Simplified-Payment-Verification (SPV) is secure under the assumption
that the chain with the most Proof-of-Work (PoW) is valid. As many
have pointed out before, and attacks like Segwit2x have shown, this is
not a safe assumption. What I propose below improves this assumption
-- invalid blocks will be rejected as long as there are enough honest
miners to create a block within a reasonable time frame. This still
doesn’t fully inoculate SPV clients against dishonest miners, but is a
clear improvement over regular SPV (and compatible with the privacy
improvements of BIP157[0]).

The idea is that a fork is an indication of potential misbehavior --
its block header can serve as a PoW fraud proof. Conversely, the lack
of a fork is an indication that a block is valid. If a fork is created
from a block at height N, this means a subset of miners may disagree
on the validity of block N+1. If SPV clients download and verify this
block, they can judge for themselves whether or not the chain should
be rejected. Of course it could simply be a natural fork, in which
case we continue following the chain with the most PoW.

The way Bitcoin currently works, it is impossible to verify the
validity of block N+1 without knowing the UTXO set at block N, even if
you are willing to assume that block N (and everything before it) is
valid. This would change with the introduction of UTXO set
commitments, allowing block N+1 to be validated by verifying whether
its inputs are present in the UTXO set that was committed to in block
N. An open question is whether a similar result can be achieved
without a soft fork that commits to the UTXO set[0][1].

If an invalid block is created and only 10% of the miners are honest,
on average it would take 100 minutes for a valid block to appear.
During this time, the SPV client will be following the invalid chain
and see roughly 9 confirmations before the chain gets rejected. It may
therefore be prudent to wait for a number of confirmations that
corresponds to the time it may take for the conservative percentage of
miners that you think may behave honestly to create a block (including
variance).

If users do not wait and happen to accept payments from an invalid
chain during this time, these payments could get reverted. This is a
weakness, but still seems preferably to continually following an
invalid chain. As long as a reasonable number of miners remains
honest, a dishonest majority can only temporarily control the network,
and their blocks (and all coins gained from it) will eventually be
rejected.

-- Ruben Somsen


[0] Olaoluwa Osuntokun, BIP 157: Client Side Block Filtering,
https://github.com/bitcoin/bips/blob/master/bip-0157.mediawiki

[1] Peter Todd, TXO commitments do not need a soft-fork to be useful,
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-February/013591.html
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