What Is AES Encryption?

AES is the symmetric encryption standard in effectively universal use — in TLS sessions, disk encryption, database encryption, cloud storage and password managers. It has withstood more than two decades of public analysis without a practical break, and the algorithm itself is not where implementations fail.

What does fail, reliably, is the mode of operation and how the surrounding values are handled.

The mode matters more than the key size

AES encrypts fixed-size blocks. A mode of operation determines how a message longer than one block is handled, and the choice has larger security consequences than moving from a 128-bit key to a 256-bit one.

Authenticated modes such as GCM provide confidentiality and integrity together, so tampering with the ciphertext is detected rather than silently producing altered plaintext. This should be the default choice.

Unauthenticated modes such as CBC protect confidentiality only. Used without a separate integrity check they permit attacks that modify decrypted output, and correctly combining CBC with authentication is exactly the kind of composition that developers get wrong.

ECB should not be used, essentially ever. It encrypts identical blocks identically, so structure in the plaintext survives into the ciphertext — the property behind the well-known demonstration where an encrypted image remains recognisable.

Never repeat the nonce

Authenticated modes require a value that must be unique for every encryption performed under a given key. Reusing one is catastrophic in a way that is easy to underestimate: with GCM, nonce reuse can expose the authentication key and allow forgery, not merely leak a relationship between two messages.

The realistic risk is not a developer deliberately reusing a value. It is a random nonce generated with insufficient entropy, a counter that resets when a service restarts, or the same key and counter space used across multiple instances that do not coordinate. All three occur in practice.

Where it fits in practice

AES protects the bulk data in almost every system you operate, with the key typically established by an asymmetric exchange such as those described in RSA or its elliptic-curve equivalents. That division — asymmetric to agree a key, symmetric to move the data — is the standard pattern.

It is also the primitive least affected by quantum computing. Symmetric encryption is weakened rather than broken, and larger keys compensate, which is why post-quantum work concentrates on key exchange and signatures rather than on AES.

Where AES appears without anyone choosing it

Most AES in your estate was never a decision. It is inside TLS sessions, disk encryption, database transparent encryption, cloud storage, backup software, password managers and the VPN. The defaults are generally sound, which is why this rarely causes problems.

The cases worth examining are the ones where a developer chose explicitly: application-level field encryption, a custom protocol, or anything encrypting data before storing it. Those are where mode selection, nonce handling and key management were decided by someone solving a different problem under time pressure.

A code search for cipher instantiation is a quick way to find them, and the results are usually a short list containing one or two genuine issues.

Where to start

Check which mode your applications use. If anything is using ECB, or CBC without authentication, that is a concrete finding with a clear remedy — and it matters considerably more than the key length anyone is quoting.

See encryption in practice. GuardsArm runs security risk assessments.