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What's the most efficient way to detect a fork in the blockchain?

My app polls for changes to user deposit addresses on each new block.

Deposits are credited to user accounts with a record in a database then the funds are transfered to cold storage.

When users get refunds those refunds come from the cold storage and a record is added to the database.

My concern is the following happening:

  • User deposits Btc and the database record is created.
  • Chain is forked from 51% attack or software incompatability.
  • User gets refund from cold storage but I no longer have access to the funds they sent so its out of my pocket.

It seems that everything but rechecking every deposit is marginally risky.

If I rely on height plus hash that can be manufactured by the attacker so the new chain looks like the old chain.

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  • I'm not completely sure what you have in mind when you say "height plus hash". But note that a different fork of the chain will necessarily have different blocks with different hashes. (Unless the attacker has managed to break sha256, in which case we all have bigger problems anyway.)
    – Nate Eldredge
    Apr 15 '14 at 15:04
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Maybe you can arrange it such that a refund always uses the TxOut of the transaction with which it came into the cold storage? Basically giving the user the same bitcoins back.

If a fork causes the funds not to go into cold storage they will not be going out either.

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Use:

bitcoin-cligetchaintips.

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