CRYPTOGRAPHY AND NETWORK SECURITY, 2 Mark UnitIII

1. What are the Key Features of Asymmetric Ciphers?

• AES is a block cipher intended to replace DES for commercial applications. It uses a 128-bit block size and a key size of 128, 192, or 256 bits.
• AES does not use a Feistel structure. Instead, each full round consists of four separate functions: byte substitution, permutation, arithmetic operations over a finite field, and XOR with a key.

2. What common mathematical constants are used in RC5?

• W :Word size in bits. RC5 encrypts 2-word blocks. 16,32,64
• r: Number of rounds. 0,1,….,255 B Number of 8-bit bytes (octets) in the secret key K. 0,1,….,255

3. What primitive operations are used in RC5?

            RC5 uses three primitive operations (and their inverse):

             • Addition: Addition of words, denoted by +, is performed modulo 2w. The inverse operation, denoted by -, is subtraction modulo 2w.

            • Bitwise exclusive-OR: This operation is denoted by “Å”.

            • Left circular rotation: The cyclic rotation of word x left by y bits is denoted by x<<>>y.

4. List important design considerations for a stream cipher.

            The encryption sequence should have a large period. The keystream should approximate the properties of a true random number stream as close as possible. The output of the pseudorandom number generator is conditioned on the value of the input key.

5. Why is it not desirable to reuse a stream cipher key?

             If two plaintexts are encrypted with the same key using a stream cipher then cryptanalysis is often quite simple. If the two ciphertext streams are XORed together the result is the OR of the original plaintexts. So it is not desirable to reuse a stream cipher key.

6. What primitive operation is used in RC4?

             The primitive operation used in RC4 is bit wise Exclusive-OR (XOR) operation.

  7. List potential locations for confidentiality attacks.

            • LANs in the same building that are interconnected with bridges and routers.

            • The wiring closet itself is vulnerable.

            • Twisted pair and coaxial cable can be attacked using either invasive taps or inductive devices that monitor electromagnetic emanation.

            • In addition to the potential vulnerability of the various communications links, the various processors along the path are themselves subject to attack.

8. What is the difference between link and end-to-end encryption?

• Link Encryption End-to-end Encryption
• Applied by sending host Applied by sending process
• Transparent to user user applies encryption
• Host maintains encryption facility User must determine algorithm
• One facility for all users User selects encryption scheme
• Can be done in hardware Software implementation
• All or no messages encrypted User chooses to encrypt, or not, for each message

9. What types of information might be derived from a traffic analysis attack?

             The following types of information can be derived from traffic analysis attack:

• Identities of partners
• How frequently the partners are communicating
• Message pattern, message length, or quantity of messages that suggest important information is being exchanged
• The events that correlate with special conversations between particular partners.

10. What is traffic padding and what is its purpose?

            Traffic padding produces cipher text output continuously, even in the absence of plaintext. A continuous random data stream is generated. When plaintext is available, it is encrypted and transmitted. When input plaintext is not present, random data are encrypted and transmitted.

11. List ways in which secret keys can be distributed to two communicating parties.

            • A can select a key and physically deliver it to B.

            • A third party can select the key and physically deliver it o A and B

            • If A and B have previously and recently used a key, one party can transmit the new key to the other, encrypted using the old key

            • If A and B each has an encrypted connection to a third party C, C can deliver a key on the encrypted links to A and B

 12. What is the difference between a session key and a master key?

           

Session Key	Master Key
A temporary encryption key used between two principals.	A long-lasting key that is used between a key distribution center and a principal for the purpose of encoding the transmission of session keys. Typically, the master keys are distributed by noncryptographic means. Also referred to as a key-encrypting key.

13. What is nonce?

            Consider A issues a request to the KDC for a session key to protect a logical connection to B. The message includes the identity of A and B and a unique identifier, N1, for this transaction, which we refer to as nonce. The nonce may be a timestamp, a counter, or a random number.

14. What is key distribution center?

            A key distribution center is responsible for distributing keys to pairs of users such as hosts, processes, and applications. Each user must share a unique key with the key distribution center for purposes of key distribution.

15. What is the difference between statistical randomness and unpredictability?

            In applications such as reciprocal authentication and session key generation the requirement is not so much that the sequence of numbers be statistically random but that the successive numbers of the sequence are unpredictable. With true random sequences each number is statistically independent of other numbers in the sequence and therefore unpredictable.

 16. What is the difference between Rijndael and AES?

             AES was developed by NIST .AES is a symmetric block cipher that is intended to replace DES.NIST selected rijndael as the proposed AES algorithm. The two researchers who developed and submitted Rijndael for the AES are the both cryptographers from Belgium.

17. Why is the middle portion of 3DES a decryption rather than an encryption?

            Decryption requires that the keys be applied in reverse order: P=Dk1[Ek1[P]] This results in a dramatic increase in cryptographic strength.The use of DES results in a mapping that is not equivalent to a single DES encryption.

18. What is the difference between the AES decryption algorithm and the equivalent inverse cipher?

            In AES decryption, we use inverse shift rows inverse sub bytes, add round key, inverse mix columns. But in equivalent inverse cipher, we interchange inverse shift rows and inverse sub bytes.

19. Define concepts of Random Number Generation.

            Random numbers play an important role in the use of encryption for various network security applications. In this section, we provide a brief overview of the use of random numbers in network security and then look at some approaches to generating random numbers.

20. Define Abelian Groups

            An abelian group G, sometimes denoted by {G, • }, is a set of elements with a binary operation, denoted by •, that associates to each ordered pair (a, b) of elements in G an element (a • b) in G, such that the following axioms are obeyed:

(A1) Closure :	If a and b belong to G, then a • b is also in G.
(A2) Associative :	a • (b • c) = (a • b) • c for all a, b, c in G.
(A3) Identity element :	There is an element e in G such that a • e = e • a = a for all a in G.
(A4) Inverse element :	For each a in G there is an element a' in G such that a • a' = a' • a = e.
(A5) Commutative :	a • b = b • a for all a, b in G.

 21. What is dual signature and it is purpose?

            The purpose of the dual signature is to link two messages that intended for two different recipients. To avoid misplacement of orders