混淆使用RNCryptor iOS发布到基于php的服务器

基于此链接:https://github.com/RNCryptor/RNCryptor

我学过这是一个加密NSData的基本方法

NSData *data = [@"Data" dataUsingEncoding:NSUTF8StringEncoding];
NSError *error;
NSData *encryptedData = [RNEncryptor encryptData:data
                                    withSettings:kRNCryptorAES256Settings
                                        password:aPassword
                                           error:&error];

这是我的代码做POST请求到基于php的服务器。

NSString *parameter = [NSString stringWithFormat:@"userid=%@&password=%@",useridStr, passwordStr];
NSData *parameterData = [parameter dataUsingEncoding:NSUTF8StringEncoding];
NSURL *url = [NSURL URLWithString: @"http://mywebiste.com/server.php"];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
[request setHTTPBody:parameterData];
[request setHTTPMethod:@"POST"];
[request addValue: @"application/x-www-form-urlencoded; charset=utf-8"  forHTTPHeaderField:@"Content-Type"];
NSURLConnection *connection = [[NSURLConnection alloc]initWithRequest:request delegate:self];

在PHP服务器中:

 <?php
    $userid = $_POST['userid'];
    $password = $_POST['password'];
    ...
 ?>

我的问题是

如果整个参数

NSString *parameter = [NSString stringWithFormat:
                         @"userid=%@&password=%@",useridStr, passwordStr];

? ?

是否有一种方法加密ONLY NSString，并像这样将它们附加到参数上?

NSString *parameter = [NSString stringWithFormat:  
                         @"userid=%@&password=%@",ENCRYPTED_USRID, ENCRYPTED_PWD];