The University of New South Wales
COMP4337/9337 Securing Wired and Wireless Networks
Assignment specifications for T1 2021 (21T1)
1. Change Log
Version 1.0 released on TBA
2. Due dates:
Mid-term report/recorded presentation submission: TBA
Final report/code/demo video submission: TBA
3. Goal and learning objectives
For this assignment, your task is to implement a hybrid digital contact tracing protocol called “DIMY: Did
I Meet You”. You should implement various components of the protocol by following the specifications
listed in this document, and reading the reference documents listed under the section references to
understand the scope and working of the DIMY protocol. You are required to use the standard setup
consisting of 1 Raspberry Pi4 module and one laptop/desktop running Linux OS (see the hardware
requirements in Section 9). Your program should run on each device and exchange messages to register
the contact between the Pi4 and the laptop/desktop. Optionally, you can select to implement the
functionality listed under the extension section to gain additional bonus marks.
3.1 Learning Objectives
On completing this assignment, you will gain sufficient expertise in the following skills:
1. Understanding and implementing several privacy-preserving and confidentiality mechanisms
such as Diffie-Hellman key exchange, Shamir Secret Sharing, Hashing and Bloom Filters.
2. Learning how UDP/TCP socket-based communications take place.
3. Integration of various technologies to achieve Confidentiality, Integrity and Privacy.
4. Experience in implementing a real protocol.
Updates to the assignment, including any corrections and clarifications, will be posted on the
course website at Teams. Please make sure that you check the subject website regularly for
4. Assignment Specifications
This section gives detailed specifications of the assignment.
4.1 COVID-19 and Contact Tracing
The outbreak of the COVID-19 pandemic has changed many aspects of everyone’s way of life. One of
the characteristics of COVID-19 is its airborne transmission, which makes it highly contagious. Moreover,
a person infected with COVID-19 can be asymptomatic, thus spreading the virus without showing any
symptoms. Anyone who comes into close contact (within 2m for at least 15 min) with an infected person
is at a high risk of contracting the coronavirus.
Contact tracing applications aim to establish the close contacts of an infected person so that they may be
tested/isolated to break the chain of infection. The digital contact tracing app is typically composed of two
main entities, the smartphones acting as clients and a back-end server. In this model, the smartphones of
two individuals with tracing apps installed would exchange some random identification code (this
identification code does not reveal any sensitive information about their actual identities) when they are
in close proximity. The back-end is typically maintained by health organisations (or the government), and
once a person is diagnosed with COVID-19, they can opt to share the local list of contacts stored on their
smartphone with the back-end server to identify at-risk users. Digital contact tracing apps are not meant
to replace the traditional manual contact tracing processes, rather, these have been designed to supplement
the contact tracing process.
4.2 DIMY Digital Contact Tracing Protocol.
Download the reference paper  from the course website and read through it to understand various
components of the DIMY protocol. Briefly, devices participating in DIMY periodically generate random
ephemeral identifiers. These identifiers are used in the Diffie-Hellman key exchange to establish a secret
key representing the encounter between two devices that come in contact with each other. After generating
their ephemeral identifiers, devices employ the “k-out-of-n” secret sharing scheme to produce n secret
shares of the ephemeral identifiers. Devices now broadcast these secret shares, at the rate of one share per
minute, through advertisement messages. A device can reconstruct the ephemeral identifiers advertised
from another device, if it has stayed in this device’s communication range for at least k minutes.
After the ephemeral identifier is re-constructed, DIMY adopts Bloom filters to store the relevant contact
information. Each device maintains a Daily Bloom Filter (DBF) and inserts all the constructed encounter
identifiers in the DBF created for that day. The encounter identifier is deleted as soon as it has been
inserted in the Bloom filter. Devices maintain DBF on a 21 days rotation basis, identified as the incubation
period for COVID-19. DBFs older than 21 days automatically get deleted.
