Peercoin called PPCoin or peer-to-peer, its the first cryptocurrency which is on a combination of the proof-of-work and of the ownerprincip based.
Peercoin is published by Sunny King (author of the book "Primecoin" in August 2012. The Peercoin is based on the technical principles and the source of Bitcoin. Also, the source-code of the peercoin is available under the MIT-license too.
The diffrence between the Bitcoin is the "hybrid mining":
In contrast to Bitcoin peercoin used a hybrid algorithm of the proof-of-work principle and ownerprincip. The so called "ownerprincip" is used to Dodge a weakness like by the Bitcoin. If by the Bitcoin it may happen when a organization get a monopoly by her mining and the network is vulnerable for the so-called 51%-attack.
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