为什么加密的字符串过长时，解密后长度丢失？

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)));
		
	}
}

最后控制台输出 的事 curity   前面部分消失了