AES Encryption Explained: What Is AES & How Does It Work?

The Advanced Encryption Standard (AES) is a symmetric encryption algorithm used by governments, corporations, and security software to protect highly-sensitive data.

AES encryption converts plain text or data into encrypted and secure ciphertext. The data is encrypted by the sender and can only be decrypted by the intended recipient.

AES is a symmetric cipher, which means it uses the same key for both encryption and decryption. AES is considered the industry standard for symmetric cipher encryption because it is fast and basically unbreakable.

If data that’s encrypted using AES is intercepted during transmission, it can’t be read or used without the encryption key.

AES was developed by The National Institute of Standards and Technology (NIST) to replace the Data Encryption Standard (DES) after it became vulnerable to modern brute force attacks.

The National Security Agency (NSA) then approved the AES encryption algorithm to protect the most sensitive government data.

Here’s a comparison of AES and DES based on their most important attributes:

Attributes	AES	DES
Year of Introduction	2001	1977
Key Length	128, 192, 256 bits	56 bits
Block Size	128 bits	64 bits
Structure	Substitution–permutation network	Feistel cipher
Security	Considered secure for most purposes	Vulnerable to brute-force attacks
Performance	Slower than DES due to larger block size, but more secure	Faster but less secure
Use Cases	Broad use in government, commercial sectors for secure data	Limited use, often replaced by AES or 3DES
Developed By	National Institute of Standards and Technology (NIST), USA	National Bureau of Standards, USA
Known Attacks	Side-channel attacks, Related-key attacks (under certain conditions)	Brute force, Differential cryptanalysis, Linear cryptanalysis

The Different Types of AES: Key Lengths Explained

In cryptography, “key length” refers to the size of the secret key used in an encryption algorithm. The key is measured in “bits” and determines the output of the encryption function.

Key length is directly related to the strength of the security provided by the encryption algorithm.

In general, the longer the key, the more secure the encrypted data. A longer key means that there are more possible keys that an attacker would have to try in order to break the encryption.

AES makes use of three different encryption key lengths:

• 128-bit key length
• 192-bit key length
• 256-bit key length

At the simplest level, these three key lengths represent increasingly more robust levels of encryption, with AES-256 being the most secure.

The numbers 128, 192, and 256 represent the number of plaintext blocks the algorithm can turn into ciphertext. For example, AES-256 can turn 256 plaintext blocks into 256 ciphertext blocks, applying different cryptographic keys to each block.

When the AES algorithm converts data into ciphertext, the conversion is done in rounds; 10 rounds for the 128-bit key length, 12 for the 192-bit, and 14 rounds for 256-bit.

Here’s a diagram explaining this process:

The more rounds of encryption the plaintext goes through, the more difficult it is for someone without the correct decryption key to access the encrypted data. That’s what makes 256-bit the most secure version of AES.

Where Is AES Encryption Used, and What For?

AES is an open standard, which means it is freely available for use by public and private companies for both commercial and non-commercial use.

Originally, AES was established by The National Institute of Standards and Technology (NIST) and ratified by the NSA to secure government and military data marked Secret or Top Secret.

Since it’s an open standard, it has since been adopted by a wide range of companies and organizations, including major corporations, VPN services, global banks, national security organizations, and the military. Companies like Google, AWS, Oracle, and IBM all use AES to secure their data.

AES is also commonly used to protect customer payment data and other sensitive information in mobile apps. WhatsApp, Snapchat, and Facebook messages are all encrypted using AES.

As of 2023, AES encryption has now become the standard for secure encrypted browsing and general file encryption.

Is AES the Best Encryption Method?

There’s a lot to consider when comparing AES to other encryption methods, including performance, compatibility, and the specific requirements of a given application.

Generally speaking, AES is considered to be one of the most secure and practical options in most cases. The “most secure” cipher can depend on the specific use-case, but the gold standard for symmetric-key encryption is generally AES-256.

AES-256 has become the industry-standard for encryption because it remains impervious to brute-force attacks, even after over 20 years.

AES is also easy to implement, making it popular with businesses and developers. It requires less computational power than most alternatives, which means it’s exceptionally efficient at encrypting large amounts of data.

