consensus-specs/specs/phase0/weak-subjectivity.md

Phase 0 -- Weak Subjectivity Guide

Table of contents

• Introduction
• Prerequisites
• Custom Types
• Constants
• Configuration
• Weak Subjectivity Checkpoint
• Weak Subjectivity Period
  • Calculating the Weak Subjectivity Period
    • compute_weak_subjectivity_period
• Weak Subjectivity Sync
  • Weak Subjectivity Sync Procedure
  • Checking for Stale Weak Subjectivity Checkpoint
    • is_within_weak_subjectivity_period
• Distributing Weak Subjectivity Checkpoints

Introduction

This document is a guide for implementing the Weak Subjectivity protections in Phase 0. This document is still a work-in-progress, and is subject to large changes. For more information about weak subjectivity and why it is required, please refer to:

• Weak Subjectivity in Ethereum Proof-of-Stake
• Proof of Stake: How I Learned to Love Weak Subjectivity

Prerequisites

This document uses data structures, constants, functions, and terminology from Phase 0 -- The Beacon Chain and Phase 0 -- Beacon Chain Fork Choice.

Custom Types

Name	SSZ Equivalent	Description
Ether	uint64	an amount in Ether

Constants

Name	Value
ETH_TO_GWEI	uint64(10**9)

Configuration

Name	Value
SAFETY_DECAY	uint64(10)

Weak Subjectivity Checkpoint

Any Checkpoint object can be used as a Weak Subjectivity Checkpoint. These Weak Subjectivity Checkpoints are distributed by providers, downloaded by users and/or distributed as a part of clients, and used as input while syncing a client.

Weak Subjectivity Period

The Weak Subjectivity Period is the number of recent epochs within which there must be a Weak Subjectivity Checkpoint to ensure that an attacker who takes control of the validator set at the beginning of the period is slashed at least a minimum threshold in the event that a conflicting Checkpoint is finalized.

SAFETY_DECAY is defined as the maximum percentage tolerable loss in the one-third safety margin of FFG finality. Thus, any attack exploiting the Weak Subjectivity Period has a safety margin of at least 1/3 - SAFETY_DECAY/100.

Calculating the Weak Subjectivity Period

A detailed analysis of the calculation of the weak subjectivity period is made in this report.

Note: The expressions in the report use fractions, whereas the consensus-specs only use uint64 arithmetic. The expressions have been simplified to avoid computing fractions, and more details can be found here.

Note: The calculations here use Ether instead of Gwei, because the large magnitude of balances in Gwei can cause an overflow while computing using uint64 arithmetic operations. Using Ether reduces the magnitude of the multiplicative factors by an order of ETH_TO_GWEI (= 10**9) and avoid the scope for overflows in uint64.

compute_weak_subjectivity_period

def compute_weak_subjectivity_period(state: BeaconState) -> uint64:
    """
    Returns the weak subjectivity period for the current ``state``. 
    This computation takes into account the effect of:
        - validator set churn (bounded by ``get_validator_churn_limit()`` per epoch), and 
        - validator balance top-ups (bounded by ``MAX_DEPOSITS * SLOTS_PER_EPOCH`` per epoch).
    A detailed calculation can be found at:
    https://github.com/runtimeverification/beacon-chain-verification/blob/master/weak-subjectivity/weak-subjectivity-analysis.pdf
    """
    ws_period = MIN_VALIDATOR_WITHDRAWABILITY_DELAY
    N = len(get_active_validator_indices(state, get_current_epoch(state)))
    t = get_total_active_balance(state) // N // ETH_TO_GWEI
    T = MAX_EFFECTIVE_BALANCE // ETH_TO_GWEI
    delta = get_validator_churn_limit(state)
    Delta = MAX_DEPOSITS * SLOTS_PER_EPOCH
    D = SAFETY_DECAY

    if T * (200 + 3 * D) < t * (200 + 12 * D):
        epochs_for_validator_set_churn = (
            N * (t * (200 + 12 * D) - T * (200 + 3 * D)) // (600 * delta * (2 * t + T))
        )
        epochs_for_balance_top_ups = (
            N * (200 + 3 * D) // (600 * Delta)
        )
        ws_period += max(epochs_for_validator_set_churn, epochs_for_balance_top_ups)
    else:
        ws_period += (
            3 * N * D * t // (200 * Delta * (T - t))
        )
    
    return ws_period

A brief reference for what these values look like in practice (reference script):

Safety Decay	Avg. Val. Balance (ETH)	Val. Count	Weak Sub. Period (Epochs)
10	28	32768	504
10	28	65536	752
10	28	131072	1248
10	28	262144	2241
10	28	524288	2241
10	28	1048576	2241
10	32	32768	665
10	32	65536	1075
10	32	131072	1894
10	32	262144	3532
10	32	524288	3532
10	32	1048576	3532

Weak Subjectivity Sync

Clients should allow users to input a Weak Subjectivity Checkpoint at startup, and guarantee that any successful sync leads to the given Weak Subjectivity Checkpoint along the canonical chain. If such a sync is not possible, the client should treat this as a critical and irrecoverable failure.

Weak Subjectivity Sync Procedure

1. Input a Weak Subjectivity Checkpoint as a CLI parameter in block_root:epoch_number format, where block_root (an "0x" prefixed 32-byte hex string) and epoch_number (an integer) represent a valid Checkpoint. Example of the format:

   0x8584188b86a9296932785cc2827b925f9deebacce6d72ad8d53171fa046b43d9:9544
   

2. Check the weak subjectivity requirements:

   • IF epoch_number > store.finalized_checkpoint.epoch, then ASSERT during block sync that block with root block_root is in the sync path at epoch epoch_number. Emit descriptive critical error if this assert fails, then exit client process.
   • IF epoch_number <= store.finalized_checkpoint.epoch, then ASSERT that the block in the canonical chain at epoch epoch_number has root block_root. Emit descriptive critical error if this assert fails, then exit client process.

Checking for Stale Weak Subjectivity Checkpoint

Clients may choose to validate that the input Weak Subjectivity Checkpoint is not stale at the time of startup. To support this mechanism, the client needs to take the state at the Weak Subjectivity Checkpoint as a CLI parameter input (or fetch the state associated with the input Weak Subjectivity Checkpoint from some source). The check can be implemented in the following way:

is_within_weak_subjectivity_period

def is_within_weak_subjectivity_period(store: Store, ws_state: BeaconState, ws_checkpoint: Checkpoint) -> bool:
    # Clients may choose to validate the input state against the input Weak Subjectivity Checkpoint
    assert ws_state.latest_block_header.state_root == ws_checkpoint.root
    assert compute_epoch_at_slot(ws_state.slot) == ws_checkpoint.epoch

    ws_period = compute_weak_subjectivity_period(ws_state)
    ws_state_epoch = compute_epoch_at_slot(ws_state.slot)
    current_epoch = compute_epoch_at_slot(get_current_slot(store))
    return current_epoch <= ws_state_epoch + ws_period

Distributing Weak Subjectivity Checkpoints

This section will be updated soon.