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/*
 * Copyright (C) 2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.gitlab.summercattle.addons.wechat.officialaccounts.security;

import java.util.Arrays;

import org.apache.commons.codec.binary.StringUtils;

/**
 * Pkcs7编码器
 * @author orange
 *
 */
public class Pkcs7Encoder {

	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
	 * @throws ESException 
	 */
	public 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 StringUtils.getBytesUtf8(tmp);
	}

	/**
	 * 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 = (int) decrypted[decrypted.length - 1];
		if (pad < 1 || pad > BLOCK_SIZE) {
			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;
	}
}