Fix(multiproofs verification): don't skip visiting intermediate hashes

[thedevbirb]

This PR fixes the logic of the function VerifyMultiproof which doesn't visit the intermediate hashes calculated while traversing the tree, resulting in the error proof is missing required nodes when verifying Merkle Multiproofs.

In more detail, what I've noticed is that when adding the intermediate generalised indices in the keys array to be visited later, they are added to the end of the array but the latter is not sorted again, which mean those indices will actually never be read. This results in skipping some required steps of calculating intermediate hashes leading to a failure of the verification.

fastssz/proof.go

Lines 80 to 91 in c98805c

	left, hasLeft := db[(k|1)^1]
	right, hasRight := db[k|1]
	if !hasRight || !hasLeft {
	return false, fmt.Errorf("proof is missing required nodes, either %d or %d", (k|1)^1, k|1)
	}
	copy(tmp[:32], left[:])
	copy(tmp[32:], right[:])
	db[getParent(k)] = hashFn(tmp)
	keys = append(keys, getParent(k))
	pos++

The solution proposed avoids a 1-line change to add sorting after every insertion due to its inefficiency but relies instead on a auxiliary array to keep track of these additional indices

[ferranbt]

Thank you for the PR!

Two things:

• Are the module updates necessary. CI is also failing with dependency issues.
• Can you also modify this multi proof test?

[ferranbt]

Can you use the types in the spec-tests folder?

[thedevbirb]

Thanks a lot for the review! I hope I have addressed it correctly