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My question was fueled by the recent question: https://crypto.stackexchange.com/questions/12582/most-important-spec-to-look-for-on-a-gpu-to-solve-scrypt-algorithms-e-g-liteco

By the time anyone see's this it'll probably be closed as off-topic, and I'm not convinced. I considered raising this in the comments, but their already racing away (and answers might end up rather more general) so I thought I'd ask here.

Why is a question about the hardware loads of a standard cryptographic algorithm off-topic?

A significant number of modern algorithms/routines are especially designed for use on certain hardware/software platforms. For example, the ease of implementation was a factor in the AES competition. More recently, several modes of operation have been proposed who's main selling point is that they may have very small hardware footprints in custom chips, or that they reuse primitives from other algorithms.

It seems to me that if the academic community feels this is a significant design principal it should certainly be on-topic here.

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No, I personally don't think the question is on-topic.

What is our site's purpose? To quote the FAQ:

Crypto Stack Exchange is for software developers, mathematicians and others interested in cryptography.

Cryptography Stack Exchange is for asking questions about the mathematics and properties of cryptographic systems, their analysis ("cryptanalysis") and subsidiary topics that generally make up cryptology, such as random number generation. As such, we welcome questions on topics such as:

  • Asymmetric and symmetric cryptographic algorithms.
  • Cryptographic protocols.
  • Cryptanalysis techniques.
  • Hash functions, hashing.
  • Entropy and information theory.
  • Random number generation.

I don't think the question fits the topics presented here at all; the best fit might be "about the mathematics and properties of cryptographic systems". The issue for me is the lack of cryptographic knowledge required to answer the question. In essence, one might as well ask "if I want to buy a graphics card that is fast at integer computation which uses a lot of memory, what should I look for?" That is, it is possible to (easily!) remove all cryptography from the question entirely without changing the answers in the slightest.

More generally, one might ask "what makes a graphics card fast, ignoring floating-point arithmetic?" I can't imagine a cryptographer being terribly interested in the question, at least not from a cryptographic point of view, and that's the crowd we are trying to appeal to.

That's not to say that hardware is off-topic in general. As you've pointed out, hardware performance is a major design goal. But hardware questions should be focused in such a way that something from cryptography plays a central role. For example, a question like "why does Serpent perform poorly in software but pretty good in hardware?" seems fine to me, if a bit broad.

To sum up my thoughts on the matter, if the hardware question is somehow related to the design of primitives or protocols, I am fine with it. As another example, I think "how does scrypt use a lot of memory" might be an on-topic question (although not a stellar question in its own right).

All-in-all, I think the question was much better suited for the Bitcoin SE.

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I think the problem with the question you've linked to is not the “hardware” aspect, but more the

… what would be the best way …

Clearly, OP asks for opinions, which points to a "shopping list" question — which is off topic. Simpler said: The core of the problem with that question resides in the "please recommend me something" area.

Besides that, it should be noted that the fact that OP cross-posted the question doesn't really help it survive around here either.

Anyway…

It's not as if hardware is generally off-topic and I personally wouldn't rule out questions about CPU or GPU load either. Yet, I think there's a difference between asking if AES will result in a higher CPU load compared to RC4, or if the question boils down to “what would be the best” GPU to do some crypto.

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    $\begingroup$ The question is far better suited for Bitcoin.SE but at the same time, what would be a good hardware solution for Scrypt cracking? Good job spotting the cross-post though (which was posted 5 days before cross-posted here and has a decent answer). I'm not convinced that what would be the best is a bad question as long as it doesn't ask to compare products (as it does). For that, the question is off-topic AFIAC. $\endgroup$ – rath Dec 25 '13 at 19:12
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    $\begingroup$ @e-sushi: I can see your point about the shopping list issue - I might well have agreed with a 'no lists' close vote, but my concern was that the reason submitted seemed to be 'this is hardware cracking not crypto'. $\endgroup$ – Cryptographeur Dec 31 '13 at 12:53
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    $\begingroup$ @figlesquidge I actually shared your concern. Yet, OP might have reasoned his/her goal is not to crack crypto. So, before diving into uncertain waters about OP's intentions, I blended that out and looked at the question again, noticing a more general problem: the "no-list" issue. Which is why I chose that one as it was the most obvious problem and it doesn't leave much room for argumentation from OP's side either — that is, unless OP would reformulate the question to be more specific (which would then again describe OP's actual intentions/goals and maybe even result in something constructive). $\endgroup$ – e-sushi Dec 31 '13 at 13:59

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