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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →Scan for outdated or missing drivers - takes under a minuteDriver Scan →In cryptography, a mode of operation is a method for using a block cipher to process data and provide a service such as confidentiality or authentication. The cipher is the underlying algorithm; the mode defines how it is applied to data beyond one fixed-size block. The phrase has other meanings in other fields, so this explanation is specifically about cryptography.
What a mode of operation does
A block cipher transforms data in fixed-size blocks. A mode specifies how to use that cipher, often through multiple calls, to handle application data and provide a defined cryptographic service. NIST states that AES has a 128-bit block size; that figure describes the size of each block, not the key length. NIST IR 8459 (September 2024) also gives Triple-DES a 64-bit block size.
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Think of the cipher as the core transformation and the mode as the method for applying it. A cipher alone does not specify how a message of arbitrary length should be processed. The mode supplies that operating framework.
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Block cipher versus mode
AES is a block cipher. CBC, CTR, and GCM are examples of modes used with block ciphers. They are not alternative names for the cipher itself, and they do not all offer the same service. NIST’s glossary defines the term in context, while its block-cipher techniques overview describes the different mode families.
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What services modes can provide
Modes are designed for different cryptographic purposes. Some are confidentiality modes: they specify how a block cipher is used to encrypt and decrypt data. Other specifications address authentication or authenticated encryption, which combines confidentiality with a means to detect unauthorized changes. GCM and CCM are examples of authenticated-encryption modes covered by NIST’s mode work. NIST’s overview distinguishes these service categories.
The five modes specified for confidentiality in NIST SP 800-38A are:
- Electronic Codebook (ECB)
- Cipher Block Chaining (CBC)
- Cipher Feedback (CFB)
- Output Feedback (OFB)
- Counter (CTR)
This is the set covered by that recommendation, not a complete list of every block-cipher mode. The publication record for NIST SP 800-38A is dated December 2001.
Why the choice and implementation matter
Do not assume that modes are interchangeable or that every mode provides authentication. A mode’s security depends on the mode and how it is used in its application. NIST explains that confidentiality modes use encryption and decryption as inverse processes and that security depends, at minimum, on keeping the key secret. A mode’s detailed operating requirements—including how it handles initialization values, nonces, or input—come from its applicable specification. NIST IR 8459 discusses application-dependent security, and NIST SP 800-38A sets out confidentiality processes.
For a real system, select and implement a mode according to the system’s security requirements and current implementation guidance; the mode name alone does not establish that a particular use is safe.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.In brief
A mode of operation is the method that tells a block cipher how to process data for a cryptographic purpose. The cipher performs the core block transformation; the mode defines how that transformation is used and what service the construction is intended to provide.
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