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Version: 0.1.25

Diffie-Hellman (FFDHE)

Finite-field Diffie-Hellman helpers, including TLS FFDHE group parameter lookup. Header: pkc/dh/noxtls_dh.h.

Algorithm overview​

FFDHE (finite-field Diffie-Hellman ephemeral) performs key agreement in multiplicative groups modulo a large prime. In TLS usage, peers use standardized groups (for example ffdhe2048/3072/4096) and derive a shared secret that is then passed to key derivation.

Pros and cons​

Pros

  • Well-studied and standardized, especially in older and regulated environments.
  • Standardized RFC 7919 groups simplify interoperability and avoid unsafe custom parameters.
  • Ephemeral mode provides forward secrecy.

Cons

  • Slower and heavier than modern elliptic-curve key agreement at similar practical security.
  • Larger key material and handshake payloads.
  • Parameter validation mistakes can be dangerous if non-standard groups are used.

When to use​

  • Use when interoperability/policy requires finite-field DH groups.
  • Prefer RFC 7919 named FFDHE groups over custom parameters.
  • For performance-first modern deployments, X25519 is usually the default alternative.

API​

noxtls_dh_ffdhe_params​

noxtls_return_t noxtls_dh_ffdhe_params(uint16_t named_group,
                                       const uint8_t **p,
                                       const uint8_t **g,
                                       uint32_t *p_len);

Get FFDHE group parameters (p, g) for named groups like ffdhe2048/3072/4096.

noxtls_dh_generate_key​

noxtls_return_t noxtls_dh_generate_key(const uint8_t *p, uint32_t p_len,
                                       const uint8_t *g, uint32_t g_len,
                                       uint8_t *private_out,
                                       uint8_t *public_out);

Generate ephemeral DH private/public key pair for a given group.

noxtls_dh_shared_secret​

noxtls_return_t noxtls_dh_shared_secret(const uint8_t *private_key,
                                        uint32_t private_len,
                                        const uint8_t *peer_public,
                                        uint32_t peer_len,
                                        const uint8_t *p,
                                        uint32_t p_len,
                                        uint8_t *secret_out);

Compute DH shared secret from local private key and peer public key.