user9512: I'm interested in mining altcoins, mainly to turn some servers into (literal) space heaters, since it's winter. One thing that really interests me is if an altcoin has been designed to only be mineable by traditional CPUs. The big thing I can think of to cause this is large memory requirements. A large memory requirement for hash processing would cause current GPUs to be slower than CPUs (due to the individual "cores" of the GPU not having enough direct memory). It would also cause ASICs and FPGAs to be very difficult to develop to be faster than CPUs, and expensive. This is because you effectively reduce the mining complexity so that it bottlenecks at the memory bus (which is much harder to significantly speed up)
I'm sure I'm not the only one to have thought of such a scheme. Is there an altcoin out there that is designed with this kind of ideal behind mining? 

user4748: Either you had a very good idea, some years behind, or you read about Litecoin and wanted to be answered with its name :)
In principle it's impossible to design an algorithm that can only be run on a generalistic CPU installed in a normal computer - you can always design purpose-specific hardware for whatever application though normally this is limited by cost-benefit reasons.
Litecoin's designers' decision to use scrypt rather than SHA-256 was precisely because they wanted mining not to be over-taken by GPUs. In the end this was a partial defeat, since GPUs do can be used to mine scrypt-based crypto-currencies, albeit the efficiencies vs. CPUs are not as enormous as with SHA-256: a GPU is between some dozens to a hundred times more efficient than a CPU, rather than several thousands more efficient.
The reason is scrypt forces you to either have large amounts of memory or be able to re-compute previous values of the hash function rapidly, thus making GPUs way less efficient for this and complicating the design of an ASIC.
Primecoin's algorithm (looking for Cullingham chains of prime-numbers) can only be done right now by a CPU. Should this über-neue crypto-currency rise as sharply in value as Bitcoin, you could see a new generation of purpose-specific chips designed solely to look for prime numbers (or to test the primality of one), which would mean the end of CPU-mining for it as well as have a deep impact both on scientific and security computing (for the good and bad, respectively).

user85: There are three problems with this idea:

It focuses on how to distribute wealth rather than how to create it, when the latter part is the hard part. If you don't create any wealth to distribute, it doesn't matter whether you have a good way to distribute it.
This would produce a very insecure crypto-currency because anyone who wanted to attack it could rent or steal general purpose computers, which are widely available. You want a mining algorithm that gives legitimate miners (who invest in the coin) an advantage over attackers, and CPUs give the advantage to attackers because botnets have lots of CPUs and general-purpose computing power can easily be rented.
It's basically economically infeasible to create a coin with profitable mining. If it's especially profitable, more people will do it until it becomes just barely profitable. Eventually, only those with some special advantage (such as the cheapest power) will do it. You can't use mining as a way to spread wealth.


user9975: Yep there are three good options for CPU mining that I know of:
XPM: PrimeCoin
QRK: Quark
PTS: Protoshare - very very new and exactly what you described.
Google search those and you'll find plenty of info.
Seems like there was a fourth but I can't remember it off the top of my head.  

user533: Litecoin is the 2nd most popular currency in the same family as Bitcoin, and:

It is based on the Bitcoin protocol but differs from Bitcoin in that
  it can be efficiently mined with consumer-grade hardware. Litecoin
  provides faster transaction confirmations (2.5 minutes on average) and
  uses a memory-hard, scrypt-based mining proof-of-work algorithm to
  target the regular computers with GPUs most people already have.

https://litecoin.org/
The intention was to allow CPU mining to be viable, but according to Wikipedia it hasn't quite met this goal:

Litecoin uses scrypt in its proof-of-work algorithm: a sequential
  memory-hard function first conceived by Colin Percival.[14] The
  original intended benefit of using scrypt was to avoid giving
  advantage to GPU, FPGA and ASIC miners over CPU miners, which occurs
  in the Bitcoin protocol. However, this turned out to be an incorrect
  assumption: GPU mining in Litecoin's implementation of scrypt is
  currently ten times more efficient than CPU mining

http://en.wikipedia.org/wiki/Litecoin

9512: <p>I'm interested in mining altcoins, mainly to turn some servers into (literal) space heaters, since it's winter. One thing that really interests me is if an altcoin has been designed to only be mineable by traditional CPUs. The big thing I can think of to cause this is large memory requirements. A large memory requirement for hash processing would cause current GPUs to be slower than CPUs (due to the individual "cores" of the GPU not having enough direct memory). It would also cause ASICs and FPGAs to be very difficult to develop to be faster than CPUs, and expensive. This is because you effectively reduce the mining complexity so that it bottlenecks at the memory bus (which is much harder to significantly speed up)</p>

