What techniques do you use when writing your own c

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Closed 7 years ago.

For years, maybe 10, I've been fascinated with cryptography. I read a book about XOR bit-based encryption, and have been hooked ever since thing.

I guess it's more fair to say that I'm fascinated by those who can break various encryption methods, but I digress.

To the point -- what methods do you use when writing cryptography? Is obfuscation good in cryptography?

I use two key-based XOR encryption, various hashing techniques (SHA1) on the keys, and simple things such as reversing strings here and there, etc.

I'm interested to see what others think of and try when writing a not-so-out-of-the-box encryption method. Also -- any info on how the pros go about "breaking" various cryptography techniques would be interesting as well.

To clarify -- I have no desire to use this in any production code, or any code of mine for that matter. I'm interesting in learning how it works through toying around, not reinventing the wheel. :)

Ian

Answer 1

I agree with not re-inventing the wheel.

And remember, security through obscurity is no security at all. If any part of your security mechanisms use the phrase "nobody will ever figure this out!", it's not secure. Think about AES -- the algorithm is publicly available, so everybody knows exactly how it works, and yet nobody can break it.

Answer 2

To contradict what everyone else has said so far, go for it! Yeah, your code might have buffer overflow vulnerabilities in it, and may be slow, buggy, etc, but you're doing this for FUN! I completely understand the recreational enjoyment found in playing with crypto.

That being said, cryptography isn't based on obfuscation at all (or at least shouldn't be). Good crypto will continue to work, even once Eve has slogged through your obfuscated code and completely understands what is going on. IE: Many newspapers have substitution code puzzles that readers try and break over breakfast. If they started doing things like reversing the whole string, yes, it'd be harder, but Joe Reader would still be able to break it, neve tuohtiw gnieb dlot.

Good crypto is based on problems that are assumed to be (none proven yet, AFAIK) really difficult. Examples of this include factoring primes, finding the log, or really any other NP-complete problem.

[Edit: snap, neither of those are proven NP-complete. They're all unproven, yet different. Hopefully you still see my point: crypto is based on one-way functions. Those are operations that are easy to do, but hard to undo. ie multiply two numbers vs find the prime factors of the product. Good catch tduehr]

More power to you for playing around with a really cool branch of mathematics, just remember that crypto is based on things that are hard, not complicated. Many crypto algorithms, once you really understand them, are mindbogglingly simple, but still work because they're based on something that is hard, not just switching letters around.

Note: With this being said, some algorithms do add in extra quirks (like string seversal) to make brute forcing them that much more difficult. A part of me feels like I read this somewhere referencing DES, but I don't believe it... [EDIT: I was right, see 5th paragraph of this article for a reference to the permutations as useless.]

BTW: If you haven't found it before, I'd guess the TEA/XTEA/XXTEA series of algorithms would be of interest.

Answer 3

Per other answers - inventing an encryption scheme is definitely a thing for the experts and any new proposed crypto scheme really does need to be put to public scrutiny for any reasonable hope of validation and confidence in its robustness. However, implementing existing algorithms and systems is a much more practical endeavor "for fun" and all the major standards have good test vectors to help prove the correctness of your implementation.

With that said, for production solutions, existing implementations are plentiful and there should typically be no reason you would need to implement a system yourself.

Answer 4

I agree with all the answers, both "don't write your own crypto algorithm for production use" and "hell yeah, go for it for your own edification", but I am reminded of something that I believe the venerable Bruce Schneier often writes: "it's easy for someone to create something that they themselves cannot break."

Answer 5

Responding to PhirePhly and tduehr, on the complexity of factoring:

It can readily be seen that factoring is in NP and coNP. What we need to see is that the problems "given n and k, find a prime factor p of n with 1 < p <= k" and "show that no such p exists" are both in NP (the first being the decision variant of the factoring problem, the second being the decision variant of the complement).

First problem: given a candidate solution p, we can easily (i.e. in polynomial time) check whether 1 < p <= k and whether p divides n. A solution p is always shorter (in the number of bits used to represent it) than n, so factoring is in NP.

Second problem: given a complete prime factorization (p_1, ..., p_m), we can quickly check that their product is n, and that none are between 1 and k. We know that PRIMES is in P, so we can check the primality of each p_i in polynomial time. Since the smallest prime is 2, there is at most log_2(n) prime factors in any valid factorization. Each factor is smaller than n, so they use at most O(n log(n)) bits. So if n doesn't have a prime factor between 1 and k, there is a short (polynomial-size) proof which can be verified quickly (in polynomial time).

So factoring is in NP and coNP. If it was NP-complete, then NP would equal coNP, something which is often assumed to be false. One can take this as evidence that factoring is indeed not NP-complete; I'd rather just wait for a proof ;-)

Answer 6

The correct answer is to not do something like this. The best method is to pick one of the many cryptography libraries out there for this purpose and use them in your application. Security through obscurity never works.

Pick the current top standards for cryptography algorithms as well. AES for encryption, SHA256 for hashing. Elgamal for public key.

Reading Applied Cryptography is a good idea as well. But a vast majority of the book is details of implementations that you won't need for most applications.

Edit: To expand upon the new information given in the edit. The vast majority of current cryptography involves lots of complicated mathematics. Even the block ciphers which just seem like all sorts of munging around of bits are the same.

In this case then read Applied Cryptography and then get the book Handbook of Applied Cryptography which you can download for free.

Both of these have lots of information on what goes into a cryptography algorithm. Some explanation of things like differential and linear cryptanalysis. Another resource is Citeseer which has a number of the academic papers referenced by both of those books for download.

Cryptography is a difficult field with a huge academic history to it for going anywhere. But if you have the skills it is quite rewarding as I have found it to be.