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The original bitcoin paper suggests a method to discard old transactions. One should compute the Merkle Tree of the entire transaction history, and store only part of the tree.

However, I didn't notice this method implemented. BuildMerkleTree stores it in memory only, and seems to assume you're holding all transaction history, and the serialization method stores the actual transactions and does not store the Merkle Tree.

Is that implemented? Can you give me a reference to where is it in the code?

If not, is it planned for a specific future release?

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bitcoin.org is where the standard client is hosted, but it's not generally used to refer to the client itself - it's usually called the standard client or the Satoshi client. AFAIK it does not currently implement pruning transactions. – Meni Rosenfeld Feb 20 at 11:36
@MeniRosenfeld Fixed, thanks. I'm not sure how exactly it should be done, and an implementation can make things clearer. Any calculation for how big a block will be with no pruning, in the next few years? – Elazar Leibovich Feb 20 at 11:41
I edited your question to include asking when is such a feature planned, hope you don't mind. (Also added a closing question mark to the title) – ripper234 Feb 20 at 12:58
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I was sure I had asked a similar question (about planning) in the past, but didn't find it now. I vaguely remmeber Gavin saying that "it's the next thing on the roadmap after multisig", but I don't have a citation, and I think it wasn't formalized anyway. For the cord, the straight answer to your question is "No, the current client does not prune history". – ripper234 Feb 20 at 13:00
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I would assume this is not implemented because of the problem that Pieter mentioned. Having a client that only stores a pruned version of the block chain can't be friendly to other clients that don't have this pruning feature implemented since it won't be able to retrieve the original block chain.

Since a variety of clients are still undergoing development allowing alternative clients the choice of how to handle their block chain data allows for new innovations. For instance, it allows analysis of how Bitcoin has behaved over time. Even if you have not recorded this from the start.

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It is not implemented. It's considered low priority right now.

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No, it requires the entire history to verify new transactions (due to how the security of a new block in the chain requires the security of all previous blocks).

Some future client may not require the whole chain and not fully verify transactions, however I would not consider these secure for large transactions because I have no clue if the "skinny client" I'm on is not stuck on a malicious fork in the chain designed to allow double spends and someone has not ran off to Mexico with my Bitcoins.

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This is not correct. You do not need the entire history to verify transactions - spent and well-buried transactions could safely be removed if it were implemented. With the current network protocol, such a pruned node is not able to provide the block chain to other full nodes, however. – Pieter Wuille Mar 31 at 12:46
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