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Saturday, April 6, 2024

bitcoin core – How does headers-first forestall disk-fill assault?


So as to add to the opposite reply, since Bitcoin Core 24.0, there may be a further safety carried out towards low-difficulty header spam: header pre-syncing.

To recapitulate, for the reason that headers-first synchronization launched in Bitcoin Core 0.10.0, blocks are by no means downloaded earlier than their headers are identified and verified to have enough work (which suggests: sufficient to inside sooner or later of the lively chain tip, and greater than the preconfigured minimal chain work). This implies we already need not fear about low-difficulty block spam anymore. The blocks are simply not downloaded until they’re a part of a series that is confirmed to be adequate.

But, a weaker drawback remained: a peer may begin giving us (a number of) chains of headers that by no means quantity to something useful. Because the headers are despatched in ahead order, there isn’t a technique to know in the beginning how good the outcome will get. The outdated resolution (checkpoints) is unsatisfactory: it depends on up to date software program with hardcoded overrides on what chain is appropriate. As of Bitcoin Core 26.0, the checkpoints stay, however have not been up to date since 2014. By now, mining has turn into a lot cheaper per hash, that an attacker may realistically begin an assault after the final checkpoint.

Since Bitcoin Core 24.0, a brand new resolution has been carried out: headers pre-syncing.

The thought is that the header synchronization (which precedes the block synchronization) is break up up in two phases:

  • In a primary part, header presyncing, headers from a peer are downloaded and verified, however not saved, as a result of we do not but know whether or not they’ll find yourself being adequate. As an alternative, solely a really compact (salted) hash of those headers is saved (in per-peer reminiscence, discarded upon disconnect).
  • In a second part, header redownloading, the identical headers are downloaded once more from the identical peer, and in comparison with the saved hash(*). If there’s a match, they’re fed to full header validation, which shops them, and can set off block obtain.

This strategy comes at a value – the header synchronization is successfully carried out twice, doubling its bandwidth value (which remains to be small in comparison with full block obtain), however removes the final reliance on checkpoints within the codebase. They will doubtless be eliminated in some future model.

(*) A easy hash would not be enough, as we want the power to confirm headers alongside the way in which, not simply the tip. The precise construction consists of a single 1-bit salted hash each ~600 blocks. To compensate for the (extraordinarily) small hash, upon redownloading, there’s a buffer of ~14000 headers which might be downloaded earlier than validation. Provided that all of the ~23 1-bit hashes in these 14000 headers match, then the start of the buffer is fed to validation. This implies each header has some 23 bits checked towards it earlier than validation, which might value an attacker tens of millions of tries to succeed towards.

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