using cryptographic keys of 128-, 192- and 256-bits, respectively.

Symmetric (also known as secret-key) ciphers use the same key for encrypting and decrypting, so the sender and the receiver must both know -- and use -- the same secret key. All key lengths are deemed sufficient to protect classified information up to the "Secret" level with "Top Secret" information requiring either 192- or 256-bit key lengths. There are 10 rounds for 128-bit keys, 12 rounds for 192-bit keys and 14 rounds for 256-bit keys -- a round consists of several processing steps that include substitution, transposition and mixing of the input plaintext and transform it into the final output of ciphertext.

AES Design
A visual chart describing the relationships between secret key, plaintext, cipher, and ciphertext.

The AES encryption algorithm defines a number of transformations that are to be performed on data stored in an array. The first step of the cipher is to put the data into an array; after which, the cipher transformations are repeated over a number of encryption rounds. The number of rounds is determined by the key length, with 10 rounds for 128-bit keys, 12 rounds for 192-bit keys and 14 rounds for 256-bit keys.

The first transformation in the AES encryption cipher is substitution of data using a substitution table; the second transformation shifts data rows, the third mixes columns. The last transformation is a simple exclusive or operation performed on each column using a different part of the encryption key. Longer keys need more rounds to complete.

AES features

The selection process for the new symmetric key algorithm was fully open to public scrutiny and comment; this ensured a thorough, transparent analysis of the designs submitted.

NIST specified the new Advanced Encryption Standard algorithm must be a block cipher capable of handling 128-bit blocks, using keys sized at 128, 192, and 256 bits; other criteria for being chosen as the next Advanced Encryption Standard algorithm included:

  • Security:Competing algorithms were to be judged on their ability to resist attack, as compared to other submitted ciphers, though security strength was to be considered the most important factor in the competition.
  • Cost:Intended to be released on a global, nonexclusive and royalty-free basis, the candidate algorithms were to be evaluated on computational and memory efficiency.
  • Implementation:Factors to be considered included the algorithm's flexibility, suitability for hardware or software implementation and overall simplicity.

Choosing AES algorithms

Fifteen competing symmetric key algorithm designs were subjected to preliminary analysis by the world cryptographic community, including the National Security Agency (NSA). In August 1999, NIST selected five algorithms for more extensive analysis. These were:

  • MARS, submitted by a large team from IBM Research
  • RC6, submitted by RSASecurity
  • Rijndael, submitted by two Belgian cryptographers, Joan Daemen and Vincent Rijmen
  • Serpent, submitted by Ross Anderson, Eli Biham and Lars Knudsen
  • Twofish, submitted by a large team of researchers from Counterpane Internet Security, including noted cryptographer Bruce Schneier

Implementations of all of the above were tested extensively in ANSI, C and Java languages for speed and reliability in the encryption and decryption processes; key and algorithm setup time; and resistance to various attacks, both in hardware- and software-centric systems. Detailed analyses were conducted by members of the global cryptographic community, including some teams that tried to break their own submissions.

After much feedback, debate and analysis, the Rijndael cipher was selected as the proposed algorithm for AES in October 2000, published by NIST as U.S. Federal Information Processing Standards (FIPS) PUB 197. The Advanced Encryption Standard became effective as a federal government standard in 2002. It is also included in the International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC) 18033-3 standard, which specifies block ciphers for the purpose of data confidentiality.

In June 2003, the U.S. government announced that AES could be used to protect classified information, and it soon became the default encryption algorithm for protecting classified information, as well as the first publicly accessible and open cipher approved by the NSA for top-secret information. The NSA chose AES as one of the cryptographic algorithms to be used by its Information Assurance Directorate to protect national security systems.

Its successful use by the U.S. government led to widespread use in the private sector, leading AES to become the most popular algorithm used in symmetric key cryptography. The transparent selection process helped create a high level of confidence in AES among security and cryptography experts. AES is more secure than its predecessors -- DES and 3DES -- as the algorithm is stronger and uses longer key lengths. It also enables faster encryption than DES and 3DES, making it ideal for software applications, firmware and hardware that may require either low latency or high throughput (such as firewalls and routers). It is used in many protocols such as Secure Sockets Layer (SSL)/Transport Layer Security (TLS) and can be found in most modern applications and devices that need encryption functionality.

Difference between AES-128 and -256

Overall, security experts consider AES safe against brute-force attacks, in which all possible key combinations are checked until the correct key is found. However, the key size employed for encryption needs to be large enough so that it cannot be cracked by modern computers, even considering advancements in processor speeds based on Moore's law.

A 256-bit encryption key is significantly more difficult for brute-force attacks to guess than a 128-bit key; however, because the latter takes so long to guess even with a huge amount of computing power, it is unlikely to be an issue for the foreseeable future, as a hacker would need to use quantum computing to generate the necessary brute force.

Still, 256-bit keys also require more processing power and can take longer to execute. When power is an issue (particularly on small devices) or where latency is likely to be a concern, 128-bit keys are likely to be a better option.

When hackers want to access a system, they will aim for the weakest point, which is typically not the encryption, regardless of whether it's a 128-bit key or a 256-bit key. The user should make sure the software under consideration does what he or she wants it to do, that it protects user data in the way it's expected to and that the overall process has no weak points. Additionally, there should be no gray areas or uncertainty about data storage and handling. If data resides in the cloud, for instance, the user should know the location of the cloud. Most importantly, the security software that has been selected should be easy to use to ensure that users have no need to perform unsecure workarounds to do their jobs.

