Bonjour,
package myrsa;
import java.io.*;
import java.math.*;
import javax.crypto.*;
import java.security.*;
import java.security.spec.*;
import java.security.interfaces.*;
import java.math.BigInteger;


/**
* Cette classe propose des méthodes permettant de crypter et décrypter des
* messages avec l'algorithme RSA. Le message doit cependant être plus petit
* que KEY_SIZE.
*/


/**
*
* @author kerim
*/
public class MyRsa {public final static int KEY_SIZE = 1024; // [512..2048]

private RSAPublicKey publicKey;
private RSAPrivateKey privateKey;


public MyRsa() {
}


public RSAPublicKey getPublicKey() {
return publicKey;
}


public byte[] getPublicKeyInBytes() {
return publicKey.getEncoded();
}


public RSAPrivateKey getPrivateKey() {
return privateKey;
}


public byte[] getPrivateKeyInBytes() {
return privateKey.getEncoded();
}


public void setPublicKey(RSAPublicKey publicKey) {
this.publicKey = publicKey;
}


public void setPublicKey(byte[] publicKeyData) {
try {
X509EncodedKeySpec publicKeySpec = new X509EncodedKeySpec(publicKeyData);
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
publicKey = (RSAPublicKey)keyFactory.generatePublic(publicKeySpec);
}
catch (Exception e) {System.out.println(e);}
}


public void setPrivateKey(RSAPrivateKey privateKey) {
this.privateKey = privateKey;
}


public void setPrivateKey(byte[] privateKeyData) {
try {
PKCS8EncodedKeySpec privateKeySpec = new PKCS8EncodedKeySpec(privateKeyData);
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
privateKey = (RSAPrivateKey)keyFactory.generatePrivate(privateKeySpec);
}
catch (Exception e) {System.out.println(e);}
}


public void generateKeyPair() {
try {
KeyPairGenerator keyPairGen = KeyPairGenerator.getInstance("RSA");
keyPairGen.initialize(KEY_SIZE, new SecureRandom());
KeyPair kp = keyPairGen.generateKeyPair();
publicKey = (RSAPublicKey)kp.getPublic();
privateKey = (RSAPrivateKey)kp.getPrivate();



}
catch (Exception e) {System.out.println(e);}
}


public byte[] crypt(byte[] plaintext) {
return crypt(new BigInteger(addOneByte(plaintext))).toByteArray();

}


public byte[] crypt(String plaintext) {
return crypt(plaintext.getBytes());
}


public byte[] decryptInBytes(byte[] ciphertext) {
return removeOneByte(decrypt(new BigInteger(ciphertext)).toByteArray());
}


public String decryptInString(byte[] ciphertext) {
return new String(decryptInBytes(ciphertext));
}


/**
* Cette méthode permet de tester le bon fonctionnement des autres.
*/
public static void main(String[] args) {
String plaintext = "issam";
System.out.println("plaintext = " + plaintext);
MyRsa rsa = new MyRsa();
rsa.generateKeyPair();

byte[] publicKey = rsa.getPublicKeyInBytes();
byte[] privateKey = rsa.getPrivateKeyInBytes();

byte[] ciphertext = rsa.crypt(plaintext);

System.out.println("ciphertext = " + new BigInteger(ciphertext));

// rsa.setPublicKey(publicKey);
// rsa.setPrivateKey(privateKey);
String plaintext2 = rsa.decryptInString(ciphertext);
System.out.println("plaintext2 = " + plaintext2);
if (!plaintext2.equals(plaintext)) System.out.println("Error: plaintext2 != plaintext");
}


private BigInteger crypt(BigInteger plaintext) {
return plaintext.modPow(publicKey.getPublicExponent(), publicKey.getModulus());
}


private BigInteger decrypt(BigInteger ciphertext) {
return ciphertext.modPow(privateKey.getPrivateExponent(), privateKey.getModulus());
}


/**
* Ajoute un byte de valeur 1 au début du message afin d'éviter que ce dernier
* ne corresponde pas à un nombre négatif lorsqu'il sera transformé en
* BigInteger.
*/
private static byte[] addOneByte(byte[] input) {
byte[] result = new byte[input.length+1];
result[0] = 1;
for (int i = 0; i < input.length; i++) {
result[i+1] = input[i];
}
return result;
}


/**
* Retire le byte ajouté par la méthode addOneByte.
*/
private static byte[] removeOneByte(byte[] input) {
byte[] result = new byte[input.length-1];
for (int i = 0; i < result.length; i++) {
result[i] = input[i+1];
}
return result;
}}