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【COMP 4901W】COMP 4901W – Homeworks 4 and 5
A. K. Goharshady
Release Date: March 21, 2022
Deadline: April 12, 2022 (23:59 HKT)
Due to the unexpected pace of the smart contracts part of the course, we had to merge
homeworks 4 and 5 together. As such, you will be given extra time (until April 12) for this
joint homework. This homework accounts for 20% of your total grade. You should submit
your main solution, i.e. a document explaining the logic behind every code, on Canvas as
a single pdf le. Your codes should be submitted as .sol les. As usual, handwritten and
scanned solutions will not be accepted since the TAs might be unable to read your handwriting.
However, you can draw your gures, if any, by hand. You are also allowed to discuss the
problems with your classmates but you have to write your solution and codes entirely on your
own. All submissions will go through a plagiarism check. If you submit the same solutions as
another student, you will both get a grade of F for the whole course. The deadline is rm and
no extensions will be granted. You will receive feedback on your submission and normally
be allowed to resubmit once. We reserve the right to disallow resubmissions if your original
submission is of a very low quality, does not compile, or shows a lack of eort.
Important: In each of the exercises, you have to provide both the smart contract and
an explanation of its ow, i.e. who calls which function in which order.
Exercise 1
This exercise accounts for 10% of your total grade.
Implement a Solidity smart contract that allows Alice and Bob to play a game of Rock–Paper–
Scissors on the Ethereum Blockchain. Each party should rst put a deposit of 1 ETH and then
2 ETH is paid to the winner of the game. If the game ties, each party receives 1 ETH back. You
can hard-code the addresses of both Alice and Bob in your contract.
? Prove that neither party can cheat in your contract.
? Prove that no third-party can tamper with the game by changing or censoring the
messages sent by either party.
Submit your code as a single le named rps.sol.
1
Exercise 2
This exercise accounts for 10% of your total grade.
Consider a simple auction in which people can bid for a specic item. Assume that the auction
runs for 24 hours and anyone can enter a bid in this period. However, the bids must remain
secret until the end of the bidding period. After the bidding ends, the winner should be
announced and everyone must be able to independently verify that no cheating has taken
place.

  1. Implement a Solidity smart contract that performs the simple auction mentioned above.
  2. What happens if the person with the highest bid refuses to pay? Provide a variant of
    your contract in which we can ensure payment by the highest bidder. Note that the bids
    should still remain hidden during the bidding period.
  3. How much gas do your contracts use? Make sure the amount of gas used by each
    participant is no more than a xed constant and nd this constant.
    Submit your codes as two les named auction1.sol and auction2.sol.

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