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1. Change Log
o Draft specifications
2. Due dates:
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 document listed under the section references to
understand the scope and working of the DIMY protocol. You can use multiple processes/threads/virtual
machines running on one laptop/desktop (with Linux OS) to setup the implementation environment.
3.1 Learning Objectives
On completing this assignment, you will gain sufficient expertise in the following skills:
1. Understanding and implementing several security mechanism for privacy-preserving, secret
sharing, key exchange and confidentiality 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.
4. Assignment Specifications
Updates to the assignment, including any corrections and clarifications, will be posted on the
course website at WebCMS. Please make sure that you check the course website regularly for
updates.
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 a close contact (within 2m for at least 15 min) with an infected person
is at a high risk of contracting the coronavirus.
Digital 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 [1] 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 ‘node’ refers to an instance of the DIMY protocol implementation
(client) running on your laptop/desktop. Your main front-end program should be named Dimy.py. Note
that you also need to implement the backend centralised server that should run on your laptop/desktop.
Your backend server code should be named DimyServer.py.
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 [1]. This is to cut down the development, testing and demo time
for the assignment. The marking guidelines appear at the end of the assignment specifications and are
provided to indicate the distribution of the marks for each component of the assignment.
Assignment Specification
We will follow most of the original specifications from the reference paper [1] except the changes that are
listed in this section. There are three major differences: 1) We will employ UDP/TCP socket-based
message passing between the nodes instead of using BLE communication. 2) We use different parameters
values described in detail later in this section. 3) You are required 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 subsection on the backend server.
In DIMY protocol, each node performs the following steps to broadcast and register a shared secret key
representing an encounter with other another node in close proximity. We have listed these in form of
tasks you will be assessed on.
Task 1: Generate a 32-Byte Ephemeral ID (EphID) after every 15 sec. Note that the reference paper
proposed a 16-Byte EphID due to limitation on the size of a Bluetooth message broadcast.
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 5 respectively.
Task 3: Broadcast these n shares @ 1 unique share per 3 seconds. For this implementation, you are not
required to use Bluetooth message advertisement, rather you can use simple UDP broadcasting to advertise
these shares. Also, you do not need to implement the simultaneous advertisement of EphIDs proposed in
the reference paper [1].
Task 3a: Implement a message drop mechanism that drops a message which is ready to be transmitted
with probability 0.5. This should be implemented at the sender. Hint: generate a random number between
0 and 1. If this number is less than 0.5, don’t transmit that message (chunk).
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 nodes have remained in contact for at least 9
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 node proceeds with applying Diffie-Hellman key exchange mechanism to arrive at the secret
Encounter ID (EncID).
Task 6: A node, 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 90-second period. A new
DBF is initiated after the 90-second period and each node stores at most 6 DBFs. DBF that is older than
9 min from the current time is deleted from the node’s 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 90-second
windows.
Task 8: Every 9 minutes, a node combines all the available DBFs into another Bloom Filter called Query
Bloom Filter (QBF).
Task 9: Each node 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 node will receive the result of
matching performed at the back-end server. The result is displayed to inform the user. You are required
to use TCP for this communication between the node and the back-end server.
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 node uploads a CBF, it stops generating the QBFs. The node will
receive a confirmation that the upload has been successful.
Task 11: This task performs simple security analysis of your implementation of the DIMY protocol.
A) List all the security mechanism proposed in the DIMY protocol and explain what purpose each of the
mechanism serves.
B) There are two types of communications in the DIMY protocol: i) Nodes communicate with each other
using UDP broadcasts. ii) Nodes communicate with the backend server using the TCP protocol. Create an
attacker node by modifying your implementation of the DIMY frontend. This code is named Attacker.py.
Assume that this node can receive all of the UDP broadcasts from other legitimate nodes. Think of one
attack that can be launched by this attacker node. Implement this attack and show how this attack affects
the DIMY nodes.
C) Now focus on the communication of nodes with the backend server. Again, think of one attack that can
be launched by the attacker node assuming the communication is not encrypted and the attacker node can
listen to any node communicating with the backend server. Explain how this attack affects the working of
the DIMY protocol. Note that you do not need to implement this 2nd type of attack on communication with
the backend server.