AES vs. RSA

AES is used widely for protecting data at rest. Applications for AES include self-encrypting disk drives, database encryption and storage encryption. On the other hand, RSA encryption is often used in web browsers to connect to websites, in virtual private network connections and in many other applications.

Unlike AES, which employs symmetric encryption, RSA is an asymmetric encryption method. Symmetric encryption involves converting plaintext to ciphertext using the same key, or secret key, to encrypt and decrypt it. On the other hand, the term asymmetric comes from the fact that there are two related secrets, or keys, used for encryption: a public and a private key. If encryption is used with the public key, decryption can only happen with the related private key, and vice versa. Typically, RSA keys are employed when there are two separate endpoints.

While RSA encryption works well for protecting the transfer of data across geographic boundaries, its performance is poor. The solution is to combine RSA encryption with AES encryption in order to benefit from the security of RSA with the performance of AES. This can be accomplished by generating a temporary AES key and protecting it with RSA encryption.

AES vs. DES

The U.S. government developed Data Encryption Standard algorithms more than 40 years ago to ensure government systems all used the same, secure standard to facilitate interconnectivity. DES had served as the linchpin of government cryptography for years until 1999, when researchers broke the algorithm's 56-bit key using a distributed computer system. In 2001, the U.S. government chose to use AES to protect classified information. DES is still used in some instances for backward compatibility.

The two standards are both symmetric block ciphers, but AES is more mathematically efficient. The main benefit of AES lies in its key length options. The time required to crack an encryption algorithm is directly related to the length of the key used to secure the communication (128-bit, 192-bit or 256-bit keys). Therefore, AES is exponentially stronger than the 56-bit key of DES. AES encryption is also significantly faster, so it is ideal for applications, firmware and hardware that require low latency or high throughput.

Attacks on AES encryption

Research into attacks on AES encryption has continued since the standard was finalized in 2000. Various researchers have published attacks against reduced-round versions of the Advanced Encryption Standard.

Researchers have found a few potential ways to attack AES encryption. In 2009, they discovered a possible related-key attack. This cryptanalysis attempted to crack a cipher by studying how it operates using different keys. The related-key attack proved to be a threat only to AES systems that are incorrectly configured.

The year 2009 also saw a known-key distinguishing attack against AES-128. A known-key was used to discern the structure of the encryption. However, the hack only targeted an eight-round version of AES-128, rather than the standard 10-round version, making the threat relatively minor.

A major risk to AES encryption comes from side-channel attacks. Rather than attempting a brute-force assault, side-channel attacks are aimed at picking up leaked information from the system. Side-channel attacks, however, may reduce the number of possible combinations required to attack AES with brute force. Side-channel attacks involve collecting information about what a computing device does when it is performing cryptographic operations and using that information to reverse-engineer the device's cryptography system. These attacks may use timing information (how long it takes the computer to perform computations), electromagnetic leaks, audio clues and optical information (for example, from a high-resolution camera) to discover extra information about how the system is processing the AES encryption. In one case, a side-channel attack was used successfully to deduce AES-128 encryption keys by carefully monitoring the cipher's shared use of the processors' cache tables.

Side-channel attacks can be mitigated by preventing possible ways data can leak. Additionally, using randomization techniques can help eliminate any relationship between data protected by the cipher and any leaked data that could be collected using a side-channel attack.

AES security

Security experts maintain that AES is secure when implemented properly. In addition, AES encryption keys need to be protected. Even the most extensive cryptographic systems can be vulnerable if a hacker gains access to the encryption key. Use of strong passwords, password managers, multifactor authentication, firewalls and antivirus software is critical to enterprise security. Employees should also be trained in ways to prevent social engineering and phishing attacks.

This was last updated in February 2020

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Do you have any concerns about theoretical attacks against AES becoming a reality? Should the industry move to accommodate alternative ciphers such as Threefish or the Salsa20 stream cipher?
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In this universe, there are no absolutes. Likewise in the world of encryption. As computing power increases at an exponential rate, ciphers become increasingly subject to being broken. There's no question that we have very robust ciphers, per se, but when these are broken, it is usually because of mistakes in their implementation.

AES will probably eventually be successfully attacked in situ. Someone will figure it out. Simply moving to alternatives will make little difference in the overall scheme of things: Crackers will someday figure out a way to break them, too. The answer to the question, "when is someday?" we have no way of predicting.

Keeping encryption robust is a cat and mouse game just like fighting malware and securing systems and networks.

Read Security Corner for more practical security advice.
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What is to prevent someone from coming in a backdoor, leaving a utility like heapmemdump running as a thread and capturing the AES keycode? This has always bothered me, that any PC connected to the Net would be open to this exploit. Why crack the key when you can snatch it right out of memory. Isn't this essentially how the Cisco routers were exploited? A recent article mentioned HP audio drivers were found to have a keylogger embedded that was writing keystrokes to a file. Any PC connected to the Net, or allowing a memory stick to be inserted, is subject to this kind of exploitation. Let me know if I am wrong.
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How could the selection process used by NIST for the Advanced Encryption Standard be improved?
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Is this practically possible by using matlab?
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Are designer and Advanced Encryption Standard stream ciphers or they use block cipher encryption?
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