Other commonly used algorithms include 3DES and RSA, but these serve different purposes. Here’s a table comparing the AES algorithm to other similar forms of encryption:

Encryption Algorithm	Type	Key Sizes	Applications	Pros	Cons
AES (Advanced Encryption Standard)	Symmetric	128-bit, 192-bit, 256-bit	Secure storage, Secure data transmission, U.S. Government for encrypting classified information	Very secure, Fast, Efficient, Approved for sensitive government data	Vulnerable to side-channel attacks in certain implementations
3DES (Triple Data Encryption Standard)	Symmetric	168 bits (effectively 112 bits due to meet-in-the-middle attacks)	Finance, Some Microsoft offerings	More secure than DES	Slower than AES, Less secure than AES, Set to be retired
RSA (Rivest–Shamir–Adleman)	Asymmetric (public key)	Up to 4096 bits	SSL/TLS, Digital signatures, Secure email	Can be used for encryption and digital signatures, Doesn’t require key sharing	Slower than symmetric algorithms, Large key sizes
ECDSA (Elliptic Curve Digital Signature Algorithm)	Asymmetric (public key)	Variable, based on the chosen curve	SSL/TLS, Bitcoin and other cryptocurrencies, SSH	More efficient than RSA at equivalent security levels	More complex, Potential issues if curve parameters aren’t chosen carefully

3DES — or “Triple DES” — is another symmetric key algorithm that was developed as a more secure version of DES. It works by running the data through the DES algorithm three times with three different keys.

3DES is slower than AES and is no longer considered as secure. While it’s still used in some sectors like finance, it’s expected to be retired soon.

RSA, on the other hand, is a public-key encryption algorithm and is used in different contexts compared to AES. It’s typically used for secure data transmission rather than data encryption at rest. The gold standard for public-key encryption is RSA-4096.

It’s important to note that the security of these algorithms also depends heavily on proper implementation and key management. AES, for instance, is considered secure, but various implementations have been subject to side-channel attacks.

It’s worth noting that this is a simplified comparison, and it does not cover all aspects of these encryption methods. Their actual security and performance can depend heavily on the circumstances of how each algorithm is implemented and used — the “best” method often depends on the specific requirements of the application in question.

To fully understand how AES encryption works, you’ll need to understand its various features and how they work together. In this section, we break them down in detail:

Substitution-permutation network (SPN)

To encrypt your data, AES uses a series of linked mathematical operations called a Substitution-Permutation Network (SPN).

As we’ve already mentioned, the different levels of AES use one or more “rounds” of encryption.
During these rounds, AES uses substitution boxes and permutation boxes to change the blocks of plaintext into ciphertext blocks.

Substitution boxes act as a substitution cipher, while permutation boxes work as a transposition cipher, shifting and mixing blocks and columns to diffuse the data.

AES-256 uses 14 rounds of transposition, substitution, and mixing to turn plaintext into ciphertext, making it an incredibly secure method of encrypting sensitive data.

Key expansion

Key expansion is connected to the use of a substitution-permutation network. It uses the initial key to generate a new key for each round, called a round key.

New round keys are generated for each round of modification, with each successive round making the encryption harder to crack.

Byte Substitution

AES employs byte substitution to further increase the complexity of its encryption process.

Byte substitution works by substituting every byte in the initial data with a code generated by a pre-established Rijndael S-box table. This nonlinear modification of the plaintext hides any relationship between it and the ciphertext.

Key length

There are three different lengths of AES encryption keys, each with its own possible number of key combinations:

• 128-bit key length: 3.4 x 1038
• 192-bit key length: 6.2 x 1057
• 256-bit key length: 1.1 x 1077

The number of possible key combinations comes into play during a brute-force attack, where a bad actor attempts to check all possible key combinations to guess the correct encryption key.

The keys used in AES-256 encryption have 1077 possible combinations, which would take around a billion years to brute-force with current computers.

Now that we’ve explained some of the AES algorithm’s key features, we can explain how it actually works to encrypt your data step-by-step:

On the very last round, step 5 is omitted, as its output does not add any further security and is considered wasteful processing.

One common use of AES is the encryption of web traffic via a Virtual Private Network (VPN).

