CMSC 414

                               	Fall 2002


Homework # 2                                    Due October 3, 2002

Note Change:

The written exercises may be handed in prior to the beginning of
class.  The programming portion still must be electronically
SUBMITted by 12:00 Noon on the above date.  Directions use of the 
SUBMIT program are still on the class webpage:
 
	http://www.cs.umd.edu/class/fall2002/cmsc414/submit/submit.txt




			Written Exercises:


1. (Problem #19 from page 290) Suppose Alice and Bob have RSA public keys
in a file on a server. They communicate regularly using authenticated,
confidential messages. Eve wants to read the messages but is unable to
crack the RSA private keys of Alice and Bob. However, she is able to break
into the server and alter the file containing Alice's and Bob's public keys.

a. How should Eve alter that file so that she can read confidential messages
sent between Alice and Bob, and forge messages from either?

b. How might Alice and/or Bob detect Eve's subversion of the public keys?


2. (Problem # 8 from page 323) Consider an RSA digital signature 
scheme (see section 10.6.2.1).  Alice tricks Bob into signing messages 
M1 and M2 such that M = M1 * M2 mod Nbob.  Show how Alice can forge Bob's
signature on M.

3. (Problem # 9 from page 323) Consider the example on page 317.  Show
that if Alice first signs the message, and then encrypts the result
Bob will be unable to successfully complete the attack he used when she
enciphered first and then signed.


4. Discuss the use of three different ciphers discussed in class and
in the textbook.  Consider a) a single substitution cipher, b) two 
substitution ciphers applied one after the other, and c) a cipher which 
is the combination of a substitution cipher followed by a transposition
cipher.  Specifically, discuss the in-class Vigenere example (still at:

http://www.cs.umd.edu/class/fall2002/cmsc414/Examples/vigenere_ex1.txt

which was encrypted with the key "BISHOP" as an example of a), the
same cipher followed by a second Vigenere encryption using the key
"UCDAVIS" as an example of b), and the original example followed by 
the rail fence transposition cipher as an example of c).  Use words like
"confusion" and "diffusion" in your analysis of the effectiveness of
each of these three ciphers.










		Programming Exercises:



You must use Java, JDK 1.3.1 . Submit a tar file including the
source code, the executable version, and all relevant documentation (as
in the previous assignment).  See the web page for format and usage 
specifications.

If the problems below seem underspecified, make a reasonable assumption
and proceed.  State your assumption in the documentation, and attempt
to justify it.  For example, although obviously insufficient in a real 
world application, assuming that the following programs will be running
on the same machine (e.g. "localhost" can be used as the internet address)
is sufficient to demonstrate proof of concept.  On the other hand, hard-
coding socket numbers is just a rookie error that we do not expect to
see in senior level CS classes.  To be polite, and not use up all the
popular sockets the night before the program is due, you may consider
using sockets that are some function of the last 3 digits of  your
class account (1XXX, 500 + XXX, etc).


1.      Write two java programs to implement Diffie-Hellman key exchange,
        using sockets.  After the exchange, each program should output
        the derived secret.  Call your programs Alice.java and Bob.java

2.	Write a third program, Eve.java, to perform a "man in the 
	middle" attack.  Each program should output the derived
	secrets, and what they _think_ the secret represents.  You
	may assume that Alice and Bob have no out of band method of
	confirming which socket(s) the other party will use.

3.	Augment your above programs, and name the new programs  A1.java, 
	B1.java, and E1,java.  The new programs should take messages 
	from standard input and encrypt them with the shared (or assumed
	shared) secret.  The sending program should echo the cipher-
	text before sending the message, and the receiving program
	should first output the ciphertext, then decrypt and output
	the corresponding plaintext.  If E1.java is not running, A1 and
	B1 should do this as advertised.  If E1 is running, she will
	carry on with her MITM attack, outputting both the ciphertext
	and the plaintext for all encryptions and decryptions she performs.
	For this assignment, E1 need only read the messages, she does not
	need to have the capability to change a message A1 send to B1
	or to forge messages from either.  Note that I will test this both
	ways, with and without the MITM program running.  Obviously, I
	will restart all programs when changing modes.


	You may encrypt with DES or RC4 (see handout).  The newer versions
	of Java provide a lot of cryptographic support, if you are not
	familiar with the most recent versions you should familiarize
	yourself before re-inventing the wheel.  OIT initially promised
	me that you would have JDK Version 1.4, which makes the 3rd
	problem trivial.  It seems like you have 1.3.1 (thus the option
	to use RC4, which is easy to code from scratch).  I will follow
	up with them about getting you JDK 1.4, but will not make any
	promises!!