Assignment 2 - Elementary Cryptography

Hands-on Activity
(DUE January 31, 2003 by midnight EST)

Laboratory Exercises on Encryption

This exercise introduces students to the concept of public-key encryption, and to the RSA public-key cryptosystem (used by permission). A student should be able to complete this lab in one hour or less.

PART I: RSA Public-Key Encryption Exercise
Click here for the lab instructions that I distribute for this exercise, as a Microsoft Word (TM) document. (modified Janaury 20, 2003)

The lab makes use of an Excel (TM) workbook with three worksheets:

1. The first worksheet allows each student to choose two small primes and a public key. The worksheet computes the associated secret key.
2. The second worksheet allows the student to encrypt a short message (up to 15 capital letters) using another student's public key.
3. The last worksheet allows the student to decrypt a message from another student using her own public key.

Click here to download the Excel workbook. (Updated November 6, 2001)

Note that a dialogue box appears when you open this workbook. Since this spreadsheet does not contain any macros, you may choose to "Disable Macros" (they were used in an earlier version of the spreadsheet). Before using the spreadsheet it is necessary to add in the Analysis Tookpak and The Analysis ToolPak VBA (from the "Tools" menu). This is explained in the lab instructions.

 

PART II: Key-Sharing Exercise
This exercise introduces students to the concept of key sharing. The instructions motivate this as a means to provide for the recovery of a lost key without revealing the entire key to any one person. I have chosen to use the Chinese Remainder Theorem for this, primarily because it's simple but also because it's mathematically interesting. Most classes are able to complete this exercise in less than one hour.

Click here for the instructions for this lab. Once again, this is a Microsoft Word document.

Once again, this exercise makes use of a Microsoft Excel (TM) workbook with two worksheets. Click here to download this workbook.

1. The first worksheet allows the student to select a three-digit key and to split it into three parts (by taking the remainder modulo three relatively prime integers).
2. The second worksheet allows another student to recover the original key from the three integers and the three remainders. This makes use of the Chinese Remainder Theorem.

This lab is a modification from a lab by Dr. Benham at Montclair University (http://www.csam.montclair.edu/~benham/enclabs/)

 

Discussion Questions

1. (DUE January 29, 2003 by midnight EST) Select one of the following questions to discuss in detail through the threaded discussion.
   • What characteristics would make an encryption absolutely unbreakable? What characteristics would make an encryption impractical to break?
   • List three kinds of data whose lifetime (amount of time for which confidentiality protection is needed( is approximately one day. List three whose lifetime is closer to one year. List three whose lifetime is closer to one century.
   • What are the risks in the U.S. government's selecting a cyrptosystem fr widspread commercial use (both inside and outside the Unisted States)? How could users from outside the United States overcome some or all of thse risks?
   • Discuss when it is appropriate and necessary to use encryption. When is it not?
   • The U.S. Department of Commerce · Bureau of Industry and Security has revised the U.S. Encryption Export Control Policy to allow encryption commodities and software with symmetric key lengths exceeding 64 bits may be exported and reexported. Discuss the advantages and disadvantages of this change in policy.
2. (DUE February 3, 2003 by midnight EST) Select a discussion from one or more of your classmates and respond to their comments.

 

Webliography Assignment
(DUE February 3, 2003 by midnight EST) Submit to the class Webliography 5 annotated links related to cryptography These will be added to our Information Assurance E-library (http://csc.ColumbusState.edu/summers/e-library/security.html) You will receive 1 point if just a new link; 1 point if annotation for one of my links; or 2 points if a new annotated link.