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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 ber-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).

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mining - Is there an altcoin that is only feasible to mine ...