S2C11: Pre-Mint + RLP #

1. Problem #

Similar to S1C12, an RLP-encoded block header must be provided. However, a preMintFlag mechanism has been added — a commitment must be pre-submitted, and then the block header must be submitted within a specified time window.

2. Cause #

The contract implements a Commit-Reveal pattern: first call preMintFlag to lock in the intent, then within a certain timeframe (within 256 blocks) submit the correct RLP block header to prove that the specified number of blocks have been "waited" for.

3. Solution #

1. Call preMintFlag multiple times (recommended 10 times to maximize success rate)
2. Wait for at least 2 blocks to be mined
3. Obtain the complete header information of the 2nd previous block from the current block number
4. Use correct RLP encoding
5. Submit within the 256-block window

# Pre-commit (multiple times)
for i in $(seq 1 10); do
  cast send $CHALLENGE "preMintFlag()" --private-key $PK --rpc-url $RPC
done

# Wait 2 blocks
sleep 10

# Submit block header
BLOCK_NUM=$(($CURRENT_BLOCK - 2))
BLOCK_HEADER=$(cast rlp $BLOCK_NUM --rpc-url $RPC)
cast send $CHALLENGE "mintFlag(bytes)" $BLOCK_HEADER --private-key $PK

4. Pitfalls Encountered #

RLP encoding correctness: The RLP encoding of OP Mainnet block headers may differ from standard Ethereum due to the Bedrock upgrade.

Commit-Reveal time window: Must complete within 256 blocks, and must wait at least 2 blocks.

Pre-mint count: A single preMintFlag may not be sufficient to lock in the correct block range; multiple calls improve the success rate.

5. Pitfall Causes #

OP's block structure has minor differences from Ethereum mainnet (e.g., the difficulty field may differ on OP). The RLP encoder needs to correctly handle all 15+ fields of OP blocks.

6. How to Resolve #

• Use ethers.utils.RLP.encode() or cast rlp for RLP encoding
• Confirm the block is within the range [block.number - 256, block.number - 2]
• Call preMintFlag 10 times to ensure sufficient time window coverage

7. Key Technical Points #