Implementation of the Double Ratchet protocol in Swift. The cryptographic operations are provided by libsodium entirely.
Alice and Bob calculate a shared secret using a secure channel. After that one party can start the conversation as soon as she gets to know the public key of the other one.
import DoubleRatchet
let sharedSecret: Bytes = ...
let info = "DoubleRatchetExample"
let bob = try DoubleRatchet(remotePublicKey: nil, sharedSecret: sharedSecret, maxSkip: 20, maxCache: 20, info: info)
// Bob sends his public key to Alice using another channel
// sendToAlice(bob.publicKey)
let alice = try DoubleRatchet(remotePublicKey: bob.publicKey, sharedSecret: sharedSecret, maxSkip: 20, maxCache: 20, info: info)
// Now the conversation begins
let message = "Hello, Bob!".bytes
let encryptedMessage = try alice.encrypt(plaintext: message)
let decryptedMessage = try bob.decrypt(message: encryptedMessage)
print(decryptedMessage.utf8String!) // Hello, Bob!