For the back-end, DIMY utilises blockchain to satisfy the immutable and decentralised storage
requirement. Once a user is diagnosed with COVID-19, they can volunteer to upload their encounter
information to the blockchain. Health Authorities (HA) then generate an authorisation access token from
the blockchain that is passed on to the device owner. The user’s device combines 21 DBFs into one Contact
Bloom Filter (CBF) and uploads this filter to the blockchain. The blockchain stores the uploaded CBF as
a transaction inside a block (in-chain storage) and appends the block to the chain.
Daily, the app will query the blockchain to perform risk-analysis, checking whether the user has come in
close contact with any person diagnosed positive. A device combines all of the locally stored DBFs (the
maximum number is limited to 21) in a single Bloom filter called the Query Bloom Filter (QBF). The
QBF is part of the query that gets uploaded to the blockchain. The blockchain matches the QBF with CBF
stored as a transaction in the blockchain and returns “matched” or “not matched” as a response. If the
response from the blockchain is negative, the device deletes its QBF. Conversely, if the user is found to
be at-risk, the user is notified, and the QBF is stored separately for further verification by HA in the follow
up manual contact tracing process.
4.3 Implementation Details
In this assignment, you will implement the DIMY protocol with a few modified parameters.
Note that in this specification, the term device refers to either Raspberry Pi4 or your laptop/desktop
running an instance of the DIMY protocol implementation. You are free to choose any programming
language out of Python, JAVA or C for your implementation. Your main front-end program should be
named Dimy.py (or Dimy.java or Dimy.c). If you choose to implement the backend centralised server as
listed in the extension section, your backend server code should be named DimyServer.py
(DimyServer.java or DimyServer.c).
This assignment specification has been modified to use TCP/IP protocol stack-based message passing
instead of BLE communication. It also uses different parameters as compared with the original
specifications listed in reference paper . This is to cut down the development, testing and demo time
for the assignment. The remainder of the specification is divided into two parts, beginning with the base
specification as the first part and the subsequent extension part adding new functionality to the base
specification. Students can choose to only attempt the base specifications or optionally can also attempt
the extension part of this assignment for bonus marks. The marking guidelines appear at the end of the
specification indicating the distribution of marks for both the base as well as the extension case.
Part 1: Base Specification
We will follow most of the original specifications from the reference paper  except the changes that are
listed in this section. There are two major differences: 1) We will employ UDP/TCP socket-based message
passing between the devices instead of using BLE communication. 2) We use different parameters values
described in detail later in this section. For the base specifications, you are not required to implement the
backend functionality. Rather, your front-end will interact with a running implementation of Blockchain-
based back-end by using the specified API calls. For the extension part, you may opt to implement a
simple centralised server acting as the back-end server instead of the Blockchain proposed in the reference
paper. For details, please go through the section on assignment extension. We use the term back-end server
to refer to default Blockchain-based implementation for base specifications and your own private
centralised server in the extension part of this assignment.
In DIMY protocol, each device performs the following steps to broadcast and register a shared secret key
representing an encounter with other another device in close proximity. We have listed these in form of
tasks you will be assessed on.
Task 1: Generate 16-Byte Ephemeral ID (EphID) after every 1 minute.
Task 2: Prepare n chunks of the EphID by using k-out-of-n Shamir Secret Sharing mechanism. For this
implementation, we use the values of k and n to be 3 and 6 respectively.
Task 3: Broadcast these n shares @ 1 unique share per 10 seconds. For this implementation, you do not
need to implement the simultaneous advertisement of EphIDs proposed in the reference paper .
Task 4: A receiver can reconstruct the advertised EphID, after it has successfully received at least k shares
out of the n shares being advertised. This means that if the devices have remained in contact for at least
30 seconds and received >= 3 shares of the same EphID, it can reconstruct the EphID. Verify the re-
constructed EphID by taking hash and comparing with the hash advertised in the chunks.
