rsaalgorithm.java

DES算法全称为Data Encryption Standard,即数据加密算法,DES算法把64位的明文输入块变为64位的密文输出块,它所使用的密钥也是64位。RSA的安全性依赖于大数分解。公钥和私

package org.encryption.rsa;

import java.math.BigInteger;
import java.util.Random;

public class RSAAlgorithm {
	private BigInteger p;
	private BigInteger q;
	private BigInteger n;
	private BigInteger e;
	private BigInteger d;

	public RSAAlgorithm() {
		init();
	}

	// rsa系统初始化,包括产生公钥(e,n)私钥(p,q,d);
	private void init() {
		System.out.println("----------开始初始化--------生成公钥密钥");
		// 用Miller-Rabin概率素数测试法产生大素数
		p = BigInteger.probablePrime(512, new Random());
		q = BigInteger.probablePrime(500, new Random());
		n = p.multiply(q);
		
		e = BigInteger.valueOf(7);
		
		BigInteger T = p.subtract(BigInteger.ONE).multiply(
				q.subtract(BigInteger.ONE));
		// (e,T)=1&&(1<e<T)==true
		while (T.gcd(e).compareTo(BigInteger.ONE) != 0) {
			e.add(BigInteger.ONE);
		}
		d = e.modInverse(T);
		System.out.println("----------初始化完成--------");
	}

	/**
	 * @param withPrivekey
	 *            用私钥加密是为true,用公钥加密时为false
	 * @return
	 */
	public BigInteger encrypt(BigInteger plainInt, boolean withPrivakey) {
		if (withPrivakey)
			return modPow(plainInt, d, n);
		else
			return modPow(plainInt, e,
					n);
	}

	/**
	 * @param withPrivekey
	 *            用私钥解密是为true,用公钥解密时为false
	 * @return
	 */
	public BigInteger decrypt(BigInteger encryptedInt, boolean withPrivakey) {
		if (withPrivakey)
			return modPow(encryptedInt, d, n);
		else
			return modPow(encryptedInt, 
					e, n);
	}

	// 核心：大数的模幂运算~~~~蒙哥玛利模幂算法(a^^b)%c
	private  BigInteger XmodPow(BigInteger a, BigInteger b, BigInteger c) {
		if (b.compareTo(BigInteger.ONE) == 0)
			return a.mod(c);
		BigInteger index = BigInteger.ONE;
		BigInteger result = a.mod(c);
		BigInteger temp = BigInteger.ONE;
		while (index.compareTo(b) == -1) {
			temp = result;
			result = result.multiply(result).mod(c);
			index = index.add(index);
		}
		if(index.compareTo(b) ==1)
		  result = temp.multiply(modPow(a,b.subtract(index.divide(BigInteger.valueOf(2))),c)).mod(c);
		return result;
	}
	//核心：大数的模幂运算~~~~二元法(a^^b)%c,效率较XmodPow()好很多
	private BigInteger modPow(BigInteger a, BigInteger b, BigInteger c) {

		String bBinaryString = ToBinaryString(b.toByteArray());
		BigInteger finalResult = BigInteger.ONE;
		BigInteger tempResult = BigInteger.ONE;
		for (int i = bBinaryString.length() - 1; i >= 0; i--) {
			if (i == (bBinaryString.length() - 1))
				tempResult= a.mod(c);
			else
				tempResult = tempResult.multiply(tempResult).mod(c);
			if (bBinaryString.charAt(i) == '1') {
				finalResult = finalResult.multiply(tempResult).mod(c);
			}
		}
		return finalResult;
	}
   


	// 判断一个大数的能否被2整除
	/*
	 * private boolean isCompsite(BigInteger bigInt) { if (bigInt.byteValue() %
	 * 2 == 0) return true; return false; }
	 */

	// 字节数组转化为字符串
	private String ToBinaryString(byte[] binary) {
		StringBuffer strBuf = new StringBuffer();
		String intStr;
		for (int i = 0; i < binary.length; i++) {
			intStr = Integer.toBinaryString(binary[i]);
			if (intStr.length() == 8)
				strBuf.append(intStr);
			else if (intStr.length() > 8) {
				strBuf.append(intStr.substring(intStr.length() - 8, intStr
						.length()));
			} else {
				char[] padding = new char[8 - intStr.length()];
				for (int j = 0; j < padding.length; j++)
					padding[j] = '0';
				strBuf.append(padding);
				strBuf.append(intStr);
			}
		}
		return strBuf.toString();
	}
}