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Multibit is clearly a faster desktop wallet since it's a thin client that doesn't download and process the entire *8GB blockchain.

My question is, what service does Multibit connect to in order to process transactions? Are there security concerns to be aware of compared to using a thick client with its own blockchain? What benefits are there using a thick client (like Bitcoin-QT) compared to a thin client?

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Multibit uses SPV, which is a peer-to-peer protocol consisting of two parts.

  • It downloads the block headers from peers (not the whole block). This allows it to verify that the blocks have a lot of hashing power behind them, and get the merkle root for each of them.
  • It asks peers for transactions matching the addresses it cares about. It can verify that the transactions belong to one of the blocks it knows about by using the merkle root.

See here for a discussion of the security tradeoffs.

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