Re: [Trans] [ct-policy] Re: Certificate Transparency Mirrors (experimental)

[Pierre Phaneuf <[e-mail]>]

On Fri, Nov 4, 2016 at 1:42 PM, Florian M. <[e-mail]> wrote:

> if, for whatever reason, a get-entries response is corrupted, the root hash
> will be incorrect. If the user has no access to intermediate STH, there is
> no way to narrow down and track the corrupted get-entries result. The user
> can only start over.

That's not entirely true, there's a strategy you can use to find where
the corruption occurred.

If the root hash of the whole Merkle tree doesn't compute correctly,
you can do a binary search for the corrupted entry, by using the
MerkleTree::RootAtSnapshot method in the C++ implementation (for
example) to compute the root hash of your tree at half the size, and
do a get-sth-consistency between that tree size and the full one. If
the proof applies correctly, then you know the entries below are good,
and you can repeat the operation, until you isolate the tree size that
stops validating with the consistency proof. You could then drop all
the entries from there on and resume from there (simpler), or maybe
just re-fetch a chunk and recompute the Merkle tree (hoping the
corruption was localised).

You can also spot check single entries using inclusion proofs.