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Pure Java implementation of libzmq
There is a newer version: 0.6.0
package org.zeromq;

import java.io.File;
import java.io.IOException;

import org.zeromq.ZMQ.Curve.KeyPair;
import org.zeromq.util.ZMetadata;

import zmq.Options;
import zmq.util.Z85;

/**
 *
    The ZCert class provides a way to create and work with security
    certificates for the ZMQ CURVE mechanism. A certificate contains a
    public + secret key pair, plus metadata. It can be used as a
    temporary object in memory, or persisted to disk.
    
    To exchange certificates, send the public file via some secure route.
    Certificates are not signed but are text files that can be verified by
    eye.
    
    Certificates are stored in the ZeroMQ Property Language format.
    

    They have two sections, "metadata" and "curve".
    

    The first contains a list of 'name = value' pairs, one per line.
    Values may be enclosed in quotes.
    

    The curve section has a 'public-key = key-value' and, for secret certificates, a
    'secret-key = key-value' line.
    

    The key-value is a {@link zmq.util.Z85 Z85-encoded CURVE key}.
 *
 */
public class ZCert
{
    private final byte[]    publicKey;                  //  Public key in binary
    private final byte[]    secretKey;                  //  Secret key in binary
    private final String    publicTxt;                  //  Public key in Z85 text
    private final String    secretTxt;                  //  Secret key in Z85 text
    private final ZMetadata metadata = new ZMetadata(); //  Certificate metadata

    public ZCert(String publickey)
    {
        if (publickey.length() == Options.CURVE_KEYSIZE) {
            // in binary-format
            publicKey = publickey.getBytes(ZMQ.CHARSET);
            publicTxt = ZMQ.Curve.z85Encode(publicKey);
        }
        else {
            assert (publickey.length() == Options.CURVE_KEYSIZE_Z85);
            // Z85-Coded
            publicKey = Z85.decode(publickey);
            publicTxt = publickey;
        }
        secretKey = null;
        secretTxt = null;
    }

    public ZCert()
    {
        KeyPair keypair = ZMQ.Curve.generateKeyPair();
        publicKey = ZMQ.Curve.z85Decode(keypair.publicKey);
        publicTxt = keypair.publicKey;
        secretKey = ZMQ.Curve.z85Decode(keypair.secretKey);
        secretTxt = keypair.secretKey;
    }

    public byte[] getPublicKey()
    {
        return publicKey;
    }

    public byte[] getSecretKey()
    {
        return secretKey;
    }

    public String getPublicKeyAsZ85()
    {
        return publicTxt;
    }

    public String getSecretKeyAsZ85()
    {
        return secretTxt;
    }

    public void apply(ZMQ.Socket socket)
    {
        socket.setCurvePublicKey(publicKey);
        socket.setCurveSecretKey(secretKey);
    }

    public ZMetadata getMetadata()
    {
        return metadata;
    }

    public void setMeta(String key, String value)
    {
        metadata.set(key, value);
    }

    private void add(ZMetadata meta, ZConfig config)
    {
        for (String key : meta.keySet()) {
            config.putValue("metadata/" + key, meta.get(key));
        }
    }

    /**
     * Saves the public  key to a file.
     * @param filename the path of the file to save the certificate into.
     * @throws IOException if unable to save the file.
     */
    public File savePublic(String filename) throws IOException
    {
        ZConfig conf = new ZConfig("root", null);
        add(metadata, conf);
        conf.addComment("   ZeroMQ CURVE Public Certificate");
        conf.addComment("   Exchange securely, or use a secure mechanism to verify the contents");
        conf.addComment("   of this file after exchange. Store public certificates in your home");
        conf.addComment("   directory, in the .curve subdirectory.");
        conf.putValue("/curve/public-key", publicTxt);
        return conf.save(filename);
    }

    /**
     * Saves the public and secret keys to a file.
     * @param filename the path of the file to save the certificate into.
     * @throws IOException if unable to save the file.
     */
    public File saveSecret(String filename) throws IOException
    {
        ZConfig conf = new ZConfig("root", null);
        add(metadata, conf);
        conf.addComment("   ZeroMQ CURVE **Secret** Certificate");
        conf.addComment("   DO NOT PROVIDE THIS FILE TO OTHER USERS nor change its permissions.");
        conf.putValue("/curve/public-key", publicTxt);
        conf.putValue("/curve/secret-key", secretTxt);
        return conf.save(filename);
    }
}