cn.felord.callback.PKCS7Encoder Maven / Gradle / Ivy

/*
 *  Copyright (c) 2023. felord.cn
 *    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
 *       https://www.apache.org/licenses/LICENSE-2.0
 *  Website:
 *       https://felord.cn
 *  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 cn.felord.callback;

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

/**
 * 提供基于PKCS7算法的加解密接口.
 */
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
	 */
	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(StandardCharsets.UTF_8);
	}

	/**
	 * 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 =  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;
	}

}