Everything You Need to Know About AES-256 Encryption

When national security is involved, strong encryption measures are taken to protect data. The Advanced Encryption Standard (AES), originally adopted by the U.S. federal government, has evolved to become an industry standard for securing data and needs to be part of every organization’s integrated risk management strategy. AES comes in 128-bit, 192-bit, and 256-bit implementations, with the 256-bit implementation being the most secure. This article explains what AES-256 encryption entails, how it works, and how secure it is. It also discusses how double encryption bolsters security and protects private content from malicious cyberattacks.

What Is AES-256 Encryption?

The Advanced Encryption Standard (AES) is a symmetric block cipher that the U.S. government selects to protect classified data. AES-256 encryption uses the 256-bit key length to encrypt as well as decrypt a block of messages. There are 14 rounds of 256-bit keys, with each round consisting of processing steps that entail substitution, transposition, and mixing plaintext to transform it into ciphertext.

The National Institute of Standards & Technology (NIST) started to develop AES in 1997 when the need arose to create an alternative standard to the Data Encryption Standard (DES). DES had started to become vulnerable to brute-force attacks.

The AES encryption standard was approved by the National Security Agency (NSA) to protect both secret and top-secret government information. It has since become an industry standard for encrypting information. It is an open standard, which means it can be used for public, private, commercial, and non-commercial implementations.

The Evolution of Encryption Standards

The landscape of data security constantly evolves, necessitating stronger encryption methods. For many years, the Data Encryption Standard (DES), adopted in 1977, was the primary symmetric encryption algorithm used by the U.S. government and various industries. However, DES utilized a relatively small 56-bit key.

As computing power increased exponentially, by the mid-1990s, DES became susceptible to brute-force attacks, where attackers could systematically try all possible keys. Recognizing this vulnerability, the U.S. National Institute of Standards & Technology (NIST) initiated a public process in January 1997 to develop a replacement. The goal was to find a new, more secure algorithm that offered better performance and flexibility.

NIST solicited candidate algorithms from cryptographers worldwide, specifying criteria such as security strength, computational efficiency, memory requirements, and flexibility across different hardware and software platforms. Fifteen candidate algorithms were submitted. After rigorous analysis and public review spanning several years, NIST announced in October 2000 that the Rijndael algorithm, developed by Belgian cryptographers Joan Daemen and Vincent Rijmen, was selected as the winner.

Rijndael was chosen for its combination of security, performance, efficiency, ease of implementation, and flexibility (supporting different key and block sizes). It officially became the Advanced Encryption Standard (AES) and was published as FIPS PUB 197 in November 2001. The open nature of the AES selection process and the resulting standard itself fostered trust and led to its rapid global adoption, making robust `aes encryption` widely available.

How Secure Is AES-256 Encryption?

AES-256 encryption is extremely secure. It is the most secure encryption algorithm available today and is used extensively in government and military applications, as well as by businesses operating in highly regulated industries. The encryption has a key size of 256 bits, which is considered virtually uncrackable—even with the most advanced computing power and algorithms. It is also the same level of security used by banks and other financial institutions to protect sensitive customer information.

AES-256 vs. Other Encryption Variants

The Advanced Encryption Standard (AES) specifies three key lengths: 128-bit, 192-bit, and 256-bit. The primary difference lies in the key size and the number of encryption rounds performed.

AES-128 uses a 128-bit key and undergoes 10 rounds of encryption. AES-192 uses a 192-bit key and 12 rounds. AES 256 encryption, the strongest variant, utilizes a 256-bit key and completes 14 rounds.

The security level increases significantly with key length; a 256-bit key offers astronomically more possible combinations (2^256) than a 128-bit key (2^128), making brute-force attacks computationally infeasible with current technology.

This increased security comes at a slight performance cost: 256 bit encryption requires more processing power and takes marginally longer to encrypt and decrypt data compared to AES-128 due to the larger key size and additional rounds.

