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HMAC

Authenticates messages by combining a hash function with a shared secret key.

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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: 7
Code example: Not ready
Visualization: Ready
Aliases: Hash-Based Message Authentication Code

Overview

HMAC wraps a cryptographic hash with inner and outer keyed hashes to verify message integrity and authenticity.

Core Concepts

security primitivemessage authentication codehash functionsecurity assumptionmessage authenticationkey materialmessage transformationmisuse resistance

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 HMAC when you need to compute a keyed message digest.
Use it for aPI request signing with inputs like shared secret key.
Use it only through reviewed libraries when the security goal matches the primitive.
Use it when the Message Authentication tradeoff is acceptable for the problem size.

When Not to Use

Avoid custom implementations for production security or when key, nonce, and padding rules are unclear.
Avoid it when the required output is not message authentication code.

What It Does

Computes a keyed message digest
Lets receivers verify that the message was not changed

Use Cases

API request signing
Message authentication
Token integrity checks

Inputs

Shared secret key
Message
Hash function

Outputs

Message authentication code

Key Formulas (1)

HMAC structure

HMAC = H((key xor opad) || H((key xor ipad) || message))

HMAC = H((key xor opad) || H((key xor ipad) || message))

Combines a secret key and hash function to authenticate a message.

Pseudocode

1.Normalize the key to the hash block size
2.Hash the inner pad with the message
3.Hash the outer pad with the inner hash
4.Send or compare the final code

Canonical Example

Input: message = 'pay 10' with configured keys, nonces, or public parameters
Output: ciphertext, tag, digest, signature, proof, or verification result
Explanation: The primitive applies its defined arithmetic, hashing, or block operation without inventing extra protocol steps.

Implementation Notes

Keep the input contract explicit: Shared secret key; Message; Hash function.
Choose hash and equality behavior together so equal keys land in the same lookup path.
Validate parameters, key sizes, nonces, salts, and error handling against the library's recommended API.
Use reviewed libraries and protocol specifications for production security-sensitive code.

Common Pitfalls

HMAC provides authentication, not encryption.
Using an unstable or poorly distributed hash function can create clustered buckets or wrong comparisons.
Confusing encryption, hashing, signatures, and authentication codes as interchangeable tools.
Handle empty, invalid, and boundary inputs before trusting message authentication code.

Interactive Mechanism Walkthrough

Graph Frontier & Edge Relaxation

Show a message passing through inner and outer keyed hash layers.

Step 1 of 4: Normalize the key to the hash block size

keyed-hash

Notes

HMAC operates in O(n) time and O(1) space within the Message Authentication family. Key governing relation: HMAC = H((key xor opad) || H((key xor ipad) || message)). Often compared with SHA-256 and ChaCha20.