Homework 3: Asymmetric Cryptography


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BBM 465: Information Security Laboratory
Information Security Lab.
Homework 3
Subject: Asymmetric Cryptography, Hashing and Digital Signatures
Language: Java
In this project, your aim will be the development of a licensing framework by utilizing the
methods of asymmetric cryptography, MD5, and digital signatures. Although the methods
that take place in symmetric cryptography are strong, they have some shortcomings such as
key publication for both receiver and sender. This could be a hard procedure if these two
counterparts live in different cities or they are geographically far away without having secure
communication. Moreover, symmetric cryptography is prone to ”man in the middle” attacks
which serve a potential vulnerability of changing the content of the message on the way to
the receiver.
On the other hand, asymmetric cryptography presents a different perspective in order to solve
these drawbacks. First, it offers to use a publicly available public key for encryption of the
message which will be only decrypted by using the private key. Note that, these two keys
form a key pair and, along with the public key, only the receiver holds the private key for
tasks of (1) decryption and (2) digital signature creation. The receiver is able to decrypt the
encrypted message and it can create a digital signature of content for further authentication
and verification purposes.
Implementation Details
As stated, in this experiment, you are expected to develop a licensing framework by utilizing
the methods of asymmetric cryptography, MD5, and digital signatures. The following lines
will state the requirements of your assignment.
ˆ Let us assume that you are supposed to create a licensing module for a highly cost
software project. According to the requirements, you must design a module that will be
located in the software itself (the Client) and a server system (the license manager ). In
practice, this scheme is implemented via a network system (e.g. client and web server).
Further, clients do communicate with a server system through web services. In this
assignment, instead, you will do a real-world simulation of this scheme by creating 2
main classes in Java namely ”Client” and ”LicenseManager”.
ˆ According to the requirements, the client module must first check the existence of the
”license.txt” in the application folder. The license.txt file involves a digital signature
signed by the license manager. If the license.txt exists, then your client module must
verify it by using the public key which is given to you at startup. Note that, the client
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BBM 465: Information Security Laboratory
holds only the public key (”public.key” as the file name) whereas the license manager
holds both the ”private.key” and ”public.key” locally.
ˆ If the ”license.txt” does not exist, then this indicates that the software has never been
licensed before. In order to license the application your first duty is to collect the
following identities:
1. the username (string)
2. the serial number (string having format of ####-####-####)
3. MAC address of the Ethernet device of the host system (string)
4. Disk serial number (string)
5. Motherboard serial number (string)
For username and serial number, you can use static text. In other words, your experiment will not ask the user to input these data by hand. Here, they are only given
to provide a perspective. For the other device-specific data, you can use third-party
codes. Nonetheless, you can not use JNI or JNA-based native services or packages.
More precisely, you should use core Java library-based solutions for this step. Once you
collect these data, you need to concatenate them by using $ as a separator character. An
example is given below for this plain text (”user-serialid-hw specific info” tuple) content.
Example of a ”user-serialid-hw specific info” tuple:
ˆ After forming the hardware and user-specific unique plain text, you must encrypt this
content via the public key (the RSA algorithm will be used as the basis). Following this
encryption, you need to obtain an encrypted version of this content.
ˆ Upon having encrypted, the encoded message must then be sent to license manager for
further processes. Once the server (the license manager) has received the encrypted
package it must decrypt the content in order to reveal the user and serial number info
along with hardware-specific ids.
ˆ Licence Manager can then do several operations such as storing these fields in a database.
However, for simplicity, we skip these stages. Rather the license manager must create
a hash of the revealed plain text content. For this purpose, you must use MD5 (i.e.
Message Digest – an irreversible message digest mechanism) to produce the hash. An
example hash content has been presented below:
Example of a MD5 hash:
ˆ Following the hash production, you must sign the hash content via the private key. For
this procedure, you can utilize the ”Signature” class (provided by the Java Security
API) through the ”SHA256WithRSA” scheme. This will create the digital signature of
the hash to be delivered to the Client.
ˆ The response (i.e. digital signature) of the license manager must be verified in the
Client module with the hash provided by Client. To do so, you must do the same
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BBM 465: Information Security Laboratory
procedure (i.e. hashing the plain user-serial-hw specific data) on the client side as well.
The verification scheme works only in this way. It is important to keep in mind that,
the verification must be carried out by using the public key.
ˆ If the verification of the digital signature is signed by the Licence Manager, then you
should prompt the user for the success of the operation and store the digital signature in
the ”license.txt”. With this step, the whole licensing stage finishes. You must prompt
the user with the message of
”Succeed. The license file content is secured and signed by the server.”
ˆ As you may question, will this whole process be executed every time? The simple
answer is NO. Instead, the only thing you must do is to check the existence of the
”license.txt” in the project folder at start-up and verify its content (i.e. build the plain
”user-serialid-hw specific info” tuple at the client side and hash it via MD5) along with
the digital signature. If the result of the verification is True then you can inform the
user by prompting
”Succeed. The license is correct.”
If the content of the ”license.txt” is destructed by an attacker, the verification fails.
In this case, you should warn the user by prompting
”The license file has been broken!!”
and re-execute the licensing process again to obtain a valid digital signature. The
output of a working instance of the desired system has been depicted in the figure below:
Figure 1: The output of a working instance in case of licensing process
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BBM 465: Information Security Laboratory
Important Notes
In this chapter, for the sake of clarification, some important notes about your implementation
have been described.
ˆ You will be provided with the public.key and private.key via the Piazza system.
ˆ Your aim will be to design a project including these functionalities.
ˆ Your code environment will be limited to 2 files named ”” and ””. Here the ”” will be the main ”Client” holding the public key
owned by the ”License Manager”. On the other hand, ”Licence Manager” can access
both of these files.
ˆ For simplicity, you can store the user name and serial id in a file (i.e. ”user serial.txt” for
retrieving them quickly. Also, you can store the username and serial id as hard-coded
in your Client class.
ˆ Figure 1 exemplifies the system messages during the events. As you can see, the license
manager known as server side events is listed as ”Server –”. Similarly, the Client side
events have been shown as ”Client – ”. You should pay attention to this format and
apply the same scheme for your own implementation.
ˆ You can only use Java’s own byte-to-string and string-to-byte helper functions. In other
words, you are prohibited to employ third-party JAR files and helper classes.
ˆ You must submit a detailed report to describe what you have done.
1. In Java, you must use standard crypto API.
2. You should prepare a report involving your approach and details of the implementation
you have coded. You must write down the names and ids of your teammates in the
report in order to be evaluated correctly.
3. You must submit the homework in groups of two.
4. You should prepare a report that describes your approach to the problem with the
details of your implementation. You must write down all group members’ names and
ids. Reports will be graded too.
5. You can ask your questions about the homework via Piazza. (
6. T.A. as himself has the right to partially change this document. However, the modifications will be announced in the Piazza system. In case, it is your obligation to check
the Piazza course page periodically.
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BBM 465: Information Security Laboratory
7. You will submit your work via the submission system.
The submission format is given below:
→ <group>
→ src /
→ src /
→ src /user serial.txt [Optional]
→ report.pdf



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