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COMP 4901W – Homework 7
This homework accounts for 10% of your total grade. You should submit your solution as a
single pdf le. 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 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 or shows a lack of eort.
1
Exercise 1
This exercise accounts for 10% of your total grade.
Alice owns 100 BTC on the Bitcoin Blockchain and Bob owns 1333 ETH on the Ethereum
Blockchain. They would like to exchange their money so that Alice gets almost 1333 ETH and
Bob gets almost 100 BTC1. However, this is complicated by two factors:
1. No party trusts the other, i.e. Bob is not willing to send his 1333 ETH unless he is assured
that he will receive 100 BTC and Alice is not willing to send her 100 BTC unless she is
assured that she will get 1333 ETH.
2. Alice and Bob are not willing to use the services of any third-party in any way. Specif-
ically, they are not willing to exchange their cryptocurrencies for at or use any bro-
ker/exchange agent of any kind.
Design a secure protocol that solves this problem and allows Alice and Bob to exchange their
money even though they have it on dierent Blockchains. Explain your protocol in detail and
argue why it is secure.
1We say “almost” because they have to pay transaction/gas fees.