Android Question Encryption functions HMACSHA256

$key       = '12345';
$raw       = 'testttttttttttttttttttttttttttttttttttttttt';
$meta      = [ 'name' => 'test-name', 'email' => 'aaa@aaaaaa.com' ];

function encrypt( $key, $plaintext, $meta = '' ) {

    // Generate valid key
    $key = hash_pbkdf2( 'sha256', $key, '', 10000, 0, true );

    // Serialize metadata
    $meta = serialize($meta);

    // Derive two subkeys from the original key
    $mac_key = hash_hmac( 'sha256', 'mac', $key, true );
    $enc_key = hash_hmac( 'sha256', 'enc', $key, true );
    $enc_key = substr( $enc_key, 0, 32 );

    // Derive a "synthetic IV" from the nonce, plaintext and metadata
    $temp = $nonce = ( 16 > 0 ? mcrypt_create_iv( 16 ) : "" );
    $temp .= hash_hmac( 'sha256', $plaintext, $mac_key, true );
    $temp .= hash_hmac( 'sha256', $meta, $mac_key, true );
    $mac = hash_hmac( 'sha256', $temp, $mac_key, true );
    $siv = substr( $mac, 0, 16 );

    // Encrypt the message
    $enc = mcrypt_encrypt( 'rijndael-128', $enc_key, $plaintext, 'ctr', $siv );
    return base64_encode( $siv . $nonce . $enc );

}