COMP26020: Programming Languages and Paradigms
Lab 5 - Solidity
Joseph Razavi and Richard Banach
1 Introduction
This lab exercise is about learning a programming language with unusual aspects from its documentation.
We focus on the Solidity programming language, in particular Solidity version 6, which you can read about
here:
https://docs.soliditylang.org/en/v0.6.0/
Solidity is a language designed to write so-called “smart contracts”. These are pieces of code which are
supposed to run on a public “blockchain” – a system which keeps a log of every event which happens, and
where no user can single-handedly affect what happens. That means that once your code is deployed, you
can no longer influence it, unless you have programmed mechanisms to do so. And if you find a bug, the
bug is there forever!
In addition, the blockchain is designed to support payments of various kinds – for instance a smart
contract has a balance of currency (called ‘wei’ for the Etherium blockchain on which Solidity contracts run)
which it must use to pay for its own computing resources. Contracts can charge each other and pay each
other for services.
Whether or not any of this is a sensible technical or social project is perhaps debatable, but it certainly
creates interesting design challenges for a programming language – and where weird programming languages
lead, let us follow!
Read about Solidity’s notion of a contract, and its execution model (the ‘Ethereum Virtual Machine’) here:
https://docs.soliditylang.org/en/v0.6.0/introduction-to-smart-contracts.html
Refer to the Solidity documentation to complete the exercises below. Aside from the above these sections
are particularly useful:
• https://docs.soliditylang.org/en/v0.6.0/solidity-by-example.html
• https://docs.soliditylang.org/en/v0.6.0/solidity-in-depth.html
If you prefer videos, I have made available on Blackboard some videos designed to help you get started.
Note these videos belong to the lab and are not part of the content of any week. Solidity will be used only
for the lab, and is not examinable.
In this lab exercise, rather than deploying our code on the real public blockchain (and having to pay
to run it!) we will use a simulated version of the Etherium Virtual Machine which is used for developing
code and testing it before deploying it for real. You must use the version provided on Blackboard; see next
section.
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2 Setup
Make sure you have downloaded Remix from Blackboard:
https://online.manchester.ac.uk/bbcswebdav/courses/I3132-COMP-26020-1231-1YR-040494/remix-d624303.zip
(If the link above does not work, check the Lab 5 folder on Blackboard for information.)
and that you can compile and run programs. To do this, you might need to click on the ‘plug’ icon on the
left hand menu, and made sure ‘Solidity compiler’ and ‘deploy and run transactions’ are enabled. This will
let you compile and run Solidity programs in Remix as seen in the videos. Remix is a browser based editor,
and has been tested for this course on Google Chrome on Linux and Windows. With other browsers you
may get strange behaviour. It is better to edit in a separate text editor and paste into Remix for testing, as
it can have problems with saving files and allowing text to be copied out of it in some browsers. Make sure
you always have a copy of your code in another editor so that you don’t lose your work. Clone the gitlab
repository
26020-lab5-S-Solidity_
where is replaced by your username. This contains the files you will need for the exercise.
3 Background
The exercises concern three contracts which should interact with each other, alongside other contracts which
we assume exist (but do not implement or worry about the implementation of). The first contract we consider is a ‘paylock’. The idea is that a supplier does some work, which can then be collected by a customer.
If the customer collects early, they get a discount, and how much discount they get depends on how early:
there are two deadlines. If they miss the second deadline they forfeit their discount altogether.
Done_2 Forfeit
Done_1 Delay
 Working Completed
Start
Signal
Collect_1_Y Collect_1_N
Collect_2_Y Collect_2_N
The blobs indicate possible states of the paylock, and the arrows represent function calls. The ‘Start’
arrow represents the constructor. The idea is that the functions should only succeed if the paylock is in
the state at the beginning of the arrow, and then the resulting state should be the one at the end. Of
course, there are other conditions: collect_1_Y should only succeed if called before the first deadline,
and collect_1_N should only succeed if called once the first deadline has passed; similar considerations
apply to the other two collect functions. Look in the file paylock.sol to see a partially finished implementation of the paylock. The first two exercises (see next section) concern only the logic of the paylock.
They are about adding features to the implementation, though we never complete a realistic implementation.
The subsequent exercises are about implementing a supplier which has to interact with both the paylock
contract and a rental contract which it needs to use to complete its work. As above, we will only model
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certain aspects of these contracts. On the one hand this makes the exercises manageable, but on the other
hand it can be confusing if not pointed out: you would naturally wonder when we would add the rest of the
necessary features!
