Hands-on Lab 6 - Encryption
Hands-on Activity
(DUE March 23, 2006 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)
- Click here to download the Excel workbook. (Updated November 6, 2001)
- The lab makes use of an Excel (TM) workbook with three worksheets:
- The first worksheet allows each student to choose two small primes and a public key. The worksheet computes the associated secret key.
- The second worksheet allows the student to encrypt a short message (up to 15 capital letters) using another student's public key.
- The last worksheet allows the student to decrypt a message from another student using her own public key.
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.
- 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).
- 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/)
Post-assignment: Answer the following questions -
- 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.
- Discuss when it is appropriate and necessary to use encryption. When is it not?