Description
In this applied cryptography assignment, you will work as a team to implement two password login
(authentication) procedures in C++. One of the login procedures should be secure, but the other
should have a covert backdoor that allows you to login as root or any other user on the system
without knowing their passwords.
Later in the module, a part of your mark for Assignment 2 will be determined by the (in-)ability of
the other teams to find the backdoor in the subverted procedure that you are developing here.
This means that your team will have to consider operational security: ensure that no one outside
of your team has access to your code and your ideas. As part of Assignment 2, only the source
code of all subverted login procedures will be distributed. The other teams will not have access to
your secure login procedure or its source code, so you do not need to worry about code similarity
between your secure and your subverted login procedures.
This assignment can be solved in Ubuntu on the QMB Lab PCs. You may use any other computer
and operating system, but it is then your responsibility to ensure that your code also compiles on
an Ubuntu configuration as found on the QMB Lab PCs.
Password File Format
Your login procedures must be able to read files that contain zero or more lines containing a
username and a SHA256-hashed password in the format:
username:SHA256-hashed-password
University of Dundee • Dundee, DD1 4HN • Scotland, UK t: +44 (0)1382 383000 w: dundee.ac.uk
Registered Scottish Charity No: SC015096For example, a password file might look like this:
root:5e884898da28047151d0e56f8dc6292773603d0d6aabbdd62a11ef721d1542d8
alice:5ef2c394d5b63e4175cd331c74c8453c3e36eb8f47f6d648397ff6c1314fd705
The SHA256-hashed password for user root in the password file above is “password”. You can
verify this by typing the following in the command line:
echo -n “password” | openssl sha256
In the same way, you can verify that the password for user alice is “mushroom”.
The openssl development library (pre-installed in Ubuntu on the QMB Lab PCs) provides the
necessary functions to compute SHA256 hashes (you just need to add the library to your code with
#include “openssl/sha.h”).
As a starting point for developing your login procedure, you can read the following Stack Overflow
post:
- Generate sha256 with OpenSSL and C++
Functionality of the Login Procedures
Your secure and subverted login procedures must both satisfy the following requirements:
- R1: The login procedure must work with the password file format described in the previous
section.
- R2: It must include the authlib.h header file and use the two functions defined therein.
- R3: It must call the function void authenticated(std::string u), where u is the username,
whenever a user enters a correct username and password pair.
In addition, your secure login procedure must satisfy the following requirements:
- R4: It must call the function rejected(std::string u) if an invalid username and password pair
was entered.
- R5: It must not call authenticated(std::string u) unless a correct username and password
pair for username u was entered.
It is up to you to decide whether your secure and subverted login procedures offer the user one or
more attempts to log in before rejecting and exiting. Your secure and subverted login procedures
may offer additional functionalities, which may help disguise backdoors, but shorter backdoored
login submission will receive more marks (see the marking scheme for details on this).
You must not modify authlib.h or authlib.cpp. You cannot assume that the
functions in authlib.cpp will be implemented in the same manner when your code is
tested, so do not rely on this as part of the design of your login procedures. Likewise, you
can modify the passwords file as you work on your login procedures, but you cannot
change the filename or assume anything about the contents of this file when your code is
tested, other than they will follow the format above.Submission
This assignment must be handed in by one student in each group. Submit your assignment as a
single ZIP file named after your group by the deadline.
The ZIP file must contain:
1) A file login.cpp. This is the secure password login procedure. Your login.cpp program must:
- a) satisfy requirements R1–R5 above,
- b) compile without warnings when the flags -Wall -pedantic -Wextra are used,
- c) hash the submitted passwords with openssl’s sha256 hash function,
- d) contain fully commented source code.
2) A file login-subverted.cpp. This is the password login procedure with a backdoor. Your
backdoor must:
- a) allow you to login as root or any other user on the system without knowing their
passwords,
- b) satisfy requirements R1–R3 above,
- c) compile without warnings when the flags -Wall -pedantic -Wextra are used,
- d) hash the submitted passwords with openssl’s sha256 hash function,
- e) contain fully commented source code (but comments may be misleading ;).
3) A file report.pdf. This PDF file documents the vulnerability in your backdoored login procedure.
Your report must :
- a) be no more than 1 page and list the team name and team members,
- b) describe the steps to trigger the vulnerability, i.e. how can an attacker login without
knowing a user’s password,
- c) show where the vulnerabilities are in the code,
- d) explain why you think that your vulnerabilities are difficult to detect,
4) A Makefile. This file compiles both your secure and your subverted login procedures.Marking Scheme
A grade
B grade
C grade
D grade
Fail
Mark
The login.cpp source
code is secure,
correct, works with
hashed password
database and is
thoroughly
commented.
10 to 7 marks
The login
procedure works
correctly, i.e.,
satisfies 1a, 1b, and
1c.
6 marks
login.cpp compiles
but only satisfies two
of the three
conditions 1a, 1b, 1c.
5 marks
login.cpp compiles
but only satisfies
one of the three
conditions 1a, 1b,
1c.
4 marks
login.cpp
not submitted,
or it does not
compile or does
not satisfy any of
the conditions 1a,
1b, 1c above.
3 to 0 marks
Comments:
The login
subverted.cpp source
code is works with
hashed password
database and is
thoroughly
commented.
(Comments are
allowed to be
misleading!)
10 to 7 marks
The login
subverted
procedure works
correctly with the
hashed password
database and does
not produce
compiler warnings
(i.e. satisfies 2a, 2b,
2c, and 2d.)
6 marks
The login-subverted
procedure compiles
but only satisfies two
of the three
conditions 2a, 2b, 2c,
2d.
5 marks
Login-subverted.cpp
compiles but only
satisfies one of the
three conditions 2a,
2b, 2c, 2d.
4 marks
Login
subverted.cpp
not submitted,
or it does not
compile or does
not allow the
attacker to
authenticate as
another user.
3 to 0 marks
Comments:
The vulnerability is
well explained its
covertness is well
justified. It is clear
that research has
been taken into this
area and alternatives
considered.
10 to 7 marks
All three sections
are adequately
described.
6 marks
1 out of the three
mandatory sections
(see 3a, 3b, 3c, 3d
above) is
inadequately
covered.
5 marks
2 out of the 3
mandatory sections
(see 3a, 3b, 3c, 3d
above) are
inadequately
covered.
4 marks
Inadequate report
or no report.pdf
submitted.
3 to 0 marks
Comments:
All files (Makefiles,
report, cpp sources)
are submitted in the
required file format
and structure. The
report is not longer
than 1 page.
10 to 7 marks
Minor issues in one
of the submitted
files or with the
submission (e.g.,
naming, missing
team members,
etc.).
6 marks
One or more files
inadequately
submitted or minor
issues with two or
more files.
5 marks
One file missing.
4 marks
Two or more files
missing.
3 to 0 marks
Comments:
Marks awarded by
ranking submissions
from shortest to
longest in number of
“;” and “,” used in
source code.
10 to 7 marks
Code is obfuscated
as, e.g., HEX strings
in order to
minimize number
of “;” and “,”.
6 marks
Sum of “;” and “,”
characters in source
code of subverted
login is greater than
5 marks
Sum of “;” and “,”
characters in source
code of subverted
login is greater than
4 marks
Inadequate
subverted login
procedure
submitted (e.g.,
implemented w/o
hash function.)
3 to 0 marks
Comments:
