对内容进行签名的实例

package com.yonge.messagedigest;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.security.InvalidKeyException;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.Signature;
import java.security.SignatureException;

import org.apache.commons.codec.binary.Base64;

public class SignUtil {

    //加密算法
    private final static String SIGNATURE_ALGORITHM      = "DSA";

    private final static String MESSAGE_DIGEST_ALGORITHM = "MD5";

    private final static String PRIVATE_KEY_FILE_NAME    = "private.key";

    private final static String PUBLIC_KEY_FILE_NAME     = "public.key";

    private static Signature    signature                = null;

    static {
        try {
            //获取签名对象
            signature = Signature.getInstance(SIGNATURE_ALGORITHM);
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
        }

    }

    /**
     * 利用DSA获取摘要，然后用base64进行编码
     * @param file 源文件
     * @return 返回摘要
     * @throws NoSuchAlgorithmException
     * @throws IOException
     */
    public static String getMessageDigest(File file) throws NoSuchAlgorithmException, IOException {
        MessageDigest messageDigest = null;
        FileInputStream fis = null;
        messageDigest = MessageDigest.getInstance(MESSAGE_DIGEST_ALGORITHM);
        byte[] buffer = new byte[1024];
        int length;
        fis = new FileInputStream(file);
        while ((length = fis.read(buffer)) > 0) {
            messageDigest.update(buffer, 0, length);
        }
        //将摘要进行base64编码
        return new String(Base64.encodeBase64(messageDigest.digest()));
    }

    /**
     * 利用DSA获取摘要，然后用base64进行编码
     * @param data
     * @return
     * @throws NoSuchAlgorithmException
     */
    public static String getMessageDigest(byte[] data) throws NoSuchAlgorithmException {
        MessageDigest messageDigest = MessageDigest.getInstance(MESSAGE_DIGEST_ALGORITHM);
        messageDigest.update(data);
        return new String(Base64.encodeBase64(messageDigest.digest()));
    }

    /**
     * 生成密钥对，并保存到文件中（公钥保存到public.key，私钥保存到private.key）
     * @return 是否生成成功
     * @throws NoSuchAlgorithmException
     * @throws IOException 
     */
    public static boolean generateKeyPair() throws NoSuchAlgorithmException, IOException {
        KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance(SIGNATURE_ALGORITHM);
        //生成公钥、私钥对
        KeyPair keyPair = keyPairGenerator.genKeyPair();

        //获取私钥
        PrivateKey privateKey = keyPair.getPrivate();

        //获取公钥
        PublicKey publicKey = keyPair.getPublic();

        //将公钥、私钥保存到本地文件中
        FileOutputStream fos_private = new FileOutputStream(new File(PRIVATE_KEY_FILE_NAME));
        ObjectOutputStream oos_private = new ObjectOutputStream(fos_private);
        oos_private.writeObject(privateKey);
        fos_private.close();
        oos_private.close();

        FileOutputStream fos_public = new FileOutputStream(new File(PUBLIC_KEY_FILE_NAME));
        ObjectOutputStream oos_public = new ObjectOutputStream(fos_public);
        oos_public.writeObject(publicKey);
        fos_public.close();
        oos_public.close();
        return true;
    }

    /**
     * 根据私钥将数据生成签名
     * @param data
     * @return 返回加密后的字节数组
     * @throws IOException 
     * @throws ClassNotFoundException 
     * @throws InvalidKeyException 
     * @throws SignatureException 
     */
    public static byte[] signByPrivateKey(byte[] data) throws IOException, ClassNotFoundException,
                                                      InvalidKeyException, SignatureException {
        PrivateKey privateKey = getPrivateKey();

        //初始化签名对象
        signature.initSign(privateKey);
        //签名操作
        signature.update(data);
        //将签名结果返回到数组中
        return signature.sign();
    }

    /**
     * 验证签名
     * @param data 
     * @param sign 签名
     * @return
     * @throws IOException
     * @throws ClassNotFoundException
     * @throws InvalidKeyException
     * @throws SignatureException
     */
    public static boolean verifyByPublicKey(byte[] data, byte[] sign) throws IOException,
                                                                     ClassNotFoundException,
                                                                     InvalidKeyException,
                                                                     SignatureException {
        PublicKey publicKey = getPublicKey();
        signature.initVerify(publicKey);
        signature.update(data);
        return signature.verify(sign);
    }

    /**
     * 获取私钥对象
     * @return
     * @throws IOException
     * @throws ClassNotFoundException
     */
    public static PrivateKey getPrivateKey() throws IOException, ClassNotFoundException {
        FileInputStream fis = null;
        ObjectInputStream ois = null;
        PrivateKey privateKey = null;
        try {
            //加载私钥文件
            fis = new FileInputStream(new File(PRIVATE_KEY_FILE_NAME));
            ois = new ObjectInputStream(fis);
            //读取对象
            privateKey = (PrivateKey) ois.readObject();
        } finally {
            //关闭流
            if (fis != null) {
                fis.close();
            }
            if (ois != null) {
                ois.close();
            }
        }
        return privateKey;
    }

    /**
     * 获取公钥对象
     * @return
     * @throws IOException
     * @throws ClassNotFoundException
     */
    public static PublicKey getPublicKey() throws IOException, ClassNotFoundException {

        FileInputStream fis = null;
        ObjectInputStream ois = null;
        PublicKey publicKey = null;
        try {
            //加载公钥文件
            fis = new FileInputStream(new File(PUBLIC_KEY_FILE_NAME));
            ois = new ObjectInputStream(fis);
            //读取对象
            publicKey = (PublicKey) ois.readObject();
        } finally {
            //关闭流
            if (fis != null) {
                fis.close();
            }
            if (ois != null) {
                ois.close();
            }
        }
        return publicKey;
    }
}