AndroidX3DH

This package implements the X3DH key agreement protocol in Swift. The cryptographic operations are provided by libsodium entirely. Libsodium is integrated via Lazysodium.

Installation

Jitpack

To integrate the library via jitpack add the jitpack repository to your root build.gradle file:

allprojects {
  repositories {
    ...
    maven { url  "https://dl.bintray.com/terl/lazysodium-maven" }
    maven { url 'https://jitpack.io' }
  }
}

You can then add the dependency to your app's build.gradle file where $VERSION specifies the specific version of the library:

dependencies {
  implementation 'com.github.TICESoftware:AndroidX3DH:$VERSION'
  implementation 'com.github.TICESoftware:AndroidHKDF:1.0.0'
  implementation "com.goterl:lazysodium-android:4.1.0@aar"
  implementation 'net.java.dev.jna:jna:5.5.0@aar'
}

Usage

Alice needs to retrieve some public keys from Bob that he has made public previously. She then calculates a shared secret and sends some information to Bob so that he can calculcate the shared secret on his side as well.

val preKeySigner = // ... Signing the key is not part of this library
val prekeySignatureVerifier = // ... and neither is verification

val bob = X3DH()
val bobIdentityKeyPair = bob.generateIdentityKeyPair()
val bobSignedPrekeyPair = bob.generateSignedPrekeyPair { /* Signer */ }
val bobOneTimePrekeyPairs = bob.generateOneTimePrekeyPairs(2)

val alice = X3DH()
val aliceIdentityKeyPair = alice.generateIdentityKeyPair()
val aliceSignedPrekey = alice.generateSignedPrekeyPair { /* Signer */ }

// [Alice fetches bob's prekey bundle ...]

val keyAgreementInitiation = alice.initiateKeyAgreement(bobIdentityKeyPair.publicKey, bobSignedPrekeyPair.keyPair.publicKey, bobSignedPrekeyPair.signature, bobOneTimePrekeyPairs.first().publicKey, aliceIdentityKeyPair, aliceSignedPrekey.keyPair.publicKey, { /* Verifier */ }, info)

// [Alice sends identity key, ephemeral key and used one-time prekey to bob ...]

val sharedSecret = bob.sharedSecretFromKeyAgreement(aliceIdentityKeyPair.publicKey, keyAgreementInitiation.ephemeralPublicKey, bobOneTimePrekeyPairs.first(), bobIdentityKeyPair, bobSignedPrekeyPair.keyPair, info)