Comp61421 – Assignment 2 Page 1

COMP61421 - Cyber Security
Assignment 2 - A Drink Vending Machine
Maximum marks available in this coursework 25

Weighting of this coursework towards the unit overall mark (%) 25%

Learning outcomes being Assessed Stated in the course unit specification

The coursework setter Dr Ning Zhang
Handout date 19th Nov 2019
Handin date 5:00pm, 12th Dec 2019
(Please submit your work via the Blackboard facility)

The Problem An advanced drinks vending machine allows a mobile user to pay for a drink using a
mobile phone billing account based on his/her fingerprint. The user is assumed to have
data related to one of his/her fingerprints registered with a server operated by the
service provider that manages the user’s billing account. To purchase a drink, the
mobile user uses his/her mobile phone to dial the number associated with the vending
machine, and the machine then displays a request for the selection of a drink and
provision of data related to the user’s fingerprint. Having received the user’s valid drink
selection and user’s fingerprint related data, the vending machine uses the fingerprint
related data to request the server of the user’s service provider to pay for the drink
selected. Here assume that the vending machine can obtain the user’s phone number
and identify the server of his/her service provider based on the number.
Upon receipt of the vending machine’s payment request, the server checks that it has a
billing account associated with the fingerprint data received and the amount of money
in the account is sufficient to pay for the drink. The server grants the payment by
debiting the user’s billing account and crediting the designated account of the vending
machine, only if the checking is positive, and informs the vending machine of its
decision. If the server grants the payment, the vending machine delivers a selected
drink. Otherwise, the vending machine terminates the purchase and informs the user by
a displayed message.
The drinks vending machine is mainly designed for a mobile user using an advanced
mobile phone with a built-in fingerprint scanner. However, sometimes the mobile user
can only get hold of an ordinary mobile phone with no built-in fingerprint scanner. In
this case, the user is allowed to download his/her fingerprint related data from the server
of the user’s service provider. This coursework only considers the latter case.
It is assumed that:
• Each user’s mobile phone offers a DES-based symmetric cryptosystem including a
secure hash function;
• The user has a password registered with the server of his/her service provider but
does not share any extra DES key with the server;
• The user does not share any DES key with the vending machine;
• The user’s mobile phone cannot run any asymmetric cryptosystem such as RSA;
• For the sake of cost-saving, the use of Kerberos has been ruled out.
The Questions You are required to perform the following tasks (you can make necessary assumptions):
1. Secure downloading of a mobile user’s fingerprint related data. This includes:
(a) Design and explain (with diagrammatical illustration) a protocol to allow the
mobile user to securely download his/her fingerprint related data from the
server of the user’s service provider to his/her mobile phone.
Note that the design of this protocol must meet the following requirements:
(i) The server transfers the fingerprint related data to the mobile user only
Comp61421 – Assignment 2 Page 2

when the server is convinced that the user is the legitimate owner of the
fingerprint related data and that the request is indeed from the claimed
user.
(ii) The confidentiality of the fingerprint related data transferred from the
server to the user must be protected.
(iii) Measures should be taken to reduce the risk of Denial of Service (DoS)
attacks on the server.
(b) Analyse the designed protocol to justify how the protocol satisfies the above
requirements 1 (a) (i), (ii) and (iii).
2. Authorised purchase of a drink by a mobile user. This includes:
(a) Design and explain a protocol (with diagrammatical illustration) to allow the
mobile user to purchase a drink based on his/her fingerprint related data
already downloaded from the server of the user’s service provider to his/her
mobile phone.
Note that the design of this protocol can omit the details of the drink
purchase (e.g. the drink price and account details of the drink vending
machine), and that the design must meet the following requirements:
(i) The mobile user authorises the drink purchase using his/her fingerprint
related data, the drink vending machine receives the authorisation but
cannot obtain any information on the user’s fingerprint data, and the
service provider’s server can verify the authenticity of the user’s
authorisation and the vending machine’s payment request.
(ii) The drink purchase authorisation of the mobile user cannot be re-used
for deceptive charging by the vending machine if it misbehaves.
(b) Analyse the designed protocol to justify how the protocol satisfies the above
requirements 2 (a) (i) and (ii).
What you should
hand in
Written report on results of all the tasks specified in the above section "The Questions",
in which all descriptions and diagrams must be word-processed.
Guidelines/Length This is an individual coursework, so it must be completed independently.
This coursework should be carried out with reference to relevant textbooks and
published articles. The length of the report should not exceed four A4 sides (i.e.
approximately no more than 2000 words).

Assessment Task Assessment Criteria Raw marks for each
problem component
Clear statement of assumptions made 2
1 Correct protocol design, clear explanation, and
convincing analysis against the specified
requirements
7
2 Correct protocol design, clear explanation, and
convincing analysis against the specified
requirements
13
Report clarity and quality (clear justifications,
protocol efficiency considerations, conciseness and
accuracy, evidence of research)
3


Additional notes:
Submissions: This is the 2nd of 2 assessed submissions for this unit.
Late Submissions: Extensions will only be granted as a result of formally processed Mitigating
Circumstances (http://documents.manchester.ac.uk/DocuInfo.aspx?DocID=427). Marks for late
submissions will be reduced in line with the following university policy
Comp61421 – Assignment 2 Page 3

(http://documents.manchester.ac.uk/display.aspx?DocID=24561).
Support: Support is available in the afternoon of Day 5 of this module teaching period. Additionally,
questions can be posted on the Blackboard forum.

End of Assignment 2