Replace jnacl with CryptoProvider interface

Author: threema-danilo

.gitignore

@@ -11,6 +11,7 @@ local.properties
 
 # Java
 build/
+out/
 
 # Server
 saltyrtc-server-python/

build.gradle

@@ -26,6 +26,7 @@ sourceCompatibility = '1.8'
 // In this section you declare where to find the dependencies of your project
 repositories {
     jcenter()
+    maven { url  "https://dl.bintray.com/terl/lazysodium-maven" }
 }
 
 // In this section you declare the dependencies for your production and test code
@@ -47,6 +48,8 @@ dependencies {
     // Test dependencies
     testCompile 'org.slf4j:slf4j-simple:1.7.+'
     testCompile 'junit:junit:4.12'
+    testCompile 'com.goterl.lazycode:lazysodium-java:3.3.0'
+    testCompile 'net.java.dev.jna:jna:5.0.0'
 }
 
 test {

docs/docs/usage.md

@@ -14,14 +14,13 @@ characteristics, therefore the crypto backend in this library is pluggable.
 
 In order to instantiate a `SaltyRTC` instance or a `KeyStore`, you need to
 provide an implementation of the `org.saltyrtc.client.crypto.CryptoProvider`
-interface. For ease of use, this library provides a pure-java implementation of
-such a CryptoProvider using [jnacl](https://github.com/neilalexander/jnacl). To
-use, simply create an instance of the `JnaclCryptoProvider`.
+interface. You can find an example implementation
+[in the testsuite](https://github.com/saltyrtc/saltyrtc-client-java/blob/master/src/test/java/org/saltyrtc/client/tests/LazysodiumCryptoInstance.java),
+but depending on your target platform and requirements you should probably provide
+your own.
 
 ```java
-import org.saltyrtc.client.crypto.JnaclCryptoProvider;
-
-final CryptoProvider cryptoProvider = new JnaclCryptoProvider();
+final CryptoProvider cryptoProvider = new MyCryptoProvider();
 ```
 
 ## The SaltyRTCBuilder
@@ -129,7 +128,7 @@ import org.saltyrtc.client.keystore.KeyStore;
 import org.saltyrtc.tasks.webrtc.WebRTCTask;
 
 final SSLContext sslContext = SSLContext.getDefault();
-final CryptoProvider cryptoProvider = new JnaclCryptoProvider();
+final CryptoProvider cryptoProvider = new MyCryptoProvider();
 final SaltyRTC client = new SaltyRTCBuilder(cryptoProvider)
         .connectTo(Config.SALTYRTC_HOST, Config.SALTYRTC_PORT, sslContext)
         .withKeyStore(new KeyStore(cryptoProvider))

src/main/java/org/saltyrtc/client/crypto/CryptoProvider.java

@@ -22,7 +22,7 @@ public interface CryptoProvider {
     int BOXOVERHEAD = 16;
 
     /**
-     * Create a public/private key pair.
+     * Create a public/private key pair. Put the key material in the provided buffers.
      *
      * @param publickey A byte array with size PUBLICKEYBYTES
      * @param privatekey A byte array with size PRIVATEKEYBYTES