Task 5: The device proceeds with applying Diffie-Hellman key exchange mechanism to arrive at the
secret Encounter ID (EncID).
Task 6: A device, after successfully constructing the EncID, will encode EncID into a Bloom filter called
Daily Bloom Filter (DBF), and delete the EncID.
Task 7: A DBF will store all EncIDs representing encounters faced during a 10-minute period. A new
DBF is initiated after the 10-minute period and each device stores at most 6 DBFs. DBF that is older than
1 hour from the current time is deleted from the device storage. Note that in original specifications DBF
stores a day worth of EncIDs, but for this demo we will use DBF to store EncIDs received in 10-minutes
Task 8: Every 60 minutes, a device combines all the available DBFs into another Bloom Filter called
Query Bloom Filter (QBF).
Task 9: Each device sends this QBF to the backend server, to check whether it has come in close contact
with someone who has been diagnosed positive with COVID-19. The device will receive the result of
matching performed at the back-end server. The result is displayed to inform the user.
Task 10: A user who is diagnosed positive with COVID-19, can choose to upload their close contacts to
the backend server. It combines all available DBF’s into a single Contact Bloom Filter (CBF) and uploads
the CBF to the backend server. Once a device uploads a CBF, it stops generating the QBFs. The device
will receive a confirmation that the upload has been successful.
o Your front-end implementation should work in the debugging mode displaying messages sent and
received, operations performed and state of Bloom filters in the terminal to illustrate that it is
o Use UDP message broadcasting to implement send and receive functionality.
o DBF, QBF and CBF are all of size 100KB and use 3 hashes for encoding.
Part 2: Extension
Your implementation for the base specifications interacts with a pre-deployed Blockchain based-server to
send CBF/QBF and receive the results for the risk analysis performed at the back-end. For this extension
part, you may opt to do your own implementation of the functionality for the back-end server to obtain 2
bonus marks**. However, you are not required to use a blockchain-based implementation, rather, you
can use a simple centralised server to interact with the front-end.
• The backend server program can be deployed in your laptop or desktop machine using TCP port
• You can provide the information regarding IP address and port No of the backend server to your
front-end program through command line arguments. For example, Dimy.py 192.168.1.100
55000, where server is running on IP 192.168.1.100 and port No 55000 Or you can opt to hard
code this information at the front-end.
• The devices establish a new TCP connection with the back-end server to transfer CBF/QBF to
Task 11 (only for the extension part): The back-end server stores all the received CBFs and can perform
matching for each QBF received from devices. It informs the device that has uploaded the QBF about the
result of matching, matched or not matched. If there is no CBF available, the back-end returns not matched.
**Condition for Bonus Marks: Please note that you can only receive bonus marks (up to 2 marks) if
you do not already have full marks in the practical component of this course (i.e. your bonus marks +
your total mark for the practical component of the course cannot exceed 70). If you have already
achieved full marks for the practical component, your bonus marks will not be added to your quiz
5. Additional Notes
• Groups: You are expected to work in groups composed of either two or three students. ALL groups
(whether same as for the labs or changed) must follow the below policy:
o Send an email to email@example.com with email subject "SWWN'21 Assignment
Group". The body of the email must include all student names and ZIDs.
o You will receive a confirmation email from the same email account which includes your group
name. The group name must be mentioned in all correspondence and when preparing the
§ Note: no change is required to groups formed on Moodle for the labs, whether you keep
the same group or change it.
o Deadline to send the email for your assignment group is TBA. Any group request after this
time will incur a penalty in the assignment mark for all group members.
• Language and Platform: You are free to use C, JAVA or Python to implement this assignment. Please
choose a language that you are comfortable with.
• You are required to develop and test the implementation on your own laptops/provided Pi4 instead of
using the CSE login servers.
• For the extension part, you are free to design your own format for messages exchanged between the
devices and the back-end server. Just make sure your front-end and back-end programs can handle
these messages appropriately.
