加密是保证数据安全的手段之1。加密是将纯文本数据转换难堪以理解的密文；解密是将密文转换回纯文本。
数据的加解密属于密码学的范畴。通常，加密和解密都需要使用1些秘密信息，这些秘密信息叫做密钥，将纯文本转为密文或转回的时候都要用到这些密钥。
对称加密指的是发送者和接收者共用同1个密钥的加解密方法。
非对称加密(又称公钥加密)指的是需要1个私有密钥1个公然密钥，两个不同的密钥的加解密体系。虽然不同，这个密钥对的这两个部份在算法上是有关联的。1个密钥将纯文本加密，另外一个将密文解密。没有1个密钥能够把加密和加密的功能全部自己完成。公钥，或用于加密数据的密钥，可以被自由分发。
RSA 是基于大整数分解的公钥加密的算法之1。RSA 代表了 Ron Rivest、Adi Shamir 和 Leonard Adleman 3人，RSA 就是他们1起提出的。
以下示例就展现了如何在 Java 中使用 RSA 算法对信息进行加解密。
java.security.KeyPairGenerator 类的实例用于产生1个 RSA 算法的公钥和私钥对，以后将其保存到文件。
javax.crypto.Cipher 类的实例用于使用上面产生的密钥对对信息进行加解密。
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.PublicKey;
import javax.crypto.Cipher;
/**
* @author JavaDigest
*
*/
public class EncryptionUtil {
/**
* String to hold name of the encryption algorithm.
*/
public static final String ALGORITHM = "RSA";
/**
* String to hold the name of the private key file.
*/
public static final String PRIVATE_KEY_FILE = "C:/keys/private.key";
/**
* String to hold name of the public key file.
*/
public static final String PUBLIC_KEY_FILE = "C:/keys/public.key";
/**
* Generate key which contains a pair of private and public key using 1024
* bytes. Store the set of keys in Prvate.key and Public.key files.
*
* @throws NoSuchAlgorithmException
* @throws IOException
* @throws FileNotFoundException
*/
public static void generateKey() {
try {
final KeyPairGenerator keyGen = KeyPairGenerator.getInstance(ALGORITHM);
keyGen.initialize(1024);
final KeyPair key = keyGen.generateKeyPair();
File privateKeyFile = new File(PRIVATE_KEY_FILE);
File publicKeyFile = new File(PUBLIC_KEY_FILE);
// Create files to store public and private key
if (privateKeyFile.getParentFile() != null) {
privateKeyFile.getParentFile().mkdirs();
}
privateKeyFile.createNewFile();
if (publicKeyFile.getParentFile() != null) {
publicKeyFile.getParentFile().mkdirs();
}
publicKeyFile.createNewFile();
// Saving the Public key in a file
ObjectOutputStream publicKeyOS = new ObjectOutputStream(
new FileOutputStream(publicKeyFile));
publicKeyOS.writeObject(key.getPublic());
publicKeyOS.close();
// Saving the Private key in a file
ObjectOutputStream privateKeyOS = new ObjectOutputStream(
new FileOutputStream(privateKeyFile));
privateKeyOS.writeObject(key.getPrivate());
privateKeyOS.close();
} catch (Exception e) {
e.printStackTrace();
}
}
/**
* The method checks if the pair of public and private key has been generated.
*
* @return flag indicating if the pair of keys were generated.
*/
public static boolean areKeysPresent() {
File privateKey = new File(PRIVATE_KEY_FILE);
File publicKey = new File(PUBLIC_KEY_FILE);
if (privateKey.exists() && publicKey.exists()) {
return true;
}
return false;
}
/**
* Encrypt the plain text using public key.
*
* @param text
* : original plain text
* @param key
* :The public key
* @return Encrypted text
* @throws java.lang.Exception
*/
public static byte[] encrypt(String text, PublicKey key) {
byte[] cipherText = null;
try {
// get an RSA cipher object and print the provider
final Cipher cipher = Cipher.getInstance(ALGORITHM);
// encrypt the plain text using the public key
cipher.init(Cipher.ENCRYPT_MODE, key);
cipherText = cipher.doFinal(text.getBytes());
} catch (Exception e) {
e.printStackTrace();
}
return cipherText;
}
/**
* Decrypt text using private key.
*
* @param text
* :encrypted text
* @param key
* :The private key
* @return plain text
* @throws java.lang.Exception
*/
public static String decrypt(byte[] text, PrivateKey key) {
byte[] dectyptedText = null;
try {
// get an RSA cipher object and print the provider
final Cipher cipher = Cipher.getInstance(ALGORITHM);
// decrypt the text using the private key
cipher.init(Cipher.DECRYPT_MODE, key);
dectyptedText = cipher.doFinal(text);
} catch (Exception ex) {
ex.printStackTrace();
}
return new String(dectyptedText);
}
/**
* Test the EncryptionUtil
*/
public static void main(String[] args) {
try {
// Check if the pair of keys are present else generate those.
if (!areKeysPresent()) {
// Method generates a pair of keys using the RSA algorithm and stores it
// in their respective files
generateKey();
}
final String originalText = "Text to be encrypted ";
ObjectInputStream inputStream = null;
// Encrypt the string using the public key
inputStream = new ObjectInputStream(new FileInputStream(PUBLIC_KEY_FILE));
final PublicKey publicKey = (PublicKey) inputStream.readObject();
final byte[] cipherText = encrypt(originalText, publicKey);
// Decrypt the cipher text using the private key.
inputStream = new ObjectInputStream(new FileInputStream(PRIVATE_KEY_FILE));
final PrivateKey privateKey = (PrivateKey) inputStream.readObject();
final String plainText = decrypt(cipherText, privateKey);
// Printing the Original, Encrypted and Decrypted Text
System.out.println("Original: " + originalText);
System.out.println("Encrypted: " +cipherText.toString());
System.out.println("Decrypted: " + plainText);
} catch (Exception e) {
e.printStackTrace();
}
}
}
原文链接：https://javadigest.wordpress.com/2012/08/26/rsa-encryption-example/。