ECE 8843 Computer Network Security (2S, 1D) 3-0-3

ECE8843 Homework Assignment 1 (HW-1) (v.1.0, 8/28/04)

Due before 8 am Friday Sept. 3. Video students add 14 days.

Instructions. Copy this document from the Web Page, or save it from your email program, as a "text" file. Edit it in a word processor to add the answers into the square brackets after each question. Save it as a "text" file, and email it back to me: (john.copeland@ece.gatech.edu).

Email the completed document as the body of a message (not as an attached document) with the Subject exactly as "HW-1" (4 characters). Please send questions or comments in a separate message, with a different Subject (e.g., "Question on HW-0"). Mail with the correct subject will be automatically filtered into the homework grading program. Including quotes or extra spaces may prevent your homework submission from being automatically graded.

I will not accept paper returns. If you can not complete the assignment on time, tell me why and turn it in as soon as possible for partial credit. Remember, home work grades count as 10% of the final grade.

Your return will be graded by a computer program that looks for your answers between square brackets. Please do not add or delete square brackets (or the ***???*** flags). The format and units of answers should be those indicated in the problem (e.g., a letter, a group of letters, or a number). Each question counts equally. Each answer within a question counts equally, but the value depends on the number of answers within the question. All letters after a number are ignored so do not use letter multipliers (e.g., M, m, k, etc.).

If the answer is a percentage, the format may be 0.52 or 52% (52 is assumed to mean 5200%). You must include the % or use a decimal fraction.

We are learning in this course how computers talk securely to each other. At present there must always be a set of rules (protocol or formatting rules) that govern the data sent so that a protocol layer or application can understand the messages it receives. In a protocol PDU every bit has to be correct and in exactly the right order. The homework submission protocol is far more "free form", but it is still far from accepting as wide a range of formatting as a human being.

See http://www.csc.gatech.edu/copeland/jac/8843/hw_tips.html", Tips on Submitting HW for more information.


--------- (Do not delete anything below this line) ------------------
***START_HW***

[     ]-NAME   Enter your name (form: last, first )

[     ]-PRISM  Enter your Prism or "mail" account code (form: "gtXNNNA" where "N" is a number, "A" is a 
letter, and X is either. Do not include "@prism.gatech.edu").

[     ]-EMAIL  Enter the email address ("account@server") where you would like to receive your graded 
homework and other class information (including quiz grades).  If on Prism or "mail.gatech.edu", leave blank.
         
#1. Break the following ciphertext:

"jvyylj_6huz2lyz6hyl6_vv6lhz46_v6mpuk62p_o6zptwsl6jvklz"

This is a Caesar-type substitution code using the following set of 37 characters (for plaintext and 
ciphertext):

"0123456789abcdefghijklmnopqrstuvwxyz_" (Hint: cut and paste the trial substitution sequence underneath)

There is no quote (") or space ( ) characters in this alphabet.  It uses underscore (_) to separate 
English words.  A character probability table derived from similar plaintext messages is as follows:

_  16.7%, e 11.1%, o  9.3%, s  9.3%, r  7.4%, t  7.4%, a  5.6%,
c   5.6%, i  5.6%, d  3.7%, n  3.7%, w  3.7%, f  1.9%, h  1.9%,
l   1.9%, m  1.9%, p  1.9%, y  1.9%

[                 ] - type in the first 14 characters of the plaintext.

There is a Excel spreadsheet that may help, http://www.csc.gatech.edu/copeland/jac/8843/tools/subs_code.xls

#2. To be effective, an encryption algorithm must require an uneconomical amount of resources to break.
Assuming no systematic shortcut is known for breaking a code, how many decryption operations, on the average, 
would be needed to break the following by a "plaintext recognition" exhaustive key search:

[     ] - A Caesar-type substitution like the above (37-characters).

[     ] - A general substitution code (37-characters) (Hint: Sterling's approximation, 
          n! =sqrt(6.28n)*(n/e)^n  e=2.719). (Answer in scientific notation, like 1.234e5).

[     ] - DES with a 56-bit key (express as a power of 2, like 2^19).

[     ] - IDEA with a 128-bit key (express as a power of 2, like 2^19).

#3. Trudi intercepts DES CBC (Cipher Block Chaining) encrypted message with 64-bit blocks c1, c2, c3, ... .
She wants to change the bit 8 of the block 6 of the decrypted message  m1, m2, m3, ... .

[   ] - Which block does she tamper with (the number)?

[   ] - Which bit does she change (the number)?

[   ] - Which decrypted message block is garbled (the number, zero if none)?

#4. Trudi intercepts DES OFB (Output Feedback Mode) encrypted message with 64-bit blocks c1, c2, c3, ... .
She wants to change the bit 8 of the block 6 of the decrypted message  m1, m2, m3, ... .

[   ] - Which block does she tamper with (the number)?

[   ] - Which bit does she change (the number)?

[   ] - Which decrypted message block is garbled (the number, zero if none)?

  ***END_HW*** (do not delete preceding flag)  (1.0 - web)