• You are encouraged to use the course discussion forum to ask questions and to discuss different
approaches to solve any issues faced during the implementation. However, you should not post any
code fragments on the forum.
6. Assignment Submission
There are two deliverable stages:
1. Midterm deliverables: Midterm report including assignment diary and a recorded presentation
2. Final deliverables: Final report including assignment diary, source code and demo video
The midterm report should include the group name, members name and zIDs, assignment diary that details
the overall plan with assigned task for each group member, progress on the implementation and issues
faced. You are also to submit a recorded presentation, maximum of 6 slides, that explains your
understanding of the DIMY protocol and what each component achieves in the protocol. You should
divide the presentation equally among the group members with each member presenting their part. These
mid-term deliverables carry 5 marks (2 marks for report and 3 marks for the presentation).
For the final deliverable, you will demonstrate your assignment with a video, and describe your method
used for implementing the specified tasks, and issues faced along with their adopted solutions, in a detailed
report (AssignmentReport.pdf see details in Section 7) to be submitted along with the video. You are also
required to submit your source code used in the demonstration. The demonstration video carries 15 marks,
while the report and code will be marked out of 10, for a total of 25 marks. The total marks for this
assignment are 30.
The video should be a screen recording showing running of each step of the assignment. We recommend
you ssh in the Pi4 from a terminal, so that you can capture the interaction between your laptop/desktop
and the Pi4 running in different terminals on the same screen. You must include each of the following
segments against Tasks 1 – 10.
Note that in the following table “show” means “A screen recording of the terminal windows (both
laptop and ssh for the raspberry Pi4)”.
Task Segment Description Marks
Task 1 Segment 1 Show the generation of the EphID at both the devices. 0.5
Segment 2 Show that 6 shares of the EphIDs are generated at each device. 1
Task 3 Segment 3-A Show the sending of the shares @ 1 share per 10 seconds over UDP. 1.5
Segment 3-B Show the receiving of shares broadcasted by the other device. 0.5
Segment 3-C Show that you are keeping track of number of shares received for each
Task 4 Segment 4-A Show the devices attempting re-construction of EphID when these
have received at least 3 shares.
Segment 4-B Show the devices verifying the re-constructed EphID by taking the hash
of re-constructed EphID and comparing with the hash value received
in the advertisement.
Task 5 Segment 5-A Show the devices computing the shared secret EncID by using Diffie-
Hellman key exchange mechanism.
Segment 5-B Show that the devices have arrived at the same EncID value. 1
Task 6 Segment 6 Show that the devices are encoding EncID into the DBF and deleting
Task 7 Segment 7-A Show that the devices are encoding multiple EncIDs into the same DBF
and show the state of the DBF after each addition.
Segment 7-B Show that a new DBF gets created for the devices after every 10
minutes. A DBF can only store maximum of 6 EncIDs.
Task 8 Segment 8 Show that after every 60 minutes, the devices combine all the available
DBFs into a single QBF.
Task 9 Segment 9-A Show that the devices send the QBF to the back-end server. For
extension, the back-end server is your own centralised server.
Segment 9-B Show that the devices are able to receive the result of risk analysis back
from the back-end server. Show the result for a successful as well as an
unsuccessful match. For extension, the back-end server is your own
Task 10 Segment 10 Show that a device can combine the available DBF into a CBF and
upload the CBF to the back-end server. For extension, the back-end
server is your own centralised server.
Show that the device is able to establish a TCP connection with the
centralised server and perform Tasks 9 and 10 successfully.
Show the terminal for the back-end server performing the QBF-CBF
matching operation for risk analysis.
For code submission, please ensure that you use the mandated file name. Your main program should be
named Dimy.py (or Dimy.java or Dimy.c). You may of course have additional header files and/or helper
files. If you are using C, then you MUST submit a Makefile/script along with your code (not necessary
with Java or Python). This is because we need to know how to resolve the dependencies among all the
files that you have provided.
