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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchSymmetric-key cryptography is cryptography in which the parties use the same secret key, or an equivalent shared secret, for the operation. The classic case is encryption: the key turns plaintext into ciphertext, and a party holding that key turns the ciphertext back into plaintext. AES is the best-known standard example.
How symmetric encryption works
A sender and a recipient first arrange to hold the same secret key. An encryption algorithm combines that key with the plaintext to produce ciphertext. Someone with the matching key can reverse the process. Anyone without it should learn nothing useful from the ciphertext.
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The security you actually get depends on the whole construction: the algorithm, the mode it runs in, how the key is generated and kept, and any required starting values (such as initialization values). The name of the algorithm alone does not settle it.
Block ciphers and AES
NIST’s glossary defines a block cipher as an invertible symmetric-key algorithm that works on fixed-length blocks and is parameterized by a secret key. “Invertible” means the same key that enciphers a block can undo it.
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AES, specified in NIST FIPS 197, is a symmetric block cipher. Its figures are:
| Property | Value | Source |
|---|---|---|
| Block size | 128 bits | NIST FIPS 197 (2001 standard, updated edition 2023) |
| Key lengths | 128, 192 or 256 bits | NIST FIPS 197 (2001 standard, updated edition 2023) |
Cipher versus mode
A block cipher only transforms one fixed-size block. A mode of operation describes how to apply the cipher to real data, which is usually longer than one block, to deliver a particular service such as confidentiality or authentication. Choosing AES is therefore only half the decision. You must also choose a mode and use it correctly.
Modes compared by the service they provide
| Mode(s) | NIST publication | Service |
|---|---|---|
| ECB, CBC, CFB, OFB, CTR | SP 800-38A (December 2001; a revision is planned) | Confidentiality only |
| CCM | SP 800-38C | Counter-mode confidentiality combined with CBC-MAC authentication |
| GCM | SP 800-38D | Authenticated encryption with associated data |
| XTS-AES | SP 800-38E | Confidentiality for stored data; no authentication of the data or its source |
The practical lesson: the five SP 800-38A modes are confidentiality modes, and by themselves they do not authenticate ciphertext. If tampering matters, use an authenticated-encryption mode such as GCM or CCM. NIST has revision-planning notes on SP 800-38C and SP 800-38D as well.
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Storage and XTS-AES
XTS-AES is aimed at encrypting data at rest on storage devices. NIST states that it does not authenticate the data or its source. NIST published a Revision 1 draft of SP 800-38E on September 3, 2026. It is a draft, so the earlier final publication remains the base document until the revision is finalized.
Keys are a separate problem
Every party must hold the same secret, and that secret has to be created, delivered, stored and protected on its own terms. Strong encryption does not solve key distribution. NIST addresses the protection of keys separately in SP 800-38F, which specifies AES Key Wrap and Key Wrap with Padding to protect the confidentiality and integrity of cryptographic keys.
Symmetric versus public-key cryptography
Symmetric cryptography relies on one shared secret. Public-key (asymmetric) cryptography instead uses a mathematically linked key pair, with one key public and one private. The shared-secret requirement is what makes key distribution the central practical difficulty of symmetric systems.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Specialist variants
Some symmetric designs fit narrower needs. NIST SP 800-38G specifies the format-preserving encryption methods FF1 and FF3, where ciphertext keeps the format of the plaintext. For general guidance on AES, modes, keys and initialization values, NIST SP 800-175B is the broader reference.
What this definition does not decide
A definition does not tell you which algorithm, library, configuration, compliance regime or key-management design suits a particular system. Those depend on the security service required and on the implementation guidance that applies to you.
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