# x25519-dalek

A pure-Rust implementation of x25519 elliptic curve Diffie-Hellman key exchange, with curve operations provided by curve25519-dalek.

This crate provides two levels of API: a bare byte-oriented `x25519` function which matches the function specified in RFC7748, as well as a higher-level Rust API for static and ephemeral Diffie-Hellman.

## Examples

Alice and Bob are two adorable kittens who have lost their mittens, and they wish to be able to send secret messages to each other to coordinate finding them, otherwise—if their caretaker cat finds out—they will surely be called naughty kittens and be given no pie!

But the two kittens are quite clever. Even though their paws are still too big and the rest of them is 90% fuzziness, these clever kittens have been studying up on modern public key cryptography and have learned a nifty trick called elliptic curve Diffie-Hellman key exchange. With the right incantations, the kittens will be able to secretly organise to find their mittens, and then spend the rest of the afternoon nomming some yummy pie!

First, Alice uses `EphemeralSecret::new()` and then `PublicKey::from()` to produce her secret and public keys:

```use rand_core::OsRng;
use x25519_dalek::{EphemeralSecret, PublicKey};

let alice_secret = EphemeralSecret::new(OsRng);
let alice_public = PublicKey::from(&alice_secret);```

Bob does the same:

```let bob_secret = EphemeralSecret::new(OsRng);
let bob_public = PublicKey::from(&bob_secret);```

Alice meows across the room, telling `alice_public` to Bob, and Bob loudly meows `bob_public` back to Alice. Alice now computes her shared secret with Bob by doing:

`let alice_shared_secret = alice_secret.diffie_hellman(&bob_public);`

Similarly, Bob computes a shared secret by doing:

`let bob_shared_secret = bob_secret.diffie_hellman(&alice_public);`

These secrets are the same:

`assert_eq!(alice_shared_secret.as_bytes(), bob_shared_secret.as_bytes());`

Voilá! Alice and Bob can now use their shared secret to encrypt their meows, for example, by using it to generate a key and nonce for an authenticated-encryption cipher.

This example used the ephemeral DH API, which ensures that secret keys cannot be reused; Alice and Bob could instead use the static DH API and load a long-term secret key.

# Installation

To install, add the following to your project's `Cargo.toml`:

``````[dependencies]
x25519-dalek = "1.1"
``````

# Documentation

Documentation is available here.

# Note

This code matches the RFC7748 test vectors. The elliptic curve operations are provided by `curve25519-dalek`, which makes a best-effort attempt to prevent software side-channels.

• crypto_box: pure Rust public-key authenticated encryption compatible with the NaCl family of encryption libraries (libsodium, TweetNaCl) which uses `x25519-dalek` for key agreement

Note that docs will only build on nightly Rust until `feature(external_doc)` is stabilized.

## Structs

 EphemeralSecret A short-lived Diffie-Hellman secret key that can only be used to compute a single `SharedSecret`. PublicKey A Diffie-Hellman public key, corresponding to an `EphemeralSecret` or `StaticSecret` key. SharedSecret The result of a Diffie-Hellman key exchange. StaticSecret A Diffie-Hellman secret key that can be used to compute multiple `SharedSecret`s.

## Constants

 X25519_BASEPOINT_BYTES The X25519 basepoint, for use with the bare, byte-oriented x25519 function. This is provided for people who cannot use the typed DH API for some reason.

## Functions

 x25519 The bare, byte-oriented x25519 function, exactly as specified in RFC7748.