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/*
 * 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 me.chanjar.weixin.common.util.crypto;

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

/**
 * 提供基于PKCS7算法的加解.
 *
 * @author tencent
 */
public class PKCS7Encoder {
  private static final Charset CHARSET = StandardCharsets.UTF_8;
  private static final 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
   */
  public static byte[] encode(int count) {
    // 计算需要填充的位数
    int amountToPad = BLOCK_SIZE - (count % BLOCK_SIZE);
    // 获得补位所用的字符
    char padChr = chr(amountToPad);
    StringBuilder tmp = new StringBuilder();
    for (int index = 0; index < amountToPad; index++) {
      tmp.append(padChr);
    }
    return tmp.toString().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
   */
  public static byte[] decode(byte[] decrypted) {
    int pad = 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
   */
  private static char chr(int a) {
    byte target = (byte) (a & 0xFF);
    return (char) target;
  }

}




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