iOS中可以替换特定位置的字符，所以替换成星号非常简单，简单扩展一下字符串，方法如下:

- (NSString *)replaceStringWithAsterisk:(NSInteger)startLocation length:(NSInteger)length {
    NSString *replaceStr = self;
    for (NSInteger i = 0; i < length; i++) {
        NSRange range = NSMakeRange(startLocation, 1);
        replaceStr = [replaceStr stringByReplacingCharactersInRange:range withString:@"*"];
        startLocation ++;
    }
    return replaceStr;
}