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Added support for MSG_FILTERED_WITNESS_BLOCK messages. #313
After much deliberation and a few attempts at other approaches to provide a workable near-term sync mechanism for thin clients that require witness data, I decided to just go with the simplest short-term path with the least amount of complications expecting BIP37 to be completely replaced eventually - hopefully in the not-very-distant future.
I believe the approach of filtering blocks on the client is a simpler and superior approach for most use cases than requiring the server to perform the filtering. I believe @Roasbeef has something written up for this that he's using for lnd. I would love to see that approach used in Bitcoin Core as well.
But given the imminent SegWit activation on the Litecoin mainnet, I believe this solution will suffice for all essential use cases of BIP37 for now - and I don't believe it's worth the effort to try to make more complex additions to BIP37 since it will eventually be entirely replaced.
Peers can request MSG_FILTERED_WITNESS_BLOCK and will receive a merkleblock structure with transactions serialized with witnesses. The merkle proof for the witnesses is not supplied. This means that the client cannot verify that the witness data is what's actually in the block. However, the attack vectors here given the actual intended use cases seem extremely slim for several reasons: