BadPaddingException RSA Encrypt then decrypt in java

Question

I have been given an RSA public Key and an RSA private key with OAEP SHA-256 padding. I am attempting to simply encrypt a random string and then decrypt it to assert the result is equalt to the original.

This is the public key i have been given:

This is the private key I have been given:

I am reading this key from a pem file "crypt_pkcs8.pem" but recive the public key as a byte array from a gRPC call.

Here is my Java code:

public class Encryption{
public static void main(String args){
    ByteString publicKey = client.getPublicKey(nonce).getRSAEncryptionKey();

    String strKeyPEM = "";
    BufferedReader br = new BufferedReader(new FileReader("crypt_pkcs8.pem"));
    String line;
    while ((line = br.readLine()) != null) {
        strKeyPEM += line+"\n";
    }
    br.close();
    byte[] pt;
    try {
        pt = "h".getBytes("UTF-8");
        byte[] ct =encrypt(publicKey, pt);
        byte[] response= decrypt(strKeyPEM, ct);

        assertEquals(pt, response);
    } catch (UnsupportedEncodingException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}
public static byte[] encrypt(ByteString rawKey ,byte[] message){
    String strippedKey=stripKey(rawKey.toStringUtf8());

    byte[] keyBytes = Base64.getDecoder().decode(strippedKey);
    System.out.println(keyBytes.length);

    Cipher cipher_RSA;
    try {
        cipher_RSA = Cipher.getInstance("RSA/ECB/OAEPWITHSHA-256ANDMGF1PADDING");
        KeyFactory keyFactory = KeyFactory.getInstance("RSA");
        X509EncodedKeySpec spec = new X509EncodedKeySpec(keyBytes);
        PublicKey pk = keyFactory.generatePublic(spec);
        System.out.println(pk.getAlgorithm()+" format : "+pk.getFormat());

        cipher_RSA.init(Cipher.ENCRYPT_MODE, pk); 
        return cipher_RSA.doFinal(message);
    } catch (NoSuchAlgorithmException | NoSuchPaddingException | IllegalBlockSizeException |
            BadPaddingException | InvalidKeyException | InvalidKeySpecException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
        return null;
    }

}

public static byte[] decrypt(String rawKey ,byte[] message){
    String strippedKey=stripPrivateKey(rawKey);

    byte[] keyBytes = Base64.getDecoder().decode(strippedKey);
    System.out.println(keyBytes.length);

    Cipher cipher_RSA;
    try {
        cipher_RSA = Cipher.getInstance("RSA/ECB/OAEPWITHSHA-256ANDMGF1PADDING");
        KeyFactory keyFactory = KeyFactory.getInstance("RSA");
        PKCS8EncodedKeySpec spec = new PKCS8EncodedKeySpec(keyBytes);
        PrivateKey pk = keyFactory.generatePrivate(spec);
        System.out.println(pk.getAlgorithm()+" format : "+pk.getFormat());

        cipher_RSA.init(Cipher.DECRYPT_MODE, pk); 
        return cipher_RSA.doFinal(message);
    } catch (NoSuchAlgorithmException | NoSuchPaddingException | IllegalBlockSizeException |
            BadPaddingException | InvalidKeyException | InvalidKeySpecException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
        return null;
    }
}
public static String stripKey(String key){
    key = key.replace("-----BEGIN RSA PUBLIC KEY-----\n", "");
    key = key.replace("-----END RSA PUBLIC KEY-----", "");
    key = key.replace("\n", "");
    return key;
}

public static String stripPrivateKey(String key){
    key = key.replace("-----BEGIN PRIVATE KEY-----", "");
    key = key.replace("-----END PRIVATE KEY-----", "");
    key = key.replace("\n", "");
    return key;
}


}

I cannot find anything obviously wrong with my actaul code so I believe this is to do with the keys not matching.

I also notice the header/footer is different on the public key and private key. From looking online this is because the public key is in PKCS#8 format.

Will I need to change the public key to pkcs#8 format as well? If so what is the easiest way to do this?

I have also been told I should be able to extract the public key (in pkcs8 format) from the private key given above. Can this be done easily?

Your public key is not in pkcs8 format. You have it in pkcs1 format. Use openssl to convert it to pkcs8. — Johann, Apr 01 '20 at 06:36

Maarten Bodewes · Answer 1 · 2017-08-10T12:06:42.323

If you can get a valid public key then the key is not in PKCS#8 format, it will be in X.509 (SubjectPublicKeyInfo) format as Java expects. However, your public and private keys do indeed not match. You can use the answer here to create the correct public key file from the private key file.

Java doesn't directly contain code to retrieve the public exponent from an RSAPrivateKey without CRT parameters. However, if you want a Java only solution you could cast the PrivateKey even further to RSAPrivateCrtKey which does contain a getPublicExponent method to retrieve the public exponent. Then you can use an RSA KeyFactory to generate a public key using RSAPublicKeySpec.

You could also parse the PEM file to retrieve the public key in Java using e.g. the Bouncy Castle API. But expect a steep learning curve if you do.

Don't forget to upvote [this answer](https://stackoverflow.com/a/24768779/589259) from James! — Maarten Bodewes, Aug 10 '17 at 12:13

BadPaddingException RSA Encrypt then decrypt in java

1 Answers1