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Curve25519 library for Java
There is a newer version: 0.5.0
/**
* Copyright (C) 2015 Open Whisper Systems
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program.  If not, see .
*/
package org.whispersystems.curve25519;

import org.whispersystems.curve25519.java.Sha512;
import org.whispersystems.curve25519.java.curve_sigs;
import org.whispersystems.curve25519.java.scalarmult;

abstract class BaseJavaCurve25519Provider implements Curve25519Provider {

  private final Sha512               sha512provider;
  private       SecureRandomProvider secureRandomProvider;

  protected BaseJavaCurve25519Provider(Sha512 sha512provider,
                                       SecureRandomProvider secureRandomProvider)
  {
    this.sha512provider       = sha512provider;
    this.secureRandomProvider = secureRandomProvider;
  }

  public abstract boolean isNative();

  public void setRandomProvider(SecureRandomProvider secureRandomProvider) {
    this.secureRandomProvider = secureRandomProvider;
  }

  public byte[] calculateAgreement(byte[] ourPrivate, byte[] theirPublic) {
    byte[] agreement = new byte[32];
    scalarmult.crypto_scalarmult(agreement, ourPrivate, theirPublic);

    return agreement;
  }

  public byte[] generatePublicKey(byte[] privateKey) {
    byte[] publicKey = new byte[32];
    curve_sigs.curve25519_keygen(publicKey, privateKey);

    return publicKey;
  }

  public byte[] generatePrivateKey() {
    byte[] random = getRandom(PRIVATE_KEY_LEN);
    return generatePrivateKey(random);
  }

  public byte[] generatePrivateKey(byte[] random) {
    byte[] privateKey = new byte[32];

    System.arraycopy(random, 0, privateKey, 0, 32);

    privateKey[0]  &= 248;
    privateKey[31] &= 127;
    privateKey[31] |= 64;

    return privateKey;
  }

  public byte[] calculateSignature(byte[] random, byte[] privateKey, byte[] message) {
    byte[] result = new byte[64];

    if (curve_sigs.curve25519_sign(sha512provider, result, privateKey, message, message.length, random) != 0) {
      throw new IllegalArgumentException("Message exceeds max length!");
    }

    return result;
  }

  public boolean verifySignature(byte[] publicKey, byte[] message, byte[] signature) {
    return curve_sigs.curve25519_verify(sha512provider, signature, publicKey, message, message.length) == 0;
  }

  public byte[] getRandom(int length) {
    byte[] result = new byte[length];
    secureRandomProvider.nextBytes(result);
    return result;
  }
}