RSA Encryption in Java / Decryption in PHP Fails

Question

I have a PHP script that I have been using do decrypt a session key that I encrypted from iOS. The encryption is done on the client using a 1024-bit public key. Decryption on the server side is done with the corresponding private key. Now I'm trying to write an encryption method for Android. Unfortunately, the decryption continues to fail, and I can't see what is wrong.

Here is the Android code:

public String encryptSessionKeyWithPublicKey(String pemString, byte[] sessionKey) {
    try {
        PublicKey publicKey = getPublicKeyFromPemFormat(pemString);
        Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1PADDING");
        cipher.init(Cipher.ENCRYPT_MODE, publicKey);
        byte[] cipherData = cipher.doFinal(sessionKey);
        return Base64.encodeToString(cipherData, Base64.DEFAULT);
    } catch (IOException ioException) {
        Log.e(TAG, "ioException");
    } catch (NoSuchAlgorithmException exNoSuchAlg) {
        Log.e(TAG, "NoSuchAlgorithmException");
    } catch (InvalidKeySpecException exInvalidKeySpec) {
        Log.e(TAG, "InvalidKeySpecException");
    } catch (NoSuchPaddingException exNoSuchPadding) {
        Log.e(TAG, "NoSuchPaddingException");
    } catch (InvalidKeyException exInvalidKey) {
        Log.e(TAG, "InvalidKeyException");
    } catch (IllegalBlockSizeException exIllBlockSize) {
        Log.e(TAG, "IllegalBlockSizeException");
    } catch (BadPaddingException exBadPadding) {
        Log.e(TAG, "BadPaddingException");
    }

    return null;
}

private PublicKey getPublicKeyFromPemFormat(String PEMString)
        throws IOException, NoSuchAlgorithmException, InvalidKeySpecException
{
    AssetManager assetManager = context.getAssets();
    InputStream inputStream = assetManager.open(PEMString);
    BufferedReader pemReader = new BufferedReader(new InputStreamReader(inputStream));

    StringBuffer content = new StringBuffer();
    String line = null;
    while ((line = pemReader.readLine()) != null) {
        if (line.indexOf("-----BEGIN PUBLIC KEY-----") != -1) {
            while ((line = pemReader.readLine()) != null) {
                if (line.indexOf("-----END PUBLIC KEY") != -1) {
                    break;
                }
                content.append(line.trim());
            }
            break;
        }
    }
    if (line == null) {
        throw new IOException("PUBLIC KEY not found");
    }

    KeyFactory keyFactory = KeyFactory.getInstance("RSA");
    return keyFactory.generatePublic(new X509EncodedKeySpec(Base64.decode(content.toString(), Base64.DEFAULT)));

}

The PHP script is fairly simple:

<?php

$passphrase = 'xxxxxxxx'; // just for testing - load from file later
$decrypted_session_key = 'unavailable';

$encrypted_session_key = base64_decode($_POST['encrypted_session_key']);

$fp = fopen('private128.pem', 'r');
if ($fp == null) { 
    $arr = array('response' => 'failure', 'message' => 'private key not readable');
    echo json_encode($arr);
    die();
}

$priv_key = fread($fp, 8192);
fclose($fp);

$res = openssl_get_privatekey($priv_key, $passphrase);
$decr_result = openssl_private_decrypt($encrypted_session_key, $decrypted_session_key, $res, OPENSSL_PKCS1_OAEP_PADDING);

if (!$decr_result) {
    $arr = array('response' => 'failure', 'message' => $decr_result);
    echo json_encode($arr);
    die();
}

// write the decrypted string to a file
$session_key_file = fopen("session_key", "w") or die("Unable to open file!");
fwrite($session_key_file, $decrypted_session_key);
fclose($session_key_file);

$arr = array('response' => 'success', 'message' => 'server confirms receipt of session key');
echo json_encode($arr);

?>

All I am trying to encrypt are 16 randomly generated bytes.

The PHP output is: {"response":"failure","message":false} which means that openssl_private_decrypt line isn't getting a correct decryption result.

Since my PHP script works with my iOS code, I do not want to change it unless absolutely necessary. Can anyone see what I should do to my Java code to align it with what is happening on the PHP side?

generally speaking, you should be just doing $priv_key = file_get_contents(...). consider what'd happen if your key file ended up being more than 8k long - you'd be loading a partial key. — Marc B
– Marc B, Commented Feb 18, 2015 at 17:10
Good point. I know that the private key file is only 1751 bytes. — AndroidDev
– AndroidDev, Commented Feb 18, 2015 at 17:15
I'm just encrypting 16 randomly generated bytes of data (128 bits). — AndroidDev
– AndroidDev, Commented Feb 18, 2015 at 17:55

t.m.adam · Accepted Answer · 2020-10-21 09:53:59Z

5

Your PHP function has OPENSSL_PKCS1_OAEP_PADDING but your java function is using RSA/ECB/PKCS1PADDING

Change your PHP decryption to OPENSSL_PKCS1_PADDING which seems to match your java encryption.

OR

switch your java encryption to RSA/ECB/OAEPWithSHA-1AndMGF1Padding

edited Oct 21, 2020 at 9:53

t.m.adam

15.4k3 gold badges34 silver badges54 bronze badges

answered Feb 18, 2015 at 18:18

greg_diesel

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3 Comments

AndroidDev Over a year ago

I knew that the padding would come into this somewhere. Thanks very much!

Scott Arciszewski Over a year ago

Go for OAEP, not PKCS1. crypto.stackexchange.com/questions/12688/…

sea-kg Over a year ago

Please change "javascript" -> "java". solution 2 was helpful! Thanks

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