The GHAST mechanism is an ingenious solution designed by Conflux to strike a balance between high performance and security.

Conflux GHAST (Greedy Heaviest Adaptive Subtrees) concept/design motivation and working principle—built to balance high performance and security

Regarding Conflux GHAST’s core concepts, design motivations, and working principles; the following is an explanation of the GHAST (Greedy Heaviest Adaptive Subtrees) mechanism:

1. Core problem: Addressing the “liveness attack”
Background

Conflux uses a Tree-Graph structure that allows parallel block production, and it confirms transactions through the heaviest-chain rule (GHOST). However, this mechanism has an inherent flaw: the liveness attack.

Attack scenario
An attacker can “cultivate” two evenly matched sub-blocks under the last common ancestor block, and use the impact on network propagation to split honest computing power into two opposing camps. Both subtrees then grow at a similar speed, preventing either side from gaining a decisive advantage, thereby stopping new blocks from being confirmed. This attack is not intended for double-spending; instead, it aims to prevent the system from confirming new transactions, disrupting the system’s “liveness.”

2. Solution: The GHAST (Greedy Heaviest Adaptive Subtrees) mechanism
To solve the above problem, the Conflux research team designed the GHAST mechanism. Its core idea is to adaptively adjust block weights so that the system can automatically “slow down” under attack—sacrificing some performance for security—while remaining fast in normal conditions.

Key points:

Three different block weights:
In the traditional heaviest-chain rule, each block has a weight of 1. GHAST introduces three weights: 0, 1, and X (where X is a relatively large number, such as 1000).
Blocks with weight 0 or X are called special blocks, while blocks with weight 1 are normal blocks.

Two types of blocks:
Normal blocks: weight is always 1, indistinguishable from ordinary blocks.
Special blocks: their weight is determined by the block’s hash. With a probability of 1/X, the weight is X; otherwise, the weight is 0. The difficulty of mining a special block is the same as mining a normal block, but its expected weight is still 1.

3. Block type is determined by the “historical tree-graph structure,” not by miners’ will:
This is a key feature of GHAST. Whether a block is normal or special is determined entirely by its “historical tree-graph structure” (i.e., the graph structure formed by all blocks before that block). Miners cannot choose it arbitrarily.
This design ensures all nodes agree on the block type, avoiding divergence in main-chain selection.

4. Adaptive logic:
Normal state: In the absence of attacks, blocks produced by honest nodes are all normal blocks (weight 1), and the system maintains fast block production.
Attack state: When the system detects abnormal behavior (e.g., signs that the tree-graph structure becomes “abnormal,” indicating a liveness attack), blocks produced by honest nodes automatically become special blocks. The weight of special blocks is either 0 or X (with an expected value of 1). This is equivalent to dynamically adjusting block difficulty, effectively reducing the system’s block production speed during the attack period, breaking the attacker’s strategy for maintaining balance, and ultimately solving the liveness attack problem.

3. Summary
The GHAST mechanism is an ingenious scheme designed by Conflux to strike a balance between high performance and security. By introducing adaptive weights, it allows the system to maintain very fast confirmation speed in normal times (averaging about 23 seconds), while automatically entering a “defense mode” against liveness attacks (by producing special blocks to reduce the effective block production speed), thereby ensuring the system’s final security. This mechanism enables Conflux to achieve high throughput while resisting complex attacks, realizing a combination of decentralization, security, and scalability.

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