[gmluke]

This is slightly tangential since you specified a conspirator on the inside, but how easy is it to break a homegrown encryption algorithm if you don't have the source code? I assume there are tools (what are they?) that will break a simple caesar cipher if you have more than a sentence or so of plain text to work with. But if you strung together 2-3 broken algorithms and your attacker doesn't know which ones, is it still trivial to decrypt?

[wolf550e]

People who can break it won't spend the time breaking your homegrown crypto, so you won't get proof it's broken. But it's still broken. If lots of money or lives of political dissidents are at stake, it will be broken.

To have really capable people work on breaking your crypto for free, you have to be an insider. You become an insider by breaking other people's crypto. You can publish a break in an insider's crypto even if you are unknown. After you publish a few such papers, you become an insider and can publish your own crypto other people will spend their time trying to break.

People can learn the state of the art and develop an alternative to the common (NIST) choices which are no worse, but also no better. Some of those are blessed as "national pride ciphers" (GOST, Camellia, SEED, etc.).

[stcredzero]

Ciphers aren't the place where security most often fails. The failures have to do with implementation. More commonly, they have to do with implementation of protocols and systems using the protocols.

[wolf550e]

I agree, but I replied to someone who talked about ciphers.

[stcredzero]

Of course. I was also (mainly) posting for the benefit of 3rd parties.

[andrewflnr]

We're specifically talking about the scenario where you have a "tiger team" (strcredzero's phrasing) trying to break it. I interpreted GP as asking just how hard a time the tiger team would have if they don't have source code.

[Taek]

I've seen a few fun articles about people breaking home grown encryption. The question is mostly about how motivated someone might be to find a problem.

Making a secure encryption algorithm requires a lot of presence of mind, and a lot of industry knowledge. If your threat model is incomplete, you lose. If you forgot one tiny thing at one tiny point in the algorithm, you lose.

If you don't have people checking your work, how do you know? If someone is determined to break your encryption, they are capable of spending a lot more time analyzing it than you spent building it. And they only need to find one mistake.

It's definitely better to use the tools that experts have spent lots of time, lots of breadth of knowledge, and lots of depth of knowledge inspecting.

[john_reel]

Those articles (and their sources) sound interesting. Think you can share any of them?

[stcredzero]

how easy is it to break a homegrown encryption algorithm if you don't have the source code?

A better question to ask is, how easy is it to break the protocols and the software using the protocols? We have secure ciphers. Those aren't where the problems in computer and network security lie.

[reitanqild]

Also interested.

Is there a general way to attack naive ciphers?

I fall of at even a proper Vigenère cipher, although I guess I could hack together a terribly inefficient python script to test all options and then print them line by line and start visually scanning for patterns :-/

[stcredzero]

Vigenère ciphers based on xor: xor the ciphertext with itself at varying offsets. Stop when you detect likely plaintext in the result.