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HMAC-DRBG

Generates deterministic random bits by updating HMAC-based internal state.

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Quick Facts

Compact metadata for implementation and discovery.

Category: Cryptography
Difficulty: Intermediate
Family: Deterministic Random Bit Generators
Tier: Core
Status: Verified
Review: Source Checked
Sources: 1
Formulas: 0
Relationships: 5
Code example: Ready
Visualization: Ready
Aliases: HMAC Deterministic Random Bit Generator

Overview

HMAC-DRBG maintains key and value state and refreshes that state with entropy, nonce, and optional personalization input. Each request produces pseudorandom bits through repeated HMAC evaluations.

Core Concepts

cryptographic random generationdeterministic random bit generatorsbit masksrandom samplingcryptography

Quick facts

Complexity

Runtime and memory behavior at a glance.

Best

O(V + E)

Average

O((V + E) log V)

Worst

O(V^2 + E)

Space

O(V + E)

When to Use

Use through a vetted implementation when a standards-based DRBG is required.
Use when entropy input, reseeding, and state lifetime can be managed explicitly.

When Not to Use

Avoid hand-rolled versions in production security code.
Avoid when entropy quality or state compromise behavior cannot be controlled.

What It Does

Initializes DRBG state from entropy and nonce input
Generates blocks through HMAC state updates
Reseeds state when required by the security policy

Use Cases

Standards-based deterministic random bit generation
Testable pseudorandom generation with explicit seeding
Systems that already depend on HMAC primitives

Inputs

Message, key, modulus, seed, or public parameters
Security parameter or nonce when required

Outputs

Cryptographic value such as signature, random bits, proof, or modular result

Pseudocode

1.Instantiate key and value state from entropy, nonce, and personalization input
2.For each generate request, update state with any additional input
3.Repeatedly compute value = HMAC(key, value) and append output bytes
4.Update state after generation
5.Reseed when required by the reseed policy

Canonical Example

Input: entropy input, nonce, and requested output length = 32 bytes
Output: 32 bytes of deterministic random output
Explanation: The generator updates HMAC state from the seed material and emits output blocks until the request is satisfied.

Implementation Notes

Keep instantiate, reseed, generate, and update operations separate.
Zero sensitive state according to the host language and platform guidance.
Use official limits for request size, reseed interval, and security strength.

Common Pitfalls

Skipping nonce, personalization, or reseed rules weakens the intended security model.
Reusing state after compromise without reseeding can leak future outputs.
Treating generated bits as secure without checking instantiation errors is unsafe.