A simple and efficient method to generate a NSString of random characters.

generateRandomString

-(NSString*)generateRandomString:(int)num {
    NSMutableString* string = [NSMutableString stringWithCapacity:num];
    for (int i = 0; i < num; i++) {
        [string appendFormat:@"%C", (unichar)('a' + arc4random_uniform(25))];
    }
    return string;
}