CODEWITHSUNDEEP
javasecurityencryptionrsa
alexsius
alexsius

Reputation: 43

Java encrypt/decrypt with RSA generating keys from modulus and exponents

I'm trying to generate a pair of keys from modulus,public exponent and private exponent to cipher a string and then decipher it, but the deciphered output is different from the input.

public class EncryptDecrypt {
public void encryptDecriptKey(){
    String text = new String("Hello");
    byte[] cipherData = null;
    byte[] cipherData2 = null;
    BigInteger modulus = new BigInteger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
    BigInteger pubExp = new BigInteger("65537", 16);
    BigInteger privateExp = new BigInteger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

    KeyFactory keyFactory = null;
    RSAPublicKeySpec pubKeySpec = new RSAPublicKeySpec(modulus, pubExp);
    RSAPublicKey key = null;
    Cipher cipher = null;
    try {
        keyFactory = KeyFactory.getInstance("RSA");
        key = (RSAPublicKey) keyFactory.generatePublic(pubKeySpec);
        cipher = Cipher.getInstance("RSA/ECB/NoPadding");
        cipher.init(Cipher.ENCRYPT_MODE, key);
        cipherData = cipher.doFinal(text.getBytes());
    } catch (IllegalBlockSizeException | BadPaddingException | InvalidKeyException | NoSuchPaddingException | NoSuchAlgorithmException | InvalidKeySpecException e) {
        e.printStackTrace();
    }



    KeyFactory keyFactory2 = null;
    RSAPrivateKeySpec privateKeySpec = new RSAPrivateKeySpec(modulus, privateExp);
    RSAPrivateKey privatekey = null;
    Cipher cipher2 = null;
    try {
        keyFactory2 = KeyFactory.getInstance("RSA");
        privatekey = (RSAPrivateKey) keyFactory1.generatePrivate(privateKeySpec);
        cipher2 = Cipher.getInstance("RSA/ECB/NoPadding");
        cipher2.init(Cipher.DECRYPT_MODE, privatekey);
        cipherData2 = cipher2.doFinal(cipherData);
    } catch (IllegalBlockSizeException | BadPaddingException | InvalidKeyException | NoSuchPaddingException | NoSuchAlgorithmException | InvalidKeySpecException e) {
        e.printStackTrace();
    }

        System.out.println(Arrays.toString(text.getBytes()));
        System.out.println(Arrays.toString(cipherData2));
        //the two outputs should be equals
}

}

Upvotes: 3

Views: 2558

Answers (1)

Devon Sutherland
Devon Sutherland

Reputation: 498

It's because you're reading the exponent as base 16 when it is base 10

BigInteger pubExp = new BigInteger("65537", 16);

This should be:

BigInteger pubExp = new BigInteger("65537", 10);

or

BigInteger pubExp = BigInteger.valueOf(65537);

Furthermore, after this fix, the result is usually left-padded with zero bytes. These need to be removed before comparing the strings.

The reason for that is that RSA operates on numbers. Because no padding method was used this padding cannot be be removed: your original plaintext may also have started with zero valued bytes. So you get back your plaintext padded up to the modulus in size (i.e. the key size).

To fix this last part use a secure padding, e.g. OAEP padding: "RSA/ECB/OAEPWithSHA1AndMGF1Padding" for encryption and decryption.

Upvotes: 5

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