AES-256 Encryption: What It Is and How to Use It
Security - 7 min read - Published 2026-08-27
AES (Advanced Encryption Standard) is a symmetric-key encryption algorithm, meaning the same key is used to both encrypt and decrypt data. It was selected by NIST in 2001 after a multi-year public competition, replacing the older DES standard, and remains the most widely deployed encryption algorithm in the world - used in everything from HTTPS connections to disk encryption to messaging apps.
What "256" Actually Means
AES supports key sizes of 128, 192, or 256 bits. AES-256 uses a 256-bit key, meaning there are 2^256 possible keys - a number so large that brute-forcing it is not considered feasible with any computing technology that exists or is anticipated in the foreseeable future, including reasonably-projected quantum computers (which would weaken it, via Grover's algorithm, to an effective strength roughly equivalent to AES-128 - still far beyond practical brute-force range).
Why Mode of Operation Matters
AES itself only defines how to encrypt a single fixed-size block of data (128 bits). To encrypt anything larger, you need a "mode of operation" that defines how successive blocks relate to each other. Older modes like ECB are now considered insecure because identical plaintext blocks produce identical ciphertext blocks, leaking patterns in the data. Modern implementations use authenticated modes like GCM (Galois/Counter Mode), which do two things at once: encrypt the data, and generate a cryptographic tag that lets the decrypting side verify the ciphertext hasn't been tampered with. GCM is the current recommended default for exactly this reason.
The Detail Everyone Forgets: The IV
Almost every AES mode, including GCM, requires an Initialization Vector (IV) - a value that should be unique for every single encryption operation performed with the same key. Reusing an IV with the same key is one of the most common real-world AES implementation mistakes, and in GCM specifically, it can catastrophically break both the confidentiality and the tamper-detection the mode is supposed to provide. A correct implementation generates a fresh, random IV every time it encrypts something, and stores or transmits that IV alongside the ciphertext (the IV itself doesn't need to be secret, just unique).
Where the Key Actually Comes From
AES needs a raw binary key, not a password someone can remember. Turning a human-chosen password into a proper AES key is a separate step, usually done with a key-derivation function like PBKDF2, scrypt, or Argon2, which deliberately adds computational cost to slow down brute-force guessing of the original password. Skipping this step and using a password directly as if it were the key is a common and serious implementation error.