Caesar Cipher written in Objective-C

NSSting+Caesar.m

- (NSString *)caesarCipher:(int)shift {
    if (self.length == 0) {
        return nil;
    }
    
    //no shift, return unencrypted string
    if (shift == 0){
        return self;
    }
    
    //create empty mutable string with capacity of the length
    NSMutableString *str = [[NSMutableString alloc] init];
    for (int i = 0; i < self.length; i++) {
        [str insertString:@" " atIndex:i];
    }
    
    for (int i = 0; i < self.length; i++) {
        //if i + shift results in going out of bounds
        if (i+shift > self.length-1) {
            int wrapIndex = abs((int)(i+shift - self.length));
            [str insertString:[NSString stringWithFormat:@"%c", [self characterAtIndex:i]] atIndex:wrapIndex];
        } else {
            [str insertString:[NSString stringWithFormat:@"%c", [self characterAtIndex:i]] atIndex:i+shift];
        }
    }
    
    NSString *result = [str stringByReplacingOccurrencesOfString:@" " withString:@""];
    return result;
}