> ## Documentation Index
> Fetch the complete documentation index at: https://docs.theblockchainlibrary.com/llms.txt
> Use this file to discover all available pages before exploring further.

# HMAC

> A construction that combines a secret key with a hash function to authenticate data integrity and origin.

# HMAC

A construction that combines a secret key with a hash function to authenticate data integrity and origin.

## Definition

A construction that combines a secret key with a hash function to authenticate data integrity and origin.

## Why it matters

Cryptography provides the mathematical foundation for keys, signatures, privacy, integrity, and verification.

## How it works

The process uses a cryptographic hash function like SHA-256 combined with a shared secret key. The sender processes the data and the key through the function to produce a unique authentication tag. The recipient performs the same calculation; if their generated tag matches the received one, the message is verified as authentic.

## Real-world example

The Lightning Network uses HMACs to authenticate messages sent between nodes, ensuring that payment routing information remains tamper-proof.

## Advantages

* Provides both integrity and authenticity
* Efficient computational performance
* Resistant to length extension attacks

## Limitations

* Requires secure key exchange
* Both parties must share the same key
* Vulnerable if the secret key is leaked

## Common misconceptions

* People often confuse HMAC with digital signatures, but they are not the same because HMAC uses a symmetric key.
* Many believe HMAC is a standalone encryption method, but it only provides integrity, not confidentiality.

## Related knowledge

* [Hash Function](/generated/v2/glossary/hash-function) — term

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**Canonical knowledge ID:** `glossary:hmac`