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There is a newer version: 5.6.4

/**
 * 对公众平台发送给公众账号的消息加解密示例代码.
 * 
 * @copyright Copyright (c) 1998-2014 Tencent Inc.
 */

// ------------------------------------------------------------------------

package org.frameworkset.wx.common.mp.aes;

import java.nio.charset.Charset;
import java.util.Arrays;

/**
 * 提供基于PKCS7算法的加解密接口.
 */
class PKCS7Encoder {
  static Charset CHARSET = Charset.forName("utf-8");
  static int BLOCK_SIZE = 32;

  /**
   * 获得对明文进行补位填充的字节.
   * 
   * @param count 需要进行填充补位操作的明文字节个数
   * @return 补齐用的字节数组
   */
  static byte[] encode(int count) {
    // 计算需要填充的位数
    int amountToPad = BLOCK_SIZE - (count % BLOCK_SIZE);
    if (amountToPad == 0) {
      amountToPad = BLOCK_SIZE;
    }
    // 获得补位所用的字符
    char padChr = chr(amountToPad);
    String tmp = new String();
    for (int index = 0; index < amountToPad; index++) {
      tmp += padChr;
    }
    return tmp.getBytes(CHARSET);
  }

  /**
   * 删除解密后明文的补位字符
   * 
   * @param decrypted 解密后的明文
   * @return 删除补位字符后的明文
   */
  static byte[] decode(byte[] decrypted) {
    int pad = (int) decrypted[decrypted.length - 1];
    if (pad < 1 || pad > 32) {
      pad = 0;
    }
    return Arrays.copyOfRange(decrypted, 0, decrypted.length - pad);
  }

  /**
   * 将数字转化成ASCII码对应的字符，用于对明文进行补码
   * 
   * @param a 需要转化的数字
   * @return 转化得到的字符
   */
  static char chr(int a) {
    byte target = (byte) (a & 0xFF);
    return (char) target;
  }

}