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1. Substantiate or refute the following statement: The cryptographic basis of the Enigma machine is transposition.

 Answer : The Enigma machines are the progression of electro-mechanical rotor cipher machines . In other way the transposition cipher is a technique of encryption by which the positions apprehended by units of plaintext (Han, J.,1999 pp.47-55).

2. As time goes on, the size (i.e., length) of cryptographic keys tends to increase. Why is this so?

Answer : As time goes on, the size (i.e., length) of cryptographic keys tends to increases because the key length (i.e.,size) is the number of bits in a key used by means of a cryptographic based algorithm (for example a cipher). The Key size/length defines the upper-bound on an algorithm's security.

3. Using the English alphabet (i.e., mod 26 arithmetic) let plaintext = {p1, p2,... , pn} and corresponding cipher text = {c1, c2,... , cn}. Suppose the encryption function is ci = pi + 10 (mod 26). You receive the cipher text message

EWEM SC K QBOKD CMRYYV. What are the decryption function and the recovered plaintext? What type of cipher is this? What are some weaknesses of this cipher?

 Answer : The sequence of ciphertextblocks c1,c2,...,cn some ciphertext block cj is errorness, 1 ≤ j<n.

4. Briefly compare and contrast stream and block ciphers.

 On the contrary, in case of contrast stream cipher technology includes encryption as well as decryption of a text byte at a time. While the block cipher does not use XOR to do this. The block ciphers uses the exact same key to encrypt every block while the stream cipher doesnot uses a same key for each byte.

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5. Consider the following plaintext message: WATER BOILS AT 212 DEGREES FAHRENHEIT

a. If this message is sent unencrypted and successfully received, what is its entropy?
Answer : zero
b. If this message is encrypted with 3DES (using an optimal set of keys) what is the encrypted message's entropy?
Answer : zero

c. If this message is encrypted with AES (using the most secure key length), what is the encrypted message's entropy?
Answer zero

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6. Name some risks that cannot be mitigated with cryptographic techniques.

Answer : Cloud computing risks that cannot be mitigated with cryptographic techniques.

7. An organization has 8 members. Using a symmetric cipher, it is desired that each member of the organization be able to communicate securely with any other member, without any other member being able to decrypt their messages. How many unique keys are required?

Answer : 2 unique keys are required.

8. How many bits would need to be added to DES's key in order to double its average brute-force cracking time?

Answer : The 64 bits would required to be additional added to DES's kind of key in order to get double its average brute-force cracking time .

7. In terms of entropy, what is the goal of cryptography?

Answer : Entropy is essentially randomness or unexpectedness of something. In case of Cryptography, this randomization should be supplied in the plaintext message to remove the structure of the plaintext message. In other way entropy must be supplied by the cipher for injection. The other goal is to used for performance and security . (Minai, A.A. and Pandian, T.D.,1998 pp.621-628)

8. In order to minimize bandwidth transmission requirements, does it make more sense to compress a message before or after it is encrypted?

Answer : Encrypting first and then compressing does not work. First compressing can spread data about plain material through a cipertext length, because Poncho is specified in the form of comment.

Specifically, pressure allows an attacker to control parts of the message that is to highlight things about other secrets, similar to behavior due to web activity due to web activity.

9. Which has the larger number of possible keys: a simple (monoalphabetic) substitution cipher for the 26-letter English alphabet, or DES? Why is DES considered to be the better cipher? Show the reasoning behind your answer.

Answer : In case of substitution cipher using 26-letter English alphabet ,we will have
? 26 * 25 * 24 * 23 * ..................... *3 * 2 * 1 = 26! Number of keys.
Then 26! is = 400000000000000000000000000 (approximately)
In DES the key length is 64 bit using these 64 bits, we can have
? 264 possible keys = 18446744073709551616.

Hence , 26! is > 264.

The substitution cipher has larger number of possible keys.

DES is considered to be the better cipher because in DES, the encryption will takes place in 16 rounds and in each round we will use a different 56 bit round key taken from 64 bit actual key.

10. A particular cipher is implemented by combining the ASCII representation of plaintext characters with pseudorandom bytes (eight-bit binary strings of 1s and 0s) using the XOR function. In the process of encrypting a message, a character in the plaintext, a capital D, is XORed with the pseudorandom byte 10010101.
a. What is the cipher text (in binary form) generated by the encryption of the character D? (Please show your work.)

Answer a.10010101.

b. How is the plaintext for this encrypted D recovered? (Please show your work.)

Answer b) The Caesar code decryption replaces the letter an additional with an inverse alphabet shift : a previous letter in the alphabet.