D) Finally, suggest measures (if possible) that can be implemented to prevent the attacks you identified in
B and C above for both types of communications.
General:
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
working correctly.
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.
o You are required to run the assignment with three nodes running the DIMY frontend (plus the
attacker node in Task 11) and one back-end server.
Back-end Server
Your client implementation interacts with a backend-server to send CBF/QBF and receive the results for
the risk analysis performed at the back-end. Note that, 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 is deployed in your laptop or desktop machine using TCP port No
55000.
You can provide the information regarding IP address and port No of the backend server to your
front-end client 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 nodes establish a new TCP connection with the back-end server to transfer CBF/QBF to the
server and receive the results of the queries.
The back-end server stores all the received CBFs and can perform matching for each QBF
received from devices. It informs the node 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.
5. Additional Notes
Groups: You are expected to work in groups composed of maximum two students. Use the same
groups that you have formed for the labs.
Use Python to implement this assignment.
You are required to develop and test the implementation on your own laptop/desktop instead of using
the CSE login servers.
You are free to design your own format for messages exchanged between the nodes 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 on Ed 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
You need to submit a report, your source code and a demo video. Only one member of the Group is
required to do the submission. Put the details of the group members in each document.
The report (AssignmentReport.pdf see details in Section 7) should include the group ID, members name
and zIDs, and an assignment diary that details weekly tasks performed by each group members. Add a
note about how to run your program detailing the steps required to compile/run your submitted code.
Moreover, describe your method used for implementing the specified tasks, and issues faced along with
their adopted solutions. For task 11, explain how the attacker node can launch your selected attacks on the
DIMY protocol.
You will demonstrate your assignment with a video. The video should be a screen recording showing
running of each step of the assignment. We recommend you run each process in a separate terminal, so
that you can capture the interaction between different terminals on the same screen. You must include
each of the following segments against Tasks 1 – 11. You can store the video on a file sharing site (keep
video private and unlisted) and share the link in the report.
You are also required to submit your source code (e.g., submit Dimy.py, DimyServer.py and Attacker.py)
used in the demonstration. The demonstration video carries 15 marks, while the report and code will be
marked out of 5, for a total of 20 marks.
For code submission, please ensure that you use the mandated file name. Your main program should be
named Dimy.py. You may of course have additional helper files.
Note that in the following table “show” means a screen recording of the terminal windows.
Task Segment Description Marks
Task 1 Segment 1 Show the generation of the EphID at the client nodes. 0.5
Task 2
Segment 2 Show that 5 shares of the EphIDs are generated at each node. 0.5
Task 3/3a Segment 3-A Show the sending of the shares @ 1 share per 3 seconds over UDP while
incorporating the drop mechanism.
0.5
Segment 3-B Show the receiving of shares broadcast by the other nodes. 0.5
Segment 3-C Show that you are keeping track of number of shares received for each
EphID. Discard if you receive less than k shares.
0.5
Task 4 Segment 4-A Show the nodes attempting re-construction of EphID when these have
received at least 3 shares.
0.5
Segment 4-B Show the nodes verifying the re-constructed EphID by taking the hash
of re-constructed EphID and comparing with the hash value received in
the advertisement.
0.5
Task 5 Segment 5-A Show the nodes computing the shared secret EncID by using Diffie-
Hellman key exchange mechanism.
0.5
Segment 5-B Show that a pair of nodes have arrived at the same EncID value. 0.5
Task 6 Segment 6 Show that the nodes are encoding EncID into the DBF and deleting the
EncID.
0.5
Task 7 Segment 7-A Show that the nodes are encoding multiple EncIDs into the same DBF
and show the state of the DBF after each addition.
0.5
Segment 7-B Show that a new DBF gets created for the nodes after every 90 seconds.
A node can only store maximum of 6 DBFs.
0.5
Task 8 Segment 8 Show that after every 9 minutes, the nodes combine all the available
DBFs into a single QBF.
0.5
Task 9 Segment 9 Show that a node can combine the available DBF into a CBF and upload
the CBF to the back-end server.
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