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X25519

Algorithm​

X25519 maps scalars and Montgomery u-coordinates to a 32-byte shared secret. In TLS 1.3, each side contributes an ephemeral key share inside ClientHello / ServerHello extensions.

Purpose​

RFC 7748 Curve25519 ECDH for TLS 1.3 x25519 key shares and general key agreement.

Rust API​

  • Crate: noxtls-crypto
  • Module path (conceptual): noxtls_crypto::pkc
  • Primary symbols:
    • X25519PrivateKey
    • X25519PublicKey
    • noxtls_x25519_shared_secret
    • noxtls_x25519_generate_private_key_auto
    • noxtls_x25519_basepoint

Functions and types:

  • noxtls_x25519_generate_private_key_auto(drbg) - Parameters: drbg supplies random bytes for scalar generation. Behavior: creates an ephemeral/private X25519 key scalar. Returns: generated X25519PrivateKey in Result.
  • X25519PrivateKey::public_key() - Parameters: private key instance. Behavior: derives Montgomery public key share from the private scalar. Returns: X25519PublicKey.
  • noxtls_x25519_shared_secret(local_private, remote_public) - Parameters: local private key and peer public key. Behavior: computes RFC 7748 X25519 shared secret (private key is consumed by value). Returns: 32-byte shared secret in Result.

Feature flags and policy​

Default.

Examples​

use noxtls_crypto::{noxtls_x25519_generate_private_key_auto, noxtls_x25519_shared_secret, HmacDrbgSha256};

let mut drbg = HmacDrbgSha256::new(b"0123456789abcdef", b"nonce", b"")?;
let alice = noxtls_x25519_generate_private_key_auto(&mut drbg)?;
let bob = noxtls_x25519_generate_private_key_auto(&mut drbg)?;
let alice_pk = alice.public_key();
let bob_pk = bob.public_key();
let s_alice = noxtls_x25519_shared_secret(alice.clone(), bob_pk)?;
let s_bob = noxtls_x25519_shared_secret(bob, alice_pk)?;
assert_eq!(s_alice, s_bob);
# Ok::<(), noxtls_core::Error>(())

Security and compatibility​

Follow RFC 7748 / TLS guidance on public-key validation and constant-time expectations for your threat model.