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package com.mizhousoft.weixin.common.util;

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

/**
 * 提供基于PKCS7算法的加解
 *
 */
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;
	}
}