• Midterm and final assignment submission documents are to be submitted via Moodle. Submission link
will be provided later on.
• Late submission penalty will be applied as follows:
o 1 day after deadline: 20% reduction
o 2 days after deadline: 40% reduction
o 3 or more days after deadline: NOT accepted
NOTE: The above penalty is applied to your obtained marks. For example, if you submit your final
assignment deliverables 1 day late and your score on the final component of the assignment is 10/25, then
your final mark will be 10 – 2.0 (20% penalty) = 8.0.
For the final deliverable, you have to submit a small report, AssignmentReport.pdf (no more than 4
pages) that must contain the following:
1. Assignment name, group name and names/IDs for all group members.
2. Executive summary that provides a brief introduction to the salient features in the assignment
3. A brief discussion of how you have implemented the DIMY protocol. Provide a list of features that
you have successfully implemented. In case you have not been able to get certain features of DIMY
working, you should also mention that in your report.
4. Discuss any design trade-offs considered and made. List what you consider is special about your
implementation. Describe possible improvements and extensions to your program and indicate how
you could realise them.
5. Indicate any segments of code that you have borrowed from the Web or other books.
6. Assignment Diary: Each group is also required to attach a 1-page assignment diary to the report. This
diary should maintain a weekly log of activities conducted by each group and should explicitly indicate
the part played by each team member in these activities. You may use any format (Gantt chart, table,
etc.) for maintaining the diary. The diary is not marked. However, if the diary is not submitted, a
penalty of 2 marks will be applied. Please attach the diary at the end of the report. Do not submit it as
a separate file. Unless specified otherwise, contribution from all members will be considered equal.
Any difficulty in working with team members must be reported to the tutor-in-charge at the earliest.
You are to write all of the code for this assignment implementation yourself. All source codes are subject
to strict checks for plagiarism, via highly sophisticated plagiarism detection software for code as well as
the submitted report. These checks may include comparison with available code from Internet sites and
assignments from previous semesters. In addition, each submission will be checked against all other
submissions of the current semester. Do not post this assignment on forums where you can pay
programmers to write code for you. We will be monitoring such forums. Please note that we take this
matter quite seriously. The LIC will decide on appropriate penalty for detected cases of plagiarism. The
most likely penalty would be to reduce the assignment mark to ZERO and reported to the school
We are aware that a lot of learning takes place in student conversations, and don’t wish to discourage
those. You are free to discuss (and are in fact strongly encouraged to do so) generic issues relevant to the
assignment on the course forum. However, refrain from posting specific code-fragments or scripts on the
forum. Students will be heavily penalized for doing so. It is important, for both those helping others and
those being helped, not to provide/accept any programming language code in writing, as this is apt to be
used exactly as is, and lead to plagiarism penalties for both the supplier and the copier of the codes. It is
OK to borrow bits and pieces of code (not complete modules/functions) from sample code out on the Web
and in books. You MUST however acknowledge the source of any borrowed code. This means providing
a reference to a book or a URL where the code appears (as comments). Also indicate in your report the
portions of your code that were borrowed. Explain any modifications you have made (if any) to the
The implementation for this assignment requires access to the following hardware:
1. Raspberry Pi4 kit.
2. A laptop/desktop running native Linux OS or a Linux VM.
Students (onshore only) will be issued one Raspberry Pi4 kit that they can use for labs and this assignment.
You are required to return the complete kit once the course is over. It is your own responsibility to arrange
an appropriate laptop/desktop for developing the application.
 DIMY: Enabling Privacy-preserving Contact Tracing. Pdf will be made available from the course
Draft V0.1: Released on 9th Feb, 2021
o Early draft specifications
Draft V0.2: Released on 18th Feb, 2021.
o Removed the requirement to implement BLE message passing between devices
o Added UDP broadcasting 学霸联盟