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This is a custom Diffie-Hellman key pair generator and a key agreement for the Curve25519 algorithm from Dan Bernstein, implemented as a Java Cryptography Extension (JCE).

The core of the Curve25519 algorithm is a fork of trevorbernard/curve25519-java.

The library contains custom KeyPairGenerator and KeyAgreement classes to get access to the algorithms without registering the JCE provider, as it may require strict JAR signatures and system permissions to do so. Of course, these custom classes are not suitable to integrate with TLS/SSL; they are provided for other custom key agreement mechanisms you may find useful.

Gradle dependency

The Gradle dependency is available via jCenter.

The minimum API level supported by this library is API 14 (Ice Cream Sandwich).

dependencies {
    compile 'br.eti.balena:ecdh-curve25519:0.1.3'

The JCE Provider is provided as a separated module (you usually don't need it):

dependencies {
    compile 'br.eti.balena:ecdh-curve25519-spi:0.1.3'


import org.junit.Test;

import javax.crypto.SecretKey;


import static;
import static org.junit.Assert.assertArrayEquals;

public class KeyExchangeTest {
    public void keyExchangeTest() throws Exception {
        Curve25519KeyPairGenerator keyPairGenerator = new Curve25519KeyPairGenerator();

        // Ana generates a key-pair as follows:
        KeyPair anaKeyPair = keyPairGenerator.generateKeyPair();

        // Now Ana saves the privateKey for later, and sends the public key to Bob.
        // The public key bytes are taken as anaKeyPair.getPublic().getEncoded().

        // Bob now generates his key pair:
        KeyPair bobKeyPair = keyPairGenerator.generateKeyPair();

        // Now Bob obtains the bobSharedSecret in this manner:
        Curve25519KeyAgreement bobKeyAgreement = new Curve25519KeyAgreement(bobKeyPair.getPrivate());
        SecretKey bobSharedSecret = bobKeyAgreement.generateSecret(ALGORITHM);

        // And, by using this shared secret, Bob can now encrypt the message.

        // At the Ana's side, the same shared secret is generated from the
        // public key sent by Bob.
        Curve25519KeyAgreement anaKeyAgreement = new Curve25519KeyAgreement(anaKeyPair.getPrivate());
        SecretKey anaSharedSecret = anaKeyAgreement.generateSecret(ALGORITHM);

        // Confirms that both shared secrets are equal.
        assertArrayEquals(anaSharedSecret.getEncoded(), bobSharedSecret.getEncoded());


Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.


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