# x25519-dalek  [![](https://img.shields.io/crates/v/x25519-dalek.svg)](https://crates.io/crates/x25519-dalek) [![](https://docs.rs/x25519-dalek/badge.svg)](https://docs.rs/x25519-dalek) [![CI](https://github.com/dalek-cryptography/curve25519-dalek/actions/workflows/x25519-dalek.yml/badge.svg?branch=main)](https://github.com/dalek-cryptography/curve25519-dalek/actions/workflows/x25519-dalek.yml)

A pure-Rust implementation of x25519 elliptic curve Diffie-Hellman key exchange,
with curve operations provided by
[curve25519-dalek](https://github.com/dalek-cryptography/curve25519-dalek).

This crate provides two levels of API: a bare byte-oriented `x25519`
function which matches the function specified in [RFC7748][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::random()` and then
`PublicKey::from()` to produce her secret and public keys:

```ignore
use x25519_dalek::{EphemeralSecret, PublicKey};

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

Bob does the same:

```ignore
# use x25519_dalek::{EphemeralSecret, PublicKey};
let bob_secret = EphemeralSecret::random();
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:

```rust
# use rand_core::OsRng;
# use x25519_dalek::{EphemeralSecret, PublicKey};
# let alice_secret = EphemeralSecret::random_from_rng(OsRng);
# let alice_public = PublicKey::from(&alice_secret);
# let bob_secret = EphemeralSecret::random_from_rng(OsRng);
# let bob_public = PublicKey::from(&bob_secret);
let alice_shared_secret = alice_secret.diffie_hellman(&bob_public);
```

Similarly, Bob computes a shared secret by doing:

```rust
# use rand_core::OsRng;
# use x25519_dalek::{EphemeralSecret, PublicKey};
# let alice_secret = EphemeralSecret::random_from_rng(OsRng);
# let alice_public = PublicKey::from(&alice_secret);
# let bob_secret = EphemeralSecret::random_from_rng(OsRng);
# let bob_public = PublicKey::from(&bob_secret);
let bob_shared_secret = bob_secret.diffie_hellman(&alice_public);
```

These secrets are the same:

```rust
# use rand_core::OsRng;
# use x25519_dalek::{EphemeralSecret, PublicKey};
# let alice_secret = EphemeralSecret::random_from_rng(OsRng);
# let alice_public = PublicKey::from(&alice_secret);
# let bob_secret = EphemeralSecret::random_from_rng(OsRng);
# let bob_public = PublicKey::from(&bob_secret);
# let alice_shared_secret = alice_secret.diffie_hellman(&bob_public);
# let bob_shared_secret = bob_secret.diffie_hellman(&alice_public);
assert_eq!(alice_shared_secret.as_bytes(), bob_shared_secret.as_bytes());
```

Voilà!  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`:

```toml
[dependencies]
x25519-dalek = "2"
```

# MSRV

Current MSRV is 1.60.

# Documentation

Documentation is available [here](https://docs.rs/x25519-dalek).

# Performance and backend selection

Performance is a secondary goal behind correctness, safety, and clarity, but we aim to be competitive with other implementations. To this end, we allow users to choose their _backend_, i.e., the underlying implementation of elliptic curve and scalar arithmetic. Different backends have different use cases. For example, if you demand formally verified code, you want to use the `fiat` backend (as it was generated from [Fiat Crypto][fiat]).

Further instructions and details regarding backends can be found in the [curve25519-dalek docs](https://github.com/dalek-cryptography/curve25519-dalek#backends).

# Note

This code matches the [RFC7748][rfc7748] test vectors.
The elliptic curve
operations are provided by `curve25519-dalek`, which makes a best-effort
attempt to prevent software side-channels.

"Secret Messages" cover image and [zine](https://shop.bubblesort.io/products/secret-messages-zine)
copyright © Amy Wibowo ([@sailorhg](https://twitter.com/sailorhg))

[rfc7748]: https://tools.ietf.org/html/rfc7748

# See also

- [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

[fiat]: https://github.com/mit-plv/fiat-crypto
[crypto_box]: https://github.com/RustCrypto/nacl-compat/tree/master/crypto_box
