[][src]Crate 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.


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:

This example is not tested
extern crate rand_os;
extern crate x25519_dalek;

use rand_os::OsRng;

use x25519_dalek::EphemeralSecret;
use x25519_dalek::PublicKey;

let mut alice_csprng = OsRng::new().unwrap();
let     alice_secret = EphemeralSecret::new(&mut alice_csprng);
let     alice_public = PublicKey::from(&alice_secret);

Bob does the same:

This example is not tested
let mut bob_csprng = OsRng::new().unwrap();
let     bob_secret = EphemeralSecret::new(&mut bob_csprng);
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:

This example is not tested
let alice_shared_secret = alice_secret.diffie_hellman(&bob_public);

Similarly, Bob computes a shared secret by doing:

This example is not tested
let bob_shared_secret = bob_secret.diffie_hellman(&alice_public);

These secrets are the same:

This example is not tested
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.


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

version = "^0.5"


Documentation is available here.


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.

"Secret Messages" cover image and zine copyright © Amy Wibowo (@sailorhg)

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



A EphemeralSecret is a short lived Diffie-Hellman secret key used to create a SharedSecret when given their PublicKey.


A PublicKey is the corresponding public key converted from an EphemeralSecret or a StaticSecret key.


A SharedSecret is a Diffie-Hellman shared secret that’s generated from your EphemeralSecret or StaticSecret and their PublicKey.


A StaticSecret is a static Diffie-Hellman secret key that can be saved and loaded to create a SharedSecret when given their PublicKey.



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.



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