Overview
CMAC is a cryptographic method that protects confidentiality, authenticity, or integrity with a defined primitive or protocol step. It should be used only as part of a well-defined protocol with correct parameters and key handling.
CoreSource checked
CMAC
CMAC protects secrecy, integrity, or authenticity with a named cryptographic primitive.
Quick Facts
Compact metadata for implementation and discovery.
- Category
- Cryptography
- Difficulty
- Intermediate
- Family
- Message Authentication
- Tier
- Core
- Status
- Verified
- Review
- Source Checked
- Sources
- 1
- Formulas
- 1
- Relationships
- 1
- Code example
- Ready
- Visualization
- Ready
Core Concepts
key materialsecurity parameterverification ruleverification
Quick facts
Complexity
Runtime and memory behavior at a glance.
Best
O(n)
Average
O(n)
Worst
O(n)
Space
O(1)
When to Use
- Use CMAC only when the protocol calls for this specific primitive or construction.
- Use it when a vetted implementation is available and parameter handling can be enforced.
When Not to Use
- Avoid custom protocol combinations around this primitive.
- Avoid when nonce, key, padding, or parameter requirements cannot be enforced.
What It Does
- Computes message authentication within Message Authentication workloads (O(n) time, O(1) space).
- Works by It should be used only as part of a well-defined protocol with correct parameters and key handling.
- Maintains key invariants across key material, security parameter, verification rule.
Use Cases
- Secure communication
- Authentication
- Key management
Inputs
- Message or secret
- Key material
- Nonce, salt, or public parameters
Outputs
- Ciphertext, tag, key, signature, or proof
- Verification result
Key Formulas (1)
MAC output
tag = AES−CMAC(key, message)
tag = AES-CMAC(key, message)Computes a message authentication tag using a block-cipher based construction.
Pseudocode
- 1.Validate keys, nonces, and required parameters
- 2.Apply the primitive's block, group, hash, or verification operation
- 3.Combine intermediate values according to the construction
- 4.Return the ciphertext, tag, key, signature, proof, or verification result
Canonical Example
- Input
- message = 'pay 10', key material = provided, nonce or public parameters = provided
- Output
- protected value or verification result
- Explanation
- CMAC applies its defined cryptographic operation without inventing extra protocol steps.
Implementation Notes
- Use vetted implementations for low-level primitives.
- Validate parameter lengths, nonce handling, and authentication failure behavior explicitly.
Common Pitfalls
- Reusing nonces, salts, or ephemeral values can break security assumptions.
- Treating a primitive as a full protocol invites key-management and authentication mistakes.
Interactive Mechanism Walkthrough
Graph Frontier & Edge RelaxationShow messages, keys, nonces, and verification values moving through the cryptographic construction.
Step 1 of 4: Validate keys, nonces, and required parameters
cryptoNotes
CMAC operates in O(n) time and O(1) space within the Message Authentication family. Key governing relation: tag = AES-CMAC(key, message). Often compared with HMAC and AES.
Sources (1)
What each reference supports for this entry.
NIST SP 800-38B
DocumentationSpecifies CMAC as a block-cipher based message authentication code, supporting CMAC.
Open sourceExplore Next
Good next jumps from the relationship graph, ranked for useful discovery.
Relationships
View full relationship pageConceptual neighborHMAC
CMAC and HMAC address nearby cryptography problems but make different implementation tradeoffs.
Tags
CryptographySecurityMessage AuthenticationSymmetric Key