Practice what you preach...

Today morning I received a mail (via listserv) from Dr Du. He wanted to fix the time slot for a make up class as he was going to attend a conference during the regular class hours. Here is the mail he sent us:

I have proposed the following 3 time slots. Unfortunately, so far, there
is an exactly one conflict for each of them. Since there is a 3-way tie, I
need to pick one randomly.

(1) 11:00 - 12:20
(2) 13:00 - 14:20
(3) 14:00 - 15:20

What makes this thing interesting is that I also have my own preference.
How can I pick one while convincing you that this is picked randomly, not
based on my preference?

To demonstrate an important application of one-way hash function, I would
like to propose a protocol to generate a number S (S will be 1, 2, or 3).
The S-th time slot described above will be picked. There is an important
requirement: S must be random; neither I nor you can control the value of
S. Let's conduct this protocol through emails.

(1) You pick a string M: Whoever reads this email first, please send me
any string you like (please send to this mailing list, so we all know the
string). Let's call this string M.

(2) I also pick a string K: I cannot tell you my string at this moment
(think about why). However, I need to tell you something, so I cannot
change my string after seeing your string M (think about why I want to
change my string). Therefore, I am sending you the md5 hash value of my
string. Here it is: 1c4bfe13bf159f26ddd23926b7376b14

(3) After receiving your string M, I will generate an HMAC_MD5 value on M,
using my K as the key. We will get a 16-byte number.

(4) Let D represent the last byte (the least significant byte) of the
HMAC_MD5 value. I will compute S = (D % 3) + 1. This result S will be our
final decision.

Question: is this protocol fair? Can anybody (you or me) control the
outcome? Please think about this, and we will talk about it in our class.

Now, I am waiting for your string. If you are the first one to read this
email, please send us a string of any length. I assume there is at least
one hard-working student in this class. If nobody sends a string by 8:00am
tomorrow. I will use M = "AAABBBCCCDDD", and generate a decision.


- Kevin

(published with permission)

Is there a better was to teach Internet security?? ... practice what you preach