The very best VPN services use AES encryption as part of their tunneling protocols because it is considered the most secure encryption standard on the market. This is one of the reasons it’s uncommon for VPN services to be hacked.

When a user connects to their VPN client, that client establishes an encrypted tunnel to their chosen VPN server.

The user’s requests (to visit a website, for example) are actioned by that server, and the data gathered is encrypted and sent via the tunnel to the client, where it is decrypted. You can find a basic overview of this process in our guide explaining how VPNs work.

The use of AES in this context provides a very high level of security. Because AES is a symmetric key encryption standard, both the VPN client and the VPN server use the same key for encrypting and decrypting data. This key is unique for each VPN session and is securely exchanged at the start of the session.

In practice, this means that if a malicious actor was able to intercept your requests and the data therein, all they would get is jumbled and unusable ciphertext.

It’s also worth noting that the use of AES is just one part of a VPN’s overall security model. Other important components include the secure exchange of the encryption key, the use of secure tunneling protocols, and different authentication methods.

Put simply, AES encryption is functionally unbreakable in most cases. Because of the sheer number of key combinations, the time and resources needed to guess all the key combinations are astronomical.

Add in the nonlinear way in which AES scrambles the plaintext into ciphertext in multiple rounds, using a discrete key for each round, and the ciphertext is impossible to decode without the decryption key.

Using the highest standard of current computing, it would take 36 quadrillion years to brute-force a 128-bit AES encryption key — the lowest level of AES encryption. By comparison, cracking DES (the standard that AES replaced) using the same machine would take just 362 seconds.

Can AES Be Decrypted?

It’s theoretically possible to decrypt AES-encrypted data. If you had infinite time and resources, it is mathematically possible to try every key combination and eventually identify the right one.

However, it’s practically impossible. Unless you’ve got a billion years to keep trying different code combinations, AES can’t be decrypted without the correct decryption key.

AES-256 is also believed to be quantum resistant, which means even quantum computers can’t reduce the time taken by enough to crack the encryption. It’s important to note that this only applies to AES-256, not AES-192 or 128.

However, there are ways that malicious actors can circumvent the protection offered by AES encryption, which we’ll explain in the next section.

There are three possible attacks that are considered effective at bypassing AES encryption:

• Side-channel attacks: Side-channel attacks monitor incorrectly-implemented computer systems for data on how an encryption algorithm is used. Data such as power consumption, number of cache accesses, computation timing, and other factors can be used to break cryptography.
• Related-key attacks: Related-key attacks occur when the hacker knows the difference between two different keys. This type of attack has been unsuccessfully tried before and resulted in the complexity of the AES key schedule being upgraded.
• Known-key attacks: Known-key attacks require the hacker to know the encryption key, which is highly unlikely. Even then, tests of a known key attack only worked on 8 out of the 10 rounds of encryption used by 128-bit key length AES, which is the lowest security form of the algorithm.

How to Ensure the Security of AES Encryption Keys

Most of the attacks that have the potential to bypass AES encryption rely on getting access to secure data, like encryption keys.

The good news is that there are simple steps you can take to prevent this from happening:

• Use strong passwords: NIST recommends that passwords should be at least 12 characters long and include a mix of upper-case letters, lower-case letters, numbers, and special characters.
• Use hardware tokens: These physical devices store the AES encryption keys, which can only be unlocked with a specific PIN or biometric authentication. This prevents the keys from being accessed by non-authorized personnel.
• Use password managers: Password managers are software programs that store AES encryption keys in an encrypted form and require a single master password to access them. This is a convenient and secure way to protect your AES encryption keys.
• Use multi-factor authentication: Multi-factor authentication (MFA) requires users to provide two or more pieces of evidence to verify their identity. This can include things like fingerprints, voice recognition, SMS verification codes, and other forms of biometric authentication.
• Deploy firewalls and anti-malware software: These security measures can help protect against outside attacks and malicious software. Firewalls can block access to specific IPs, while anti-malware software can scan for malware and prevent it from entering the system.
• Security awareness training for employees: Encryption keys should never be shared with anyone, and all employees should understand the importance of security best practices when it comes to handling sensitive data. Security awareness training can help ensure that everyone is aware of the risks and how to protect against them.