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CS251 Final Exam 2021

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You may use any (non-human) resource to answer the questions. You may not collaborate with others.

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CS251 Final Exam 2021

Due Wednesday, Dec. 8, 2021, 11:59pm on gradescope

Instructions:

You may use any (non-human) resource to answer

the questions. You may not collaborate with others.

Your Name:

SUNet ID: @stanford.edu

In accordance with both the letter and the spirit

of the Stanford Honor Code, I neither received nor

provided any assistance on this exam.

Signature:

• The exam has 6 questions totaling 100 points.

• You have until the deadline to complete them.

• Please submit your answers on Gradescope (D5GKRX)

• Keep your answers concise.

1 /18

2 /20

3 /20

4 /16

5 /15

6 /11

Total /100

Page 1 of 12

Problem 1. [18 points]: Questions from all over.

A) Brieﬂy explain why a Rollup system stores all transaction data on chain? What would

go wrong if transaction data were discarded and not stored anywhere?

B) Consider the following Solidity code:

pragma solidity ^0.8.0;

contract ERC20 is IERC20 {

mapping(address => uint256) private _balances;

event Transfer(address indexed from, address indexed to, uint256 value);

function _transfer(address sender, address recipient, uint256 amount) {

emit Transfer(sender, recipient, amount);

}}

Suppose this code is deployed in two contracts: a contract at address X and a contract

at address Y . Which of the following can read the state of _balances in contract X?

Circle the correct answer(s).

A Code in the _transfer() function in contract ERC20 at address X

B Code in the _transfer() function in contract ERC20 at address Y

C An enduser using etherscan.io

C) Continuing with part (B), which of the following can read the log entry Transfer

emitted when the function _transfer() is called? Circle the correct answer(s).

A Code in the function getBalance() deﬁned in contract ERC20 at address X

B Code in the function getBalance() deﬁned in contract ERC20 at address Y

C An enduser using etherscan.io

Page 2 of 12

D) Two Ethereum transactions, tx

and tx

, are submitted to the network at the same time.

Transaction tx

has maxPriorityFee set to y and transaction tx

has maxPriorityFee

set to 2y. Will tx

necessarily be executed on chain before tx

? Justify your answer. You

can assume that maxFee for both tx

and tx

is greater than baseFee + maxPriorityFee.

E) Alice wants to buy a car from dealer Bob. She sends 1 BTC to Bob’s Bitcoin address.

Bob waits for a transaction where (i) the input is from Alice’s address, and (ii) one of

the outputs is a UTXO bound to Bob’s address with a value of 1 BTC. As soon as Bob

sees this transaction on the Bitcoin blockchain, he gives Alice the keys, and she drives

away. Is this safe? Could Alice get the car for free? If so, explain why. If not, explain

what Bob should do to ensure that he gets paid.

F) Alice owns a brand new Tesla Model Y. Can she currently use her car as collateral for

a loan in the Compound system? (without selling the car) If yes, explain how. If no,

explain why not.

Page 3 of 12

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