// Use RSA/NONE/NoPadding as algorithm and BouncyCastle as crypto provider Cipher asymmetricCipher = Cipher.getInstance("RSA/NONE/NoPadding", "BC"); // asume, that publicKeyBytes contains a byte array representing // your public key X509EncodedKeySpec publicKeySpec = new X509EncodedKeySpec(publicKeyBytes); KeyFactory keyFactory; keyFactory = KeyFactory.getInstance(publicKeySpec.getFormat()); Key key = keyFactory.generatePublic(publicKeySpec); // initialize your cipher asymmetricCipher.init(Cipher.DECRYPT_MODE, key); // asuming, cipherText is a byte array containing your encrypted message byte[] plainText = asymmetricCipher.doFinal(cipherText);