CRYPTOGRAPHY AND NETWORK SECURITY, 2 Mark Unit I

1. Why we need the cryptography and Network Security?

            To prevent our data from  different threats and attacks is called Network Security, during this prevention we are using some methodology is called cryptography.

2. Diffeneiate cryptography and steganography

           

cryptography	steganography
The branch of cryptology dealing with the design of algorithms for encryption and decryption, intended to ensure the secrecy and/or authenticity of messages.	Methods of hiding the existence of a message or other data. This is different than cryptography, which hides the meaning of a message but does not hide the message itself.

3. Define cryptology.

The study of secure communications, which encompasses both cryptography and cryptanalysis.

4. What is the Role of cryptanalysis?

The branch of cryptology dealing with the breaking of a cipher to recover information, or forging encrypted information that will be accepted as authentic.

5. What is difference between Encryption and Decryption?

Encryption	. Decryption
The conversion of plaintext or data into unintelligible form by means of a reversible translation, based on a translation table or algorithm. Also called enciphering.	The translation of encrypted text or data (called ciphertext) into original text or data (called plaintext). Also called deciphering.

6. What are the essential ingredients of a symmetric cipher?

 A symmetric cipher encryption has five ingredients. They are:

• Plaintext

• Encryption algorithm

• Secret key

• Cipher text

• Decryption algorithm 

7. Distinguish Threat And Attack

           

Threat	Attack
A potential for violation of security, which exists when there is a circumstance, capability, action, or event that could breach security and cause harm. That is, a threat is a possible danger that might exploit a vulnerability.	An assault on system security that derives from an intelligent threat; that is, an intelligent act that is a deliberate attempt (especially in the sense of a method or technique) to evade security services and violate the security policy of a system.

8. List the types of Attacks.

o   Active Attacks

o   Passive Attacks

9. Define Passive Attacks.

Passive attacks are in the nature of eavesdropping on, or monitoring of, transmissions. The goal of the opponent is to obtain information that is being transmitted. Two types of passive attacks are release of message contents and traffic analysis.

10. Define Active Attacks.

            Active attacks involve some modification of the data stream or the creation of a false stream and can be subdivided into four categories: masquerade, replay, modification of messages, and denial of service.

11. What is Authentication? list out its types.

            The authentication service is concerned with assuring that a communication is authentic.

                        The Types are,

·         Peer entity authentication: Provides for the corroboration of the identity of a peer entity in an association. It is provided for use at the establishment of, or at times during the data transfer phase of, a connection. It attempts to provide confidence that an entity is not performing either a masquerade or an unauthorized replay of a previous connection.

·         Data origin authentication: Provides for the corroboration