RSA encryption Android and decryption in Java: javax.crypto.BadPaddingException: Decryption error

Question

I have an Android app which encrypts a text using RSA 2048 bit key.

Encryption and Decryption are working fine on the app.

My problem is that i am trying to encrypt on the application and then send the cipher to an external Java program for decryption.

Here is how it is done:

public static PrivateKey getPrivateKey(String key) {
    try {
        /* Add PKCS#8 formatting */
        byte[] byteKey = Base64.getDecoder().decode(key.getBytes());
        ASN1EncodableVector v = new ASN1EncodableVector();
        v.add(new ASN1Integer(0));
        ASN1EncodableVector v2 = new ASN1EncodableVector();
        v2.add(new ASN1ObjectIdentifier(PKCSObjectIdentifiers.rsaEncryption.getId()));
        v2.add(DERNull.INSTANCE);
        v.add(new DERSequence(v2));
        v.add(new DEROctetString(byteKey));
        ASN1Sequence seq = new DERSequence(v);
        byte[] privKey = seq.getEncoded("DER");

        PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(privKey);
        KeyFactory kf = KeyFactory.getInstance("RSA");
        return kf.generatePrivate(keySpec);
    } catch (Exception e) {
        e.printStackTrace();
    }
    return null;
}

public static String decrypt(String cipherString,PrivateKey privateKey){
    try{
        Cipher cipher = Cipher.getInstance("RSA");
        cipher.init(Cipher.DECRYPT_MODE, privateKey);
        byte[] plainByte = cipher.doFinal(Base64.getDecoder().decode(cipherString));
        return Base64.getEncoder().encodeToString(plainByte);
    }
    catch (Exception e) {
        e.printStackTrace();
    }
    return null;
}

But when i am trying to decryption the cipher i am getting error:

javax.crypto.BadPaddingException: Decryption error
    at sun.security.rsa.RSAPadding.unpadV15(RSAPadding.java:380)
    at sun.security.rsa.RSAPadding.unpad(RSAPadding.java:291)
    at com.sun.crypto.provider.RSACipher.doFinal(RSACipher.java:363)
    at com.sun.crypto.provider.RSACipher.engineDoFinal(RSACipher.java:389)
    at javax.crypto.Cipher.doFinal(Cipher.java:2165)
    at RSA.decrypt(RSA.java:94)
    at RSA.main(RSA.java:116)

what am i missing?

UPDATE i regenerated a new pair of keys within Java itself and got rid of the part:

/* Add PKCS#8 formatting */
byte[] byteKey = Base64.getDecoder().decode(key.getBytes());
ASN1EncodableVector v = new ASN1EncodableVector();
v.add(new ASN1Integer(0));
ASN1EncodableVector v2 = new ASN1EncodableVector();
v2.add(new ASN1ObjectIdentifier(PKCSObjectIdentifiers.rsaEncryption.getId()));
v2.add(DERNull.INSTANCE);
v.add(new DERSequence(v2));
v.add(new DEROctetString(byteKey));
ASN1Sequence seq = new DERSequence(v);
byte[] privKey = seq.getEncoded("DER");

but I'm stuck with the same error!

Answer 1

Your padding is either not being set or set wrong.

Try changing Cipher cipher = Cipher.getInstance("RSA"); to Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");

If that doesn't work you can see here for other padding modes.