S1C9: Password Protected #

1. Problem #

The contract has a password-protected function. You need to find the correct password to call mintFlag.

2. Cause #

The password is stored in the contract's storage. Since all data on the blockchain is public, you can directly read the storage slot to obtain the password.

3. Solution #

1. Use cast storage to read the contract's storage slots
2. Find the slot where the password is stored
3. Use that password to call mintFlag

# Read storage slots (try different slots)
for slot in 0 1 2 3 4 5; do
  cast storage <CHALLENGE> $slot --rpc-url $RPC
done
# Use the discovered password
cast send <CHALLENGE> "mintFlag(string)" "found_password"

4. Pitfalls Encountered #

Password Format: The password may be stored across multiple slots (if longer than 32 bytes), requiring proper concatenation.

5. Why the Pitfall Occurred #

In Solidity, if a string type is <= 31 bytes in length, it is stored in a single slot (short string optimization). If longer, the current slot stores the length, and the data is stored in consecutive slots starting from keccak256(slot).

6. How to Resolve #

First check if the rightmost byte of the slot value equals length * 2 (short string encoding). If not, read the consecutive slots starting from keccak256(slot).

7. Technical Takeaways #