请问为什么长度会丢失

public  class DH {

    public static void main(String[] args)throws Exception{

        //初始化发送方密钥----------------------------------------------------------------

        KeyPairGenerator keyPairGenerator=KeyPairGenerator.getInstance("DH");

        keyPairGenerator.initialize(1024);

        KeyPair senderKeyPair=keyPairGenerator.generateKeyPair();

        byte[] senderPbkEnc=senderKeyPair.getPublic().getEncoded();//得到发送方公钥的  byte数组形式;>>>>>>>>>>>>>>>>>-传递给接受方

        //使用发送方密钥中的参数 初始化接收方的密钥载体------------------------------------------------

        KeyFactory keyFactory=KeyFactory.getInstance("DH");

        X509EncodedKeySpec x509EncodedKeySpec=new X509EncodedKeySpec(senderPbkEnc); //根据发送方给定的编码密钥创建一个符合编码标准的 X509EncodedKeySpec

        PublicKey publicKey=keyFactory.generatePublic(x509EncodedKeySpec);   //密钥工厂根据 标准密钥规范的编码 生成公钥

        DHParameterSpec dhParameterSpec=DHPublicKey.class.cast(publicKey).getParams();  //从公钥中获取参数 

        KeyPairGenerator keyPairGenerator2=KeyPairGenerator.getInstance("DH");

        keyPairGenerator2.initialize(1024);

        keyPairGenerator2.initialize(dhParameterSpec); //利用发送方传递的公钥中获取的参数 初始化密钥生成器

        KeyPair receiverkeyPair=keyPairGenerator2.generateKeyPair(); //生成秘钥

        PrivateKey receiverPrivateKey=receiverkeyPair.getPrivate(); //获得私钥

        //获得公钥bytes  此公钥用于传递给发送方

        byte[] receiverPbkBytes=receiverkeyPair.getPublic().getEncoded();//>>>>>>>>>>>>>>>>>>>>>>传递给发送方

        //构建接收方密钥

        KeyAgreement keyAgreement=KeyAgreement.getInstance("DH");

        keyAgreement.init(receiverPrivateKey);

        keyAgreement.doPhase(publicKey, true);

        SecretKey receiverSecretKey=keyAgreement.generateSecret("DES"); //接收方密钥构建完成

        //构建发送方密钥---------------------------------------------------------------

        KeyFactory keyFactory2=keyFactory.getInstance("DH");

        X509EncodedKeySpec x509EncodedKeySpec2=new X509EncodedKeySpec(receiverPbkBytes);

        PublicKey senderPublicKey=keyFactory2.generatePublic(x509EncodedKeySpec2);

        KeyAgreement keyAgreement2=KeyAgreement.getInstance("DH");

        keyAgreement2.init(senderKeyPair.getPrivate());

        keyAgreement2.doPhase(senderPublicKey, true);

        SecretKey senderSecretKey =keyAgreement2.generateSecret("DES"); //发送方密钥构建完成

        //进行加解密

        String str="imooc security";

        Cipher cipher=Cipher.getInstance("DES");

        cipher.init(Cipher.ENCRYPT_MODE, senderSecretKey);

        cipher.update(str.getBytes());

        byte[] bs=cipher.doFinal();

        System.out.println("DH加密后:"+Hex.encodeHexString(bs));

        cipher.init(Cipher.DECRYPT_MODE, receiverSecretKey);

        System.out.println("解密后:"+new String(cipher.doFinal(bs)));

    }

}