4 Exercises
The implementation of the paylock which you are given does not model the passage of time. To do this, we
will add a tick function, representing the passage of one unit of time. We shall assume for the moment that
the tick function is going to be called by a neutral third party, who we trust to call it at a regular interval.
For now we also trust all other contracts in the universe not to call this function. (And assume that the
blockchain updates quickly enough that this is a reasonable model of time! This is not how one would deal
with time in a real smart contract system.)
EXERCISE 1: (2 marks)
Add an int variable clock and a tick function which models the passage of time. Modify the various
collect functions to adhere to the deadlines, where we consider the first deadline to happen if the clock
has reached 4 units of time or more, and the second deadline to be when the clock has increased by
4 units of time or more from when collect_1_N was called.
We now need to make sure this tick function can only be called by the agreed third party.
EXERCISE 2: (2 marks)
Add an address variable timeAdd to the contract. Add an argument to the constructor and set the
value of timeAdd to that argument. Now modify tick so that it can only be called by someone from
the address timeAdd .
Tip: when testing your code, copy one of the addresses from the ‘Account’ dropdown menu and paste
it into the constructor argument. That should make it easier to experiment.
Look in the file supplier.txt and paste its contents at the end of paylock.sol . Note how the Supplier
contract interacts with the paylock, indicating to the paylock when it has finished its task. In the next
exercise, we will make it interact with the Rental contract too. The idea is that in order to finish its job,
the Supplier must rent a resource, then return it, before calling finish will succeed.
EXERCISE 3: (2 marks)
Add functions aquire_resource and return_resource which must be called in that order to the
Supplier contract. To do this you will need to add new local variables. Add a local variable
representing an instance of the Rental contract, and allow the address of an instance of Rental to
be passed as an argument to the constructor. Modify the aquire_resource and return_resource
functions so that they call the appropriate functions of the Rental contract.
Tip: Since the constructor of Supplier requires the addresses of a Paylock and a Rental, make sure
you deploy instances of those first when testing.
We will now make our model of the Rental contract somewhat more realistic, by requiring the payment
of a deposit which is returned once the rented resource is re- turned. For the purposes of the lab we assume
that the deposit is 1 wei.
Since the Rental contract is not supposed to assume that it is being called be a Supplier, it should
assume that the contract it is connected to implements a receive function; you can read about this in the
Solidity language documentation:
https://docs.soliditylang.org/en/v0.6.0/contracts.html#receive-ether-function.
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Since we are not allowed to assume the calling contract is a Supplier, it is also useful to look at the
functions which can be applied to any address:
https://docs.soliditylang.org/en/v0.6.0/types.html#members-of-addresses .
In fact, our intention is to make as few assumptions about the other contract as possible, so we will use
the low-level .call() function. Find out how to make this work and attach a value to it.
EXERCISE 4: (2 marks)
Modify the Rental contract in the following way. First find the commented line
//CHECK FOR PAYMENT HERE
and replace it with something which prevents the function from succeeding unless proper payment is
made. You will also have to make the functions payable. Then find the commented line
//RETURN DEPOSIT HERE
and replace it with a single use of the .call function which returns the deposit. Modify the Supplier
contract so that it has a receive function, and make sure that Rental does not assume that the
contract which calls its functions is an instance of Supplier. Modify the external function calls made
by Supplier to Rental so that they transfer the deposit as appropriate.
At this point you should copy the file paylock.sol to supplier2.sol and work in supplier2.sol .
The rental contract as implemented has a security flaw (which is described in the ‘Reentrancy’ section of
chapter 9 of Antonopoulos’s book Mastering Etherium (available online from the library, and also at
https://github.com/ethereumbook/ethereumbook/blob/develop/09smart-contracts-security.asciidoc
EXERCISE 5: (1 mark)
Modify the Supplier contract to take advantage of this security flaw to take more Ether belonging
to the Rental contract than it has sent to the contract, if more ehter is available. Make sure this
work is saved in the file supplier2.sol
At this point you should copy the file supplier2.sol to suppler3.sol and work in supplier3.sol .
EXERCISE 6: (1 mark)
Re-order the lines of the retrieve_resource function of the Rental contract so that the vulnerability
above is fixed. Make sure this work is saved in the file supplier2.sol
Note: You need only prevent the attack described here while preserving correct functionality; you do
not need to solve any other security flaws.
5 Submission
Submission is by gitlab, following the same procedure as the other labs for this unit. Ensure that you have
pushed a commit containing your submission (i.e. make sure you have added all files to the repository),
tagged with the tag lab5-submission , by 6pm on 03/05.
Check SPOT to make sure your submission has been received correctly, and contact me (Joe) if you
notice any strange behaviour from SPOT.

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