<p>I'm sure I'm not the only one to have thought of such a scheme. Is there an altcoin out there that is designed with this kind of ideal behind mining? </p>
 4748: <p>Either you had a very good idea, some years behind, or you read about Litecoin and wanted to be answered with its name :)</p>

<p>In principle it's impossible to design an algorithm that can <em>only</em> be run on a generalistic CPU installed in a normal computer - you can always design purpose-specific hardware for whatever application though normally this is limited by cost-benefit reasons.</p>

<p>Litecoin's designers' decision to use scrypt rather than SHA-256 was precisely because they wanted mining not to be over-taken by GPUs. In the end this was a partial defeat, since GPUs do can be used to mine scrypt-based crypto-currencies, albeit the efficiencies vs. CPUs are not as enormous as with SHA-256: a GPU is between some dozens to a hundred times more efficient than a CPU, rather than several thousands more efficient.</p>

<p>The reason is scrypt forces you to either have large amounts of memory or be able to re-compute previous values of the hash function rapidly, thus making GPUs way less efficient for this and complicating the design of an ASIC.</p>

<p>Primecoin's algorithm (looking for Cullingham chains of prime-numbers) can only be done right now by a CPU. Should this über-neue crypto-currency rise as sharply in value as Bitcoin, you could see a new generation of purpose-specific chips designed solely to look for prime numbers (or to test the primality of one), which would mean the end of CPU-mining for it as well as have a deep impact both on scientific and security computing (for the good and bad, respectively).</p>
 85: <p>There are three problems with this idea:</p>

<ol>
<li><p>It focuses on how to distribute wealth rather than how to create it, when the latter part is the hard part. If you don't create any wealth to distribute, it doesn't matter whether you have a good way to distribute it.</p></li>
<li><p>This would produce a very insecure crypto-currency because anyone who wanted to attack it could rent or steal general purpose computers, which are widely available. You want a mining algorithm that gives legitimate miners (who invest in the coin) an advantage over attackers, and CPUs give the advantage to attackers because botnets have lots of CPUs and general-purpose computing power can easily be rented.</p></li>
<li><p>It's basically economically infeasible to create a coin with profitable mining. If it's especially profitable, more people will do it until it becomes just barely profitable. Eventually, only those with some special advantage (such as the cheapest power) will do it. You can't use mining as a way to spread wealth.</p></li>
</ol>
 9975: <p>Yep there are three good options for CPU mining that I know of:</p>

<p>XPM: PrimeCoin</p>

<p>QRK: Quark</p>

<p>PTS: Protoshare - very very new and exactly what you described.</p>

<p>Google search those and you'll find plenty of info.</p>

<p>Seems like there was a fourth but I can't remember it off the top of my head.  </p>
 533: <p>Litecoin is the 2nd most popular currency in the same family as Bitcoin, and:</p>

<blockquote>
  <p>It is based on the Bitcoin protocol but differs from Bitcoin in that
  it can be efficiently mined with consumer-grade hardware. Litecoin
  provides faster transaction confirmations (2.5 minutes on average) and
  uses a memory-hard, scrypt-based mining proof-of-work algorithm to
  target the regular computers with GPUs most people already have.</p>
</blockquote>

<p><a href="https://litecoin.org/">https://litecoin.org/</a></p>

<p>The intention was to allow CPU mining to be viable, but according to Wikipedia it hasn't quite met this goal:</p>

<blockquote>
  <p>Litecoin uses scrypt in its proof-of-work algorithm: a sequential
  memory-hard function first conceived by Colin Percival.[14] The
  original intended benefit of using scrypt was to avoid giving
  advantage to GPU, FPGA and ASIC miners over CPU miners, which occurs
  in the Bitcoin protocol. However, this turned out to be an incorrect
  assumption: GPU mining in Litecoin's implementation of scrypt is
  currently ten times more efficient than CPU mining</p>
</blockquote>

<p><a href="http://en.wikipedia.org/wiki/Litecoin">http://en.wikipedia.org/wiki/Litecoin</a></p>
