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The Advanced Encryption Standard (AES) is a symmetric-key block cipher defined by the U.S. National Institute of Standards and Technology (NIST) in FIPS 197. It encrypts and decrypts data in fixed 128-bit blocks. AES-128, AES-192 and AES-256 all use that same block size; their numbers describe the length of the secret key.
What AES means
AES is a cryptographic algorithm used to provide encryption and decryption. NIST specifies the Rijndael algorithm as a symmetric block cipher: the same secret key is used for the corresponding encryption and decryption operations. NIST describes block ciphers as foundational to cryptographic services, particularly those that provide confidentiality.
AES is not a password, a device, or a complete secure-storage or communications system. It is the cipher at the core of a system, whose broader security also depends on how it is implemented and used.
What AES-128, AES-192 and AES-256 mean
The number in each name is the key length in bits, not the block size. In every standardized AES variant, the cipher processes a 128-bit block of data.
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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 being transformed. So AES-256 has a 256-bit key and a 128-bit block—not a 256-bit block.
What AES does not specify
FIPS 197 defines the cipher, not every detail needed to encrypt arbitrary files or protect a network connection. Systems use modes of operation or protocols to apply a block cipher to larger amounts of data and meet particular security goals. The AES definition alone therefore does not establish that an application or device is secure, nor does choosing a longer key automatically make the whole system more secure.
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- WORKS WITH 1000+ ACCOUNTS: Compatible with Google, Microsoft, and Apple. A single Security Key NFC secures 100 of your favorite accounts, including email, password managers, and more.
- 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.
- TRUSTED PASSKEY TECHNOLOGY: Uses the latest passkey standards (FIDO2/WebAuthn & FIDO U2F) but does not support One-Time Passwords. For complex needs, check out the YubiKey 5 Series.
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Where AES came from and which edition is current
NIST began the AES development effort in 1997, selected Rijndael in 2000 and first published FIPS 197 in 2001. The current publication is FIPS 197 Update 1, issued May 9, 2023. NIST says the update made no technical changes to the algorithm; it improved editorial presentation, terminology, diagrams and references. See NIST’s FIPS 197 Update 1 announcement and publication for the current standard.
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