AES-128 offers a strong balance of security and performance, suitable for many applications like general file encryption or securing web traffic where speed is a factor. AES-192 provides an intermediate level. AES-256 is preferred for maximum security, often required for protecting highly sensitive data, government classified information (up to TOP SECRET), long-term data archiving, and meeting stringent regulatory requirements.

Compared to asymmetric algorithms like RSA, AES (being symmetric) is significantly faster, making it ideal for encrypting large volumes of data, while RSA is often used for key exchange or digital signatures.

Is AES Encryption Symmetric or Asymmetric?

AES encryption is a symmetric cryptography algorithm. This means that the encryption and decryption process uses the same key for both processes. AES has been the standard for symmetric encryption for the last few decades, and is still widely used today for its secure encryption capabilities. AES is fast and secure, making it a popular choice for encrypting files and other sensitive data.

What Are the Encryption Features of AES?

 AES consists of several main features:

Substitution-permutation (SP) Network

AES-256 encryption is based on a substitution-permutation network, also known as an SP network. The encryption works on an SP network structure rather than a Feistel cipher structure that uses the same basic algorithm for both encryption and decryption. 

Key Expansion

The algorithm takes a single key up during the first stage. This is later expanded to multiple keys used in each round. 

Byte Data

The AES encryption algorithm operates on byte data instead of bit data. This means that it treats the 128-bit block size as 16 bytes during the encryption process. 

Key Length

The number of rounds of encryption to be carried out depends on the key length being used to encrypt data. The 256-bit key size has 14 rounds.

Common AES Encryption Modes

There are a number of AES encryption modes organizations can choose from to protect their sensitive data at rest. These are some of the most common:

• Electronic Codebook (ECB): The simplest mode. Each block of plaintext is encrypted independently using the same key. Identical plaintext blocks result in identical ciphertext blocks, revealing patterns. Security Risk: Not recommended for most applications due to its deterministic nature.
• Cipher Block Chaining (CBC): Each plaintext block is XORed with the previous ciphertext block before encryption. An Initialization Vector (IV) is used for the first block to ensure uniqueness. Security: More secure than ECB as it hides patterns. Widely used, often seen as `aes-256-cbc`. Requires padding for the last block. Vulnerable to padding oracle attacks if not implemented carefully.
• Cipher Feedback (CFB): Turns AES into a self-synchronizing stream cipher. Encrypts the previous ciphertext block (or IV for the first block) and XORs the result with the current plaintext block. Allows encryption of data units smaller than the block size.

Output Feedback (OFB): Also turns AES into a synchronous stream cipher. Generates a keystream by repeatedly encrypting an IV, which is then XORed with the plaintext. Transmission errors do not propagate.

• Counter (CTR): Another way to create a stream cipher. Encrypts successive values of a “counter” (combined with a nonce) to generate a keystream, which is XORed with the plaintext. Allows parallel encryption/decryption and random access to blocks. Highly regarded for performance and flexibility.
• Galois/Counter Mode (GCM): An authenticated encryption mode. Provides both confidentiality (like CTR mode) and data authenticity/integrity checking. Uses a universal hash function for authentication. Widely recommended for its security and efficiency, often used in protocols like TLS 1.2/1.3 and IPsec. Requires a unique nonce for each encryption with the same key.

How Does AES-256 Encrypt Your Data?

Since AES is a symmetric key cipher, it uses the same secret key for both encryption and decryption. This means that both the sender and receiver of the data in question need a copy of the secret key. Symmetric keys are better suited for internal transfers, unlike asymmetric keys, which are best for external transfers. Symmetric key ciphers, such as AES, are faster and more efficient to run since they require less computational power than asymmetric key algorithms.

Additionally, AES uses block ciphers, where the plaintext is divided into sections called blocks. AES uses a 128-bit block size, whereby data is divided into 4-by-4 arrays that contain 16 bytes. Each byte contains 8 bits, with the total bits in every block being 128. In AES, the size of encrypted data remains the same. This means that 128 bits of plaintext yield 128 bits of ciphertext.

