Sensitive data: NSString VS NSMutableString (iPhone)

Question

I have some sensitive data I want to clear directly after use. Currently, the sensitive data is in the form of NSString. NSString is in my understanding immutable, meaning that I can't really clear the data. NSMutableString seems more appropriate, though, as it is mutable and has methods like replaceCharactersInRange and deleteCharactersInRange. I have no knowledge of the implementation details so I wonder if NSMutableString would serve my purpose?

Answer 1

I would be afraid NSMutableString would try to optimize and leave the string in memory. If you want more control try allocating your own memory then create an NSString with it. If you do that you can overwrite the memory before you release it.

char* block = malloc(200);
NSString* string = [[NSString alloc] initWithBytesNoCopy:length:encoding:freeWhenDone];
//use string
memset(block, 0, 200);// overwrite block with 0
[string release];
free(block);