DRBG

Deterministic Random Bit Generator (AES-CTR-DRBG) per NIST SP 800-90A. Header: noxtls_drbg.h.

Algorithm overview

This module implements AES-CTR-DRBG from NIST SP 800-90A. A DRBG expands entropy input into a stream of pseudorandom output with controlled reseeding. Security depends on correct entropy, proper instantiation, and reseed policy.

Pros and cons

Pros

• Standardized design with clear security model and operational guidance.
• Deterministic generation is efficient and suitable for repeated key material requests.
• Supports selectable entropy source and custom entropy callback integration.

Cons

• Not a replacement for true entropy; weak entropy input undermines security.
• Requires operational discipline (reseed intervals, source health, failure handling).
• More complexity than using OS CSPRNG directly when no portability policy requires DRBG control.

When to use

• Use when you need a portable internal CSPRNG with explicit lifecycle control.
• Good fit for embedded/controlled environments where entropy plumbing is managed.
• If platform CSPRNG is trusted and available, direct OS random APIs may be simpler.

Types

drbg_aes_type_t

DRBG security strength:

• DRBG_AES128
• DRBG_AES192
• DRBG_AES256

noxtls_entropy_source_t

Entropy source selection:

• NOXTLS_ENTROPY_SOURCE_AUTO
• NOXTLS_ENTROPY_SOURCE_WINDOWS_CSPRNG
• NOXTLS_ENTROPY_SOURCE_UNIX_URANDOM
• NOXTLS_ENTROPY_SOURCE_CUSTOM
• NOXTLS_ENTROPY_SOURCE_DUMMY

noxtls_entropy_cb_t

typedef noxtls_return_t (*noxtls_entropy_cb_t)(uint8_t *entropy_buffer, uint32_t entropy_len);

Custom entropy callback type.

drbg_state_t

DRBG runtime state (V, Key, reseed counter, AES variant, lengths, instantiated flag).

API

noxtls_drbg_get_entropy

noxtls_return_t noxtls_drbg_get_entropy(uint8_t *entropy_buffer, uint32_t entropy_len);

Fill buffer with entropy from the configured source.

noxtls_drbg_set_entropy_source

void noxtls_drbg_set_entropy_source(noxtls_entropy_source_t source);

Select entropy source backend.

noxtls_drbg_get_entropy_source

noxtls_entropy_source_t noxtls_drbg_get_entropy_source(void);

Get current entropy source backend.

noxtls_drbg_set_entropy_callback

void noxtls_drbg_set_entropy_callback(noxtls_entropy_cb_t cb);

Set custom entropy callback.

noxtls_drbg_get_entropy_callback

noxtls_entropy_cb_t noxtls_drbg_get_entropy_callback(void);

Get custom entropy callback.

drbg_instantiate

noxtls_return_t drbg_instantiate(drbg_state_t *state,
                                 drbg_aes_type_t aes_type,
                                 const uint8_t *entropy_input,
                                 uint32_t entropy_len,
                                 const uint8_t *nonce,
                                 uint32_t nonce_len,
                                 const uint8_t *personalization_string,
                                 uint32_t pers_len);

Instantiate DRBG state. Entropy/nonce/personalization inputs are optional depending on the calling model.

drbg_generate

noxtls_return_t drbg_generate(drbg_state_t *state,
                              uint8_t *output_buffer,
                              uint32_t requested_bits,
                              const uint8_t *additional_input,
                              uint32_t add_input_len);

Generate pseudo-random output bits. Additional input is optional.

drbg_reseed

noxtls_return_t drbg_reseed(drbg_state_t *state,
                            const uint8_t *entropy_input,
                            uint32_t entropy_len,
                            const uint8_t *additional_input,
                            uint32_t add_input_len);

Reseed DRBG state.

drbg_update

noxtls_return_t drbg_update(drbg_state_t *state,
                            const uint8_t *provided_data,
                            uint32_t provided_data_len);

Update DRBG internal state with provided data.

noxtls_drbg_uninstantiate

noxtls_return_t noxtls_drbg_uninstantiate(drbg_state_t *state);

Clear DRBG state and mark uninstantiated.