In all encryption, each unit of data is replaced by a different unit according to the security key used. AES is a substitution-permutation network that uses a key expansion process where the initial key is used to come up with new keys called round keys. The round keys are generated over multiple rounds of modification. Each round makes it harder to break the encryption. The AES-256 encryption uses 14 such rounds.

AES works by having the initial key added to a block using an exclusive or (XOR) cipher. This is an operation that is built into processor hardware. In the block, each byte of data is substituted with another, following a predetermined table. The rows of the 4-by-4 array are shifted, with the bytes in the second row being moved one space to the left. Bytes in the third row are moved two spaces, and the ones in the fourth row moved three spaces. The columns are then mixed, combining the four bytes in each column, and the round key is added to the block. The process is repeated for each round, yielding a ciphertext that is completely different from the plaintext.

This encryption algorithm features the following advantages:

• Using a different key for every round yields a much more complex result
• Byte substitution modifies the data in a nonlinear way, thus hiding the relationship between plaintext and ciphertext.
• Shifting rows and mixing columns diffuses data, thus transposing bytes. This further complicates the encryption.

The result of these processes is secure data exchange. The same process is repeated in reverse for the decryption process.

AES-256 Decryption Process

AES ciphertexts can be restored to the initial state by the help of inverse encryption. As we have seen above, the AES uses symmetric encryption, meaning that the secret key used for encryption is the same one used for decryption.

In the case of AES-256 decryption, the process begins with the inverse round key. The algorithm then reverses every action, namely: shifting rows, byte substitution, and column mixing, until it deciphers the original message.

Is AES-256 Encryption Crackable?

AES-256 encryption is virtually uncrackable using any brute-force method. It would take millions of years to break it using the current computing technology and capabilities.

However, no encryption standard or system is completely secure. In 2009, a cryptanalysis discovered a possible related-key attack. In such an attack, attackers try to crack a cipher by observing how it operates by using different keys. Luckily, experts have since concluded that such a threat can only happen in AES systems that are not configured correctly.

Since it is almost impossible to crack the AES cipher using a brute-force method, the main risk to this standard is side-channel attacks. In these attacks, attackers try to pick up information leaking from a system to discover how the encryption algorithms work. However, this can only happen in non-secure systems. A sound AES-256 implementation safeguards a system from side-channel attacks.

Much as the AES-256 standard is very secure, a vulnerable system can lead to an attacker gaining the secret key itself. A zero-trust security approach ensures that organizations trust and verify digital communications that exchange data. Further, organizations must take a defense-in-depth approach to security that employs multi-factor authentication, hardened infrastructure, and proactive and integrated incident response.  Incoming sensitive content communications must be verified using data loss prevention, antivirus, and anti-malware capabilities, while outbound sensitive content communications should also leverage data loss prevention. These capabilities should be involved in any cyber risk management approach.

The open nature of the AES-256 standard makes it one of the most secure encryption standards. Cybersecurity experts are on the constant lookout for potential vulnerabilities, and when a vulnerability is discovered, users are notified and action is taken to address the issue.

Practical Applications of AES-256 Encryption

AES 256 encryption is ubiquitous in modern digital systems due to its robust security and efficiency. Key application areas include:

• Government and Military: Protecting classified information, secure communications, and tactical systems, often mandated for sensitive data.
• Finance: Securing financial transactions, customer account data, and internal banking systems to prevent fraud and comply with regulations like PCI DSS.
• Healthcare: Encrypting Protected Health Information (PHI) both at rest (in databases, EHRs) and in transit (secure messaging, telemedicine) to comply with HIPAA regulations.
• Cloud Storage and Services: Major providers like AWS, Google Cloud, and Azure use AES-256 to encrypt customer data stored on their platforms, offering encryption at rest and sometimes in transit.
• Telecommunications: Securing mobile communications, voice-over-IP (VoIP), and network traffic through protocols like TLS/SSL and IPsec which frequently employ AES.
• Consumer Technology: Encrypting data on laptops (full-disk encryption like BitLocker, FileVault), smartphones, securing Wi-Fi networks (WPA2/WPA3), protecting messages in apps like WhatsApp and Signal, and securing files in compression tools (e.g., 7-Zip, WinZip).
• Software Development: Used within applications to protect sensitive configuration data, user credentials, and application data.

The strength of AES-256 makes it suitable where long-term data confidentiality is critical. However, the security relies heavily on correct implementation, including proper key management and mode selection, often integrated within broader security frameworks incorporating authentication and data integrity checks.

Examples of Where AES-256 Encryption Is Currently Being Used

Following are some of the use cases for AES-256 encryption:

1. U.S. government entities such as the NSA, the military, and many other entities use AES encryption for secure communication and storage of data.
2. Many devices, applications, and networks today use AES-256 encryption to protect data at rest and in transit. Many SSDs employ AES encryption algorithms.
3. All data stored in the Google Cloud is encrypted using the AES-256 standard by default.
4. AWS, Oracle, and IBM also use the AES-256 encryption standard.
5. WhatsApp messages are encrypted using the AES-256 encryption standard.

Implementing AES-256 Encryption: Best Practices

While AES 256 encryption itself is incredibly strong, its real-world security depends entirely on proper implementation. We encourage you to consider the following best practices:

1. Use Standard Libraries: Avoid implementing cryptographic algorithms yourself. Use well-vetted, standard cryptographic libraries provided by your programming language or platform (e.g., OpenSSL, Bouncy Castle, Java Cryptography Architecture, .NET Cryptography). These libraries are rigorously tested and updated.
2. Secure Key Management: This is paramount. Practice secure aes key generation using a cryptographically secure pseudo-random number generator (CSPRNG). Store keys securely, ideally using Hardware Security Modules (HSMs) or secure key vaults provided by cloud platforms. Implement strict access controls for keys. Establish policies for key rotation (changing keys periodically) and secure key distribution. Never hardcode keys in source code or configuration files.
3. Choose Appropriate Modes: Select an encryption mode suitable for your use case. Avoid ECB mode entirely. Prefer authenticated encryption modes like GCM or CCM which provide both confidentiality and integrity. If using modes like CBC or CTR, implement separate integrity checks (e.g., HMAC).
4. Handle IVs/Nonces Correctly: Initialization Vectors (IVs) or nonces must be unique for each encryption operation performed with the same key. For CBC mode, IVs should also be unpredictable (random). Reusing IVs/nonces can severely compromise security. Store the IV/nonce alongside the ciphertext; it doesn’t need to be secret, just unique.
5. Protect Against Side-Channel Attacks: Be aware of potential information leakage through timing variations, power consumption, or cache hits, especially in sensitive environments. Use constant-time implementations provided by reputable libraries where possible.
6. Combine with Other Security Measures: Encryption is only one part of a secure system. Use strong authentication, authorization, secure transport protocols (like TLS), and regular security audits to build a defense-in-depth strategy. Ensure compliance requirements (e.g., FIPS 140-2/3 for government applications) are met if applicable.

AES-256 for Sensitive Content Communications

Organizations must protect and keep data private when it is at rest and in motion. Encryption must be employed in both instances. For data at rest, AES-256 encryption is often the best option, whereas transport layer security (TLS) encryption creates secure sockets layer (SSL) tunnels to protect private content. Private data that requires encryption includes personally identifiable information (PII), protected health information (PHI), financial records, and corporate strategic product, marketing, and sales plans.

Kiteworks encrypts each piece of content with a unique, strong key at the file level and with a different strong key at the disk-level volume. This ensures that each file is double encrypted. Further, file keys, volume keys, and other intermediate keys are encrypted when stored.

Kiteworks uses a passphrase entered by an administrator to generate a super key it uses in the encryption of all stored keys. Thus, when an administrator rotates the passphrase on a regular basis, as recommended, the process is quick and efficient because only the keys need to be re-encrypted and not all content.

Those wanting more detail about Kiteworks’ key encryption approach across different sensitive content communication channels—email, file sharing, managed file transfer, web forms, and application programming interfaces (APIs)—can schedule a custom demo tailored to their environment.

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