11. A hacker with access to a super computer using brute force is able to crack a 3DES message (using its most secure keying option) in 1 hour. How many hours will it take her to crack an AES message (that uses its most secure keying option)?

Answer : Total hours 1 x 10 x 100 x 1000 hours are required to crack an AES message . Whatsoever breakthrough might crack128-bit will probably.

12. Substantiate or refute the following: As the cipher text sample size becomes smaller, cryptanalysis becomes easier.

Answer : Caesar cipher is almost certainly the easiest to break all the ciphers. While the shift should have a number between 1 and 25 (i.e , 0 or 26 will result in an unchanged plaintext) We can just do every effort for each possibility and see which results in a piece of understandable text is.

13. The following cipher text is a monoalphabetic cipher:


Decrypt this message, and briefly describe your cryptanalysis methodology. In particular, list features of the cipher text that hindered or helped your decryption process.

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After placing the first two most commonly used letters, We need to use other "personality traits" of the letters to decrypt this important message. It may include looking at the common pairs of letters (or digraphs), there are no two letter words; There are only few letters that appear as a couple (SS, EE, TT, GG, OO and FF are the most common). Now we frame a possible word of the given set of letters, and if tested letters make arguments in other words then do the test.

14. Non-repudiation and authentication are terms sometimes used interchangeably. Are they different? If so, how do they differ?

Answer : Non-repudiation is the assurance that a person can not deny anything. Generally, the non-counter-reference refers to the ability to ensure that a party can not refuse to send authenticity or signature of a sign on a document for contract or communication that they had generated (Reed, C., 1989 pp.649-660).

15. Ciphers are broadly categorized as using transposition, substitution or trap-door functions. Please provide an example of each type of cipher.

Answer : A trap-door function is a function that is easy to calculate in one direction, yet it is difficult to calculate in the opposite direction without specific information (to find its inverse), which is called as "trapdoor". Trap-door functions are mostly used in cryptography.

16. Why wasn't John von Neumann a proponent for using pseudorandom number generators for use in stream ciphers?

Answer : The problem with the approach as suggested by John von Neumann is that the pattern repeat itself after certain iterations and it is easy to predict the encrypted text.

Methoff is called as the middle class number, where we take any number and then we square that number first and then we end the numbers as a "random number" between the resulting number, after Use that number as the seed for the next recurrence.

To take an example we can consider 1111 it will give us 1234321 ot 0123431. Now the middle number will be 2343 and we will repeat this. In the next square middle number will be 4896.

Also the computer present at that point was not able to handle big calculations and therefore it was not good to use pseudorandom number generators in steam ciphers.

17. How many different possible substitutions are there for a simple Caesar cipher?

Answer : There are total 25 keys possible substitutions for a simple Caesar cipher.

18. What is a known weakness of Enigma ciphers.

Answer : The security of Enigma ciphers did contain fundamental weaknesses that proved helpful to cryptanalysts. A letter could not at all be encrypted to itself, a significance of the reflector.

19. A cipher that uses a one-time pad is considered unbreakable. If this is the case, why aren't one-time pads more widely used?

Answer : A cipher that uses a One-time pad (OTP) is considered as unbreakable, also known as Vernam-cipher or the perfect cipher, it sometime used in crypto algorithm .

20. Other than brute-force, what other cryptanalysis tools are commonly used to crack encrypted messages.

Answer : A web based cryptanalysis tools are commonly used to crack encrypted messages.

21. What CAIN cryptographic services do each of the following provide: (1 point each)
a. Symmetric ciphers
b. Asymmetric ciphers
c. Hashing functions
d. Steganography

Answer : The CAIN cryptographic services provides Symmetric ciphers. (Barresi, A., Razavi, K., Payer, M. and Gross, T.R.,2015).

22. The development of AES was done in a completely open manner, including detailed information regarding the algorithm used for the cipher. Was this a good idea? Why or why not?

Answer : The development of AES was that the secret to ultimately ensure the secrecy is developing an algorithm in an open manner. The strength of the algorithm is to improves under the intense public scrutiny of the global cryptographic community

23. Fill in the blank: A ____requires that the cipher alphabet changes throughout the encryption process.
a. monoalphabetic substitution cipher
a. polyalphabetic substitution cipher
b. quantum cipher
c. alphanumeric shift cipher

Answer : A polyalphabetic substitution cipher which requires that the cipher alphabet changes throughout the complete encryption route.
Reason and explanation for this polyalphabetic substitution cipher is the cipher alphabet varies all through the encryption.

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