Fix multiple equal selection signatures in Aggregator Committee

[MatheusFranco99]

Overview

Context

For the sync committee Ethereum duty, 512 validators are selected to submit head votes, distributed through 4 subnets, and a fraction of them are selected to become sync committee contributors. While a validator can belong to multiple subnets, it expected to be assigned to a subnet only once. Still, on small devnets with few validators, it's possible for a validator to be assigned multiple times to the same subnet.

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Issue and Fix

Issue: In the pre-consensus phase of the aggregator committee duty, selection proof signatures are created to check whether the validator is a sync committee contributor or not. If it's assigned more than once to the same subnet, our current implementation would sign the same selection data multiple times and add them to the partial-signature message, which is redundant as the signature is the same.

Fix: We simply sign once per subnet and let the message have unique signatures per (validator index, subnet).

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Test

A new test is added with specification:

• Runner process a duty with validator sync committee indices {0,0,1,129,129,130}. Subnet is computed as index/(512/4). Thus, they map to subnets {0,0,0,1,1,1}.
• The runner's pre-consensus message should have only 2 PartialSignatureMessage (one root for 0 and another for 1).
• We compare the runner's output with a message with only the 2 signatures mentioned before.

Testing Utils Changes

• TestingContributionProofIndexes is changed from {0, 1, 2} to {0, 129, 257} so we can have different subnets for the default tests.
• contributionProofMsg (the core testing function for creating partial-signature messages for sync committee contribution) is extended with an argument for validator sync committee indices.

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SSV repo PR: ssvlabs/ssv#2681

[greptile-apps]

Too many files changed for review. (886 files found, 500 file limit)

[nkryuchkov]

@greptileai can you review only these files?
ssv/aggregator_committee.go
types/testingutils/beacon_node_aggregator_committee_duty.go
types/testingutils/beacon_node_sync_committee.go
types/testingutils/ssv_msgs_sync_committee_contribution.go

[GalRogozinski]

Still, on small devnets with few validators

@MatheusFranco99 just a note that this issue isn't just for devnet.
It could also happen on mainnet with low probability

[shane-moore]

agree that this solution alleviates the issue