Using built in functions

Question

I am developing a Windows Form Application in C#.I have heard that one should not use built in methods and functions in code since hackers have deep understanding of such built in methods and know how to fail them Instead one should always use his/her own functions and methods and if not then call built in functions intelligently from those newly made functions.How much is that true?

A supporting example in favour of my argument is that I have seen developer always develope there own made encryption algorithm like AES,DES,RC4 and Hash functions since they believe that built in encryption algorithm have many times backdoor in them.

Answer 1

What?! No, no, no! Whoever told you this is just wrong.

There is a common fallacy that published source code is more vulnerable to "h4ckerz" because it is available for anyone to spot the flaws in. However, I'm glad you mentioned crypto, because this is an area where this line of reasoning really stands out as the fallacy it is.

One of the most popular questions of all time on https://security.stackexchange.com/ is about a developer (in the OP he was given the pseudonym "Dave") who shared this fear of published code. Dave, like the developer you saw, was trying to homebrew his own encryption algorithm. Here's one of the most popular comments in that thread:

Dave has a fundamentally false premise, that the security of an algorithm relies on (even partially) its obscurity - that's not the case. The security of a hashing algorithm relies on the limits of our understanding of mathematics, and, to a lesser extent, the hardware ability to brute-force it. Once Dave accepts this reality (and it really is reality, read the Wikipedia article on hashing), it's a question of who is smarter - Dave by himself, or a large group of specialists devoted to this very particular problem. (emphasis added)

As a matter of fact, as it stands now, the top two memes on Security.SE are "Don't roll your own" and "Don't be a Dave".

While this has all been about crypto, this applies in general to most open-source software. The chance that a backdoor will get found and fixed goes up with each new set of eyes laid on the code. This should be a simple and uncontroversial premise: the more people are looking for something, the higher the chance it will be found. Yes, this applies to malicious users looking for exploits. However, it also applies to power users, white hat hackers, security researchers, cryptographers, professional developers, and others working for "good", which generally (hopefully) outnumber those working for "evil". This also implicitly relies on the false premise that hackers need to see the source code to do bad things. This should be obviously false based on the sheer number of proprietary systems whose source code has never been published (various Microsoft and Adobe programs come to mind) which have been inundated with vulnerabilities for years. Maybe having source code to read makes the hacker's job easier, but maybe not -- is it easier to pore over source code looking for an attack vector or to just use scanning tools and scripts against a compiled binary?

tl;dr Don't be a Dave. Rolling your own means you have to be the best at everything to succeed, instead of taking a sampling of the best the community has to offer.

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Heartbleed

In your comment, you rebut:

Then why was the Heartbleed bug in openSSL not found and corrected [earlier]?

Because no one was looking at it. That's the sad truth. Here's the difference -- what happened once someone did find it? Now tens of thousands of security researchers, crypto experts, and others are looking at it. Suppose the same kind of vulnerability existed in one of the proprietary products I mentioned earlier, which it very well could. Once it's caught (if it's caught), ask yourself:

• Could the team of programmers at the company responsible benefit from the help of the entire worldwide community of security experts, cryptographers, and other analysts right now?
• If a bug this critical were discovered (and that's a big if!) in your software, would you be prepared to deal with the fallout caused by your custom implementation?

Answer 2

Unless you know of specific failure modes or weaknesses of the built-in methods your application would use and know how to minimize or eliminate them, it is probably better to use the methods provided by the language or library designers, which will often be both more efficient and more secure than what an average programmer would come up with on the fly for a particular project.

Your example absolutely does not support your view: developing your own encryption algorithm without some serious background in the domain and review by cryptanalysts, and then employing it in security-critical code, is a recipe for disaster. Even developing your own custom implementation of an industry standard encryption algorithm can present problems, and almost certainly will if you are inexperienced at it.