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Simple ECDH handshake protocol in Rust, based on X25519 and ChaCha20-Poly1305
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README.md

nnshake

A Rust implementation of a simple Elliptic Curve Diffie-Hellman (ECDH) channel-binding handshake protocol, based on X25519 and ChaCha20-Poly1305, for establishing forward-secure encrypted and authenticated sessions between endpoints such as clients and servers.

This is experimental software. It has not received formal security review and should not be entrusted with sensitive data. Use at your own risk.

Overview

This crate implements functions for performing a two-phase handshake between two endpoints. The phases of the handshake are:

  1. kx (ephemeral ECDH key exchange/agreement)
  2. auth (Peer authentication)

The kx phase establishes an encrypted session in the form of a symmetric keypair shared by the two endpoints. This keypair can be used for bidirectional encrypted data transfer. However, at the end of this phase, the session is entirely unauthenticated—the endpoints may be talking to an MITM attacker rather than to each other.

The auth phase allows each endpoint to confirm the identity of the other using an authentication method. This phase is tied to the kx phase using channel binding. In addition to a shared symmetric keypair, the kx phase produces a shared secret channel binding token. This token is used in the auth phase to link the authentication with the encrypted session. This two-phase approach separates the concerns of establishing encrypted sessions and authentication, allowing for a range of different authentication mechanisms.

This library does not perform any I/O; it just accepts byte slices from the user for input and output. As such, it can easily be embedded in higher-level protocols and network applications.

The following cryptographic primitives are used in the current version:

  • X25519 (RFC 7748) for ECDH key agreement
  • ChaCha20-Poly1305 (RFC 7539) for AEAD symmetric encryption
  • HKDF (RFC 5869) with HMAC-SHA-512 (using fixed-length salt) for key derivation
  • BLAKE2b for hashing of user-supplied additional data

There is, by design, no provision made for negotiating or using different primitives. The ring library is used for all primitives except BLAKE2b.

Protocol

Call the endpoints Alice (A) and Bob (B). Assume that Alice is the initiator of the session.

kx phase

  1. Alice generates an ephemeral ECDH key a for this session with corresponding public key a.pub.

  2. Alice sends a.pub to Bob.

  3. When Bob receives a.pub, he generates an ephemeral key b.

  4. Bob sends b.pub to Alice.

That is:

A: a.pub → B
B: b.pub → A

  1. Both parties compute k = ECDH(a, b.pub) = ECDH(b, a.pub).

  2. Both parties use k to derive:

    • a shared pair of symmetric keys (up, dn). The upstream key up is used for encryption by the session initiator Alice (hence for decryption by Bob), and vice-versa for the downstream key dn.
    • A shared channel-binding token t.

auth phase

This phase allows one or both endpoints to authenticate the other via some authentication mechanism that makes use of the channel-binding token t derived in the kx phase. Many different such mechanisms are possible, and they can be flexibly combined in different ways.

Example 1: Authentication Using a Static Public Key

Suppose Alice is a client and Bob is a server with a well-known static ECDH public key bs.pub. Alice can verify she is really talking to Bob as follows:

Let E_k(m) denote symmetric encryption of message m with key k.

  1. Alice generates a random challenge code r and an ephemeral ECDH challenge key c / c.pub. This key will be used only to encrypt a single message to Bob.

  2. Alice computes cs = ECDH(c, bs.pub).

  3. Alice sends E_up(c.pub, E_cs(r ^ t)) to Bob. That is, she takes the XOR of the challenge code and the channel binding token, encrypts it under the key cs, prepends c.pub, and sends to Bob (encrypted under the upstream session key up).

  4. Bob receives and unwraps this message, computes cs = ECDH(bs, c.pub), unwraps the inner message r ^ t with cs, and XORs it with t, yielding r.

  5. Bob sends E_dn(r) to Alice.

  6. Alice receives and unwraps this message, and verifies that the received value of r matches the one she generated in Step 1.

That is:

A: E_up(c.pub, E_cs(r ^ t)) → B
B: E_dn(r) → A

This protocol works because if Mallory is an MITM between Alice and Bob, then she can't unwrap the inner message (because it's encrypted using Bob's static public key bs.pub); and if she forwards it to Bob, then the token Bob uses to perform the XOR in Step 4 will be the binding token for his channel with Mallory, instead of the token he shares with Alice. The response he sends in Step 5 will thus fail Alice's verification in Step 6.

(This protocol was proposed by @eternaleye and inspired by TCPcrypt.)

Example 2: Authentication Using a Pre-Shared Key

Suppose Alice and Bob share an authentication key K. Then one party can authenticate to the other by sending E(H(K, t)), where E is the session encryption and H(key, message) is a suitable cryptographic keyed hash or HMAC function.

Example 3: Hash Puzzles

Suppose Alice is requesting a service from Bob. Bob may wish to rate-limit such requests and ensure "good faith" on the part of clients to mitigate denial-of-service attacks. Bob can "authenticate" Alice by sending a hash puzzle to Alice that requires knowledge of the binding token t to solve, ensuring any solution he receives is really provided by Alice.

Example Usage

Currently this crate only implements static public key authentication. Here's a basic example that shows the complete handshake for both client and server:

extern crate nnshake;

// A static keypair can be generated with examples/static-keygen
const SERVER_STATIC_PRIVKEY: [u8; 32] = [
    228, 172, 49, 122, 10, 86, 15, 63,
    30, 82, 44, 209, 15, 142, 38, 144,
    20, 110, 164, 245, 17, 109, 59, 27,
    158, 22, 80, 64, 178, 92, 10, 138,
];
const SERVER_STATIC_PUBKEY: [u8; 32] = [
    85, 215, 107, 196, 200, 26, 154, 112,
    186, 205, 170, 96, 156, 87, 242, 31,
    134, 36, 10, 205, 133, 18, 230, 211,
    38, 179, 240, 57, 75, 251, 43, 122,
];

fn main() {
    use nnshake::{Random, Client, Server};
    let rng = Random::new();

    // Message 1: Client generates key and sends pubkey to server
    let mut c = Client::new(&rng).expect("Client keygen failed");

    // Message 2: Server receives client pubkey, generates key and
    // sends pubkey to client
    let mut s = Server::new(&rng).expect("Server keygen failed");

    // Both parties compute ECDH key exchange to derive a shared session key
    s.kx(c.public_key()).expect("S: Key exchange failed");
    c.kx(s.public_key()).expect("C: Key exchange failed");

    // Message 3: Client generates a challenge message to authenticate
    // server based on its static public key, and sends to server
    let mut challenge_frame = [0u8; 96];
    c.challenge(&SERVER_STATIC_PUBKEY, &mut challenge_frame)
        .expect("C: Challenge generation failed");

    // Message 4: Server receives challenge, generates response, and
    // sends to client
    let mut response_frame = [0u8; 48];
    s.solve_challenge(&SERVER_STATIC_PRIVKEY, &mut challenge_frame, &mut response_frame)
        .expect("S: Challenge solution failed");

    // Client receives and validates server's response message
    c.check_response(&mut response_frame).expect("C: Invalid response");

    // Client and server now share a pair of keys (upstream, downstream)
    // that can be used for symmetric AEAD data transfer
    let (c_up_key, c_dn_key) = c.finish().expect("C: Finish failed");
    let (s_up_key, s_dn_key) = s.finish().expect("S: Finish failed");

    assert_eq!((*c_up_key, *c_dn_key), (*s_up_key, *s_dn_key));
}

Notes

  • The API is simple and hard to misuse. Each of the main handshake methods can only be called when the handshake is at the appropriate step, and upon success, each transitions the handshake to the next step.

  • When the handshake finishes, a pair (upstream, downstream) of keys is returned. The intent is that upstream is used for client→server encryption (encryption by the client and decryption by the server), and that downstream is used conversely.

  • The library itself does not use the upstream and downstream keys, but rather derives its own for use in the challenge and response steps. This is to ensure that all AEAD nonce values can safely be used by the user after the handshake finishes.

  • Support is provided for supplying additional data during the handshake via the update_ad_tx() and update_ad_rx() methods. These methods should be used at every step to include any transmitted and received cleartext message headers in the respective hash state. This provides security against alteration of the cleartext headers of messages in transit. If such alteration occurs, the tx hash of the sender will fail to match the rx hash of the receiver. Since this hash is passed as additional data during symmetric AEAD encryption and decryption, non-matching hashes will cause a handshake failure when attempting to decrypt Message 3 or Message 4. See the simple-ad example.

  • Some care is taken to ensure that stack memory containing sensitive key material is cleared after use. The upstream and downstream keys returned to the user when the handshake finishes are wrapped in a struct that clears its contents when dropped. This struct implements Deref, so the array of key bytes can be accessed with the * operator.

  • A static-keygen tool is provided in the examples that can be used to generate static ECDH keypairs. This tool will print the private and public keys in Base64 and hex format.

Building

Currently (Oct 2017) the ring crate does not support static ECDH keys. Therefore, building this crate requires applying a small patch to a local copy of ring. This patch just makes a few ring datatypes public instead of private:

$ git clone https://github.com/doomsparkles/nnshake
$ cd nnshake
$ git clone https://github.com/briansmith/ring
$ (cd ring ; git apply ../ring.diff)
$ cargo build --examples

License

Distributed under the MIT License.

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