zkevm: keccak worst-case #1497
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zkevm: keccak worst-case #1497
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jsign
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kevaundray
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Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com>
jsign
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Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com>
marioevz
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Co-authored-by: Mario Vega <marioevz@gmail.com>
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* zkEVM: add keccak attack Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com> * lints Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com> * only leave 36M gas limit Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com> * lints Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com> * feedback Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com> * Update tests/zkevm/test_worst_compute.py Co-authored-by: Mario Vega <marioevz@gmail.com> * cleanup Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com> * lints Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com> --------- Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com> Co-authored-by: Mario Vega <marioevz@gmail.com>
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This PR adds a test that builds a block to maximize the number of keccak permutations for a given gas limit (considering the max contract size).
There are two main aspects of the test design.
Find the optimal input size
The metric we care about is gas per permutation, not simply max opcode calls. Although the dynamic cost of
KECCAK256is linear in input length, memory expansion is quadratic, so there is a sweet spot for minimal cost per permutation.Instead of baking magic numbers, the test does a scan for the optimal cost and then does the attack with it. This is much more transparent on how this optimal length is found, plus it sounds also more future-proof.
Another way of understanding the above is by looking at this chart I created from that same script, with a more detailed output:
The “attack” loop
Once the optimal length is known, I create a loop that drives as many permutations as the block gas limit allows. The loop shouldn't be a tight loop, but quite the opposite -- run as many calls as possible within the max contract size, so the "JUMP"-like gas overhead is amortized as much as possible.
The general structure is:
Note that for 36M you can fit the
(....)without a loop since the max you can add fits within 24KiB contract limit. But for bigger gas limits, you run short with 24KiB and need the loop!zkVM cycles: 36m gas -> 632 million cycles