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The Advanced Encryption Standard (AES) is a symmetric-key block cipher: it encrypts and decrypts data in fixed 128-bit blocks using a secret key. The U.S. National Institute of Standards and Technology (NIST) specifies three versions—AES-128, AES-192 and AES-256—whose numbers refer to key length, not block size.
What AES means
AES is the name of a cryptographic standard, not a password, device or complete encryption system. Its specification, NIST FIPS 197, defines the Rijndael algorithm as a symmetric block cipher. “Symmetric” means the same secret key is used to encrypt and decrypt data. A block cipher transforms data in fixed-size blocks; AES’s block size is 128 bits.
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NIST describes block ciphers as foundational to cryptographic services, particularly those that provide confidentiality. AES is the standardized algorithm within that broader work; it does not, by itself, specify every part of a secure storage or communications system.
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The number after “AES” gives the key length in bits. All three variants process the same 128-bit block size.
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| Variant | Key length | Block size |
|---|---|---|
| AES-128 | 128 bits | 128 bits |
| AES-192 | 192 bits | 128 bits |
| AES-256 | 256 bits | 128 bits |
A key is the secret input that determines the cipher’s transformation. A block is the fixed-size piece of data it transforms. So AES-256 has a 256-bit key—not a 256-bit block. Although the Rijndael family supports other block sizes and key lengths, FIPS 197 standardizes only these three AES key lengths, each with a 128-bit block.
What AES does—and does not—define
AES defines the cipher used to encrypt and decrypt blocks. Applications that encrypt longer data or need particular security properties also rely on additional choices, such as how the cipher is used and how keys are managed. The AES definition alone does not describe those choices or guarantee that a complete product or system is secure.
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- POWERFUL SECURITY KEY: The Security Key NFC is the essential physical passkey for protecting your digital life from phishing attacks. It ensures only you can access your accounts.
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- FAST & CONVENIENT LOGIN: Plug in your Security Key NFC via USB-A and tap it, or tap it against your phone (NFC) to authenticate. No batteries, no internet connection, and no extra fees required.
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- BUILT TO LAST: Made from tough, waterproof, and crush-resistant materials. Manufactured in Sweden and programmed in the USA with the highest security standards.
AES is not asymmetric or public-key encryption: it is a symmetric-key cipher. Nor is AES-256 automatically a guarantee that an application using it is more secure overall; implementation and use matter beyond the cipher itself.
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NIST began its AES development effort in 1997, selected Rijndael in 2000 and first published FIPS 197 in 2001. The current edition, FIPS 197 Update 1, was published on May 9, 2023. NIST says the update made no technical changes to the algorithm; it revised the standard’s editorial presentation, terminology, diagrams and references. See NIST’s announcement of the 2023 update.
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