JAVA每日一题10

?? 题目：用JAVA做一个数字签名的例子。

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package test;import java.security.*;import javax.crypto.*;public class DigitalSignatureExample{public static void main(String[] args) throws Exception{// 检查输入参数，得到明文if (args.length != 1){System.err.println("Usage: java DigitalSignature1Example text");System.exit(1);}byte[] plainText = args[0].getBytes("UTF8");// 得到MD5消息摘要对象并且计算明文摘要MessageDigest messageDigest = MessageDigest.getInstance("MD5");System.out.println("\n" + messageDigest.getProvider().getInfo());messageDigest.update(plainText);byte[] md = messageDigest.digest();System.out.println("\nDigest: ");System.out.println(new String(md, "UTF8"));// 生成RSA密钥对System.out.println("\nStart generating RSA key");KeyPairGenerator keyGen=KeyPairGenerator.getInstance("RSA");keyGen.initialize(1024);KeyPair key = keyGen.generateKeyPair();System.out.println("Finish generating RSA key");// 得到RSA cipher同时列出算法的提供人Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");System.out.println("\n" + cipher.getProvider().getInfo());// 将信息加密并且利用RSA私钥进行签名System.out.println("\nStart encryption");cipher.init(Cipher.ENCRYPT_MODE, key.getPrivate());byte[] cipherText = cipher.doFinal(md);System.out.println("Finish encryption: ");System.out.println(new String(cipherText, "UTF8"));// 利用RSA公钥进行解密System.out.println("\nStart decryption");cipher.init(Cipher.DECRYPT_MODE, key.getPublic());byte[] newMD = cipher.doFinal(cipherText);System.out.println("Finish decryption: ");System.out.println(new String(newMD, "UTF8"));// 重新建立信息明文System.out.println("\nStart signature verification");messageDigest.reset();messageDigest.update(plainText);byte[] oldMD = messageDigest.digest();// 检查签名前后数据是否相同int len = newMD.length;if (len > oldMD.length){System.out.println("Signature failed, length error");System.exit(1);}for (int i = 0; i < len; ++i){if (oldMD[i] != newMD[i]){System.out.println("Signature failed, element error");System.exit(1);}}System.out.println("Signature verified");}}

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