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Answer the following with mathematical expressions (like 26×26) or numbers, calculated using the calculator in Windows’ Accessories (or any other that’s handy).
1.a.i. Suppose an exhaustive search attack were launched on a ciphertext known to have been generated using the traditional Vigenère square and a key exactly five characters long. How many different keys would there be in the universe of possibilities requiring test?
Answer: 265=26x26x26x26x26
1.a.ii. If, instead, the key length were exactly six, how many additional keys would this put into the universe?
Answer: total number of keys for a key of exactly six letters is 266=26x26x26x26x26x26
The number of additional keys is: 266 – 265 = 265 (26 – 1) = 25*265
1.a.iii. If the attacker knew that keys could be no less than four characters long but no more than six, how many different keys would be in the universe?
Answer: We know that for 4 letters there are 264 variants, for 5 letters – 265, for 6 letters – 266 combinations. Total number of possible keys: 264 + 265 + 266 = 264 * (1 + 26 + 26×26) = 264 * 703
1.b. Encrypt the plaintext happiness with the key abba. Do not pad the plaintext with additional characters to use-up trailing characters in the final iteration of the key.
Answer:
h a p p i n e s s
a b b a a b b a a
Result (encoded):
h b r p i o f s s

2. Use the message authentication code program to compute the MAC for the message happiness using abba as the key. Follow the action of the program to confirm that each stage conforms to the process was depicted above. Goal: to understand what the program’s display is showing you.
Answer:
Firstly, the secret key is set

Secondly, the message is set.

After setting key and message, first stage is computed: first block of text is encoded using the key.

The second stage includes encoding cipherblock 2 by the second block as a key, and encoding the result by the appropriate part of the secret key. By such double encoding it is guaranteed that MAC ensures identity (since it depends on all parts of the message) and authenticity (since all parts are encrypted using the secret key, which is supposed to be known by the sender and the recipient only).

Third stage is the same process completed with cipherblock 2 and block 3 of the text.

2.b. Here are several messages and their MACs. Each message authentication code was computed with the key sunshine. Which ones check as authentic?
2.b.i. Message: It is nice to be important but more important to be nice.
Message authentication code: WEH

Message is authentic.

2.b.ii.
Message: Opportunity is missed by most people because it is dressed in overalls and looks like work.
Message authentication code: WLG

Message is authentic/

2.b.iii. Message: It’s not whether you get knocked down, it’s whether you get up.
Message authentication code: UCR

Message is authentic.

2.c. Suppose that Alice sends Bob the message in 2.b.i. and its MAC in an email.
(i) What does Bob do to confirm that message he received is the message that was sent?
Answer: Bob has to calculate the MAC using the same secret key as Alice, and compare the result with the MAC she has sent. If the MACs are equal, it means that the message is the same that was sent by Alice.
(ii) What gives Bob confidence that the message actually came from Alice?
Answer: Bob can be confident that it was exactly Alice sending this message because the secret key is not disclosed to other people (it is supposed that the secret key is known only by the sender and the recipient). Then, since MACs are equal, this means that the message was encoded using the common secret key; thus, authenticity is verified.

2.d. Assume that computing message authentication codes on bit strings is no more difficult than computing message authentication codes on strings of characters, and that it works in the same way.
How could a piece of software work that would detect whether fields in a database had been tampered with (or, alternately, that executable files on your computer had not become infected by a virus)? Be sure to explain our confidence that an attacker could not “cover his tracks”.
Answer: n order to ensure integrity of fields in the database (or executable files) it is necessary to use a certain secret key (which should not be disclosed to other people or available by other means from outside), calculate MACs using this key and store them along with the data itself. Then, in order to check integrity it will be necessary to recalculate MAC using the same secret key and compare it with the stored MAC. If they are different, then the field in the database (or the file) have been changed. This method does not allow the attacker to cover his tracks because he does not know the secret key, and cannot change MACs appropriately.

3.a. Use the hash generator at http://www.whatsmyip.org/hash_generator/ to generate the MD5 hashes for the messages in 2.b.i., 2.b.ii., and 2.b.iii. Copy and paste these onto your answers document.
Answer:
Message: It is nice to be important but more important to be nice.
Md5 hash: e6961239a0fc158e284f72163975ca4c

Message: Opportunity is missed by most people because it is dressed in overalls and looks like work.
Md5 hash: d2d24f4c2e108c42ae5a2be43988572e

Message: It’s not whether you get knocked down, it’s whether you get up.
Md5 hash: ea10b6902c5dd382f2d2f14978371818

3.b. Use the hash generator at http://md5.md to compute the MD5 message digests for the same three messages. Copy and paste these onto your answers document.
Message: It is nice to be important but more important to be nice.
Md5 hash: e6961239a0fc158e284f72163975ca4c

Message: Opportunity is missed by most people because it is dressed in overalls and looks like work.
Md5 hash: d2d24f4c2e108c42ae5a2be43988572e

Message: It’s not whether you get knocked down, it’s whether you get up.
Md5 hash: ea10b6902c5dd382f2d2f14978371818

3.c. Bob could take his message, compute its hash, and then encrypt the hash using a key he shares with Alice. Finally, he’d send Alice both the message and the encrypted hash.
What does Alice do, when she receives this message, to confirm its integrity as well as assure herself that the message came from Bob, and not an imposter?
In this case I will suggest that public-key encryption is used for encryption of hash functions, and that Bob has encrypted hash value using Alice’s public key. Then, in order to check the authenticity and integrity of the message, Alice would need to decrypt hash with her secret key, and then compute md5 hash of the message that she has received. If these two results coincide, then the message is both authentic and is exactly the same as Bob sent it. In case the hashes do not coincide, it is necessary to ask Bob to re-send the message because something happened to it.
3.d. Another approach to using a secret key in conjunction with a hash could be used for constructing a hashed message authentication code. It employs the technique suggested by the MS-CHAP, explained on pages 127-128, and diagrammed on page 142.
Using this approach, what would Bob do to send a message to Alice that requires the security of an integrity and an authenticity check? [Hint: A secret key may be substituted for the password, and a message may be substituted for the challenge.]
What would Alice do, upon receiving the message, to verify its integrity?
In order to ensure the authenticity and integrity of the message, Bob would need to send Alice the message first; then, append the secret key to the message and compute its hash. The resulting hash should be sent to Alice.
Thus, Alice would need to do the same on her machine in order to check authenticity and integrity, i.e. add the secret key to the message, hash the result and compare it with the hash she received from Bob.

 
References
Panko, Raymond R. (2009). Corporate computer and network security. Pearson.
Schneier, Bruce. (2000). Secrets and lies: digital security in a networked world. John Wiley.
Online MD5 Generator. Available from http://md5.md/
Hash generator.