有效负载阅读器从PHP到Objective-c

Question

I have this code below that process an ASCII character in PHP and transform it in a real message:

$message = '';

$len = ord($buffer[1]) & 127;

$masks = null;
$data = null;

if ($len === 126) {
    $masks = substr($buffer, 4, 4);
    $data = substr($buffer, 8);
}
elseif($len === 127) {
    $masks = substr($buffer, 10, 4);
    $data = substr($buffer, 14);
} else {
    $masks = substr($buffer, 2, 4);
    $data = substr($buffer, 6);
}

for ($index = 0; $index < strlen($data); $index++) {
    $message. = $data[$index] ^ $masks[$index % 4];
}

I'm trying to do the same thing in objective-c, for this:

NSData * buffer = self.data;

NSString * mystring = [
    [NSString alloc] initWithData: buffer encoding: NSASCIIStringEncoding];

unichar len = [mystring characterAtIndex: 1] & 127;

NSString * masks = nil;
NSString * data = nil;

NSString * message = @"";

if (len == 126) {
    masks = [mystring substringWithRange: NSMakeRange(4, 4)];
    data = [mystring substringToIndex: 8];
} else if (len == 127) {
    masks = [mystring substringWithRange: NSMakeRange(10, 4)];
    data = [mystring substringToIndex: 14];
} else {
    masks = [mystring substringWithRange: NSMakeRange(2, 4)];
    data = [mystring substringToIndex: 6];
}

for (int index = 0; index < [data length]; index++) {
    [message stringByAppendingString: [NSString stringWithFormat: @"%@",
    data[index] ^ masks[index % 4]]];
}

This code has a little problem, I'm receiving this error message:

Expected method to read array element not found on object of type 'NSString*'

Why and how can I solve this problem?

Answer 1

When assigning to len you use characterAtIndex: to obtain a character from mystring .

Later when trying to access characters in data and masks you instead try to use [] indexing, and the compiler advises you:

Expected method to read array element not found on object of type 'NSString*'

Use characterAtIndex: , as you did when indexing mystring .

Note that characterAtIndex returns back a unichar - the type of len - and this is 16-bits not an 8-bit byte. You may be better off using the NSString method cStringUsingEncoding , or one of its siblings, and processing you data as a C string or array of bytes.

HTH