Suppose I want to start an AltCoin, and set it up for merged mining with Bitcoin. Will it require the AltCoin end-users to also download the Bitcoin blockchain to verify the hashes embedded in the blocks, or do only the miners need a copy of both chains?

up vote 8 down vote accepted

Those running a miner against a pool don't. The pool operator would need both.

As stated in the comments below, normal users of the alternate system will not need the bitcoin chain but will need the alternate chain, and normal users of bitcoin will need the bitcoin chain but not the alternate chain. Once the block is mined, nobody cares what other chains may have been involved.

  • And how about normal end-users? Can they verify a full copy of the AltCoin blockchain, without needing a full copy of the Bitcoin blockchain? – Muis Jul 24 '12 at 20:13
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    @Joshua No. Merge mining requires building valid blocks in the Bitcoin blockchain which link to valid blocks in the alt-coin blockchain. A copy of both blockchains are needed. The only reason why pool miners don't need a copy of the blockchain is because the pool server is doing the block validation and creation for all pool members. The requirement is still there it just happens that the pool server is doing the work for the pool members. The entity creating & validating blocks must have access to both blockchains. That is unavoidable. – DeathAndTaxes Jul 24 '12 at 21:04
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    @Joshua: No. Normal users of one coin don't care whether the blocks are or aren't valid in the other block chain. With merged mining, it is perfectly normal and common for blocks to be valid in one block chain and not in the other (for example, if the difficulties are different and the hash falls between the two). Users of a particular coin only care if the block is valid in their coin's block chain. – David Schwartz Jul 24 '12 at 22:16
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    @DavidSchwartz In bitcoin.stackexchange.com/questions/273/… you wrote that some Bitcoin hashes are added to the Namecoin blocks, so why does this stuff gets added if end-users will never be able to verify those? – Muis Jul 25 '12 at 0:15
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    @Joshua: End users do verify them. End users of Namecoin can verify them with only the Namecoin chain because they're added to the Namecoin blocks. End users of Bitcoin can verify them with only the Bitcoin chain because they're Bitcoin hashes. (Namecoin users don't need to do any further verification of them. It doesn't matter to Namecoin whether they're valid Bitcoin hashes or random garbage. It's still a valid Namecoin block.) – David Schwartz Jul 25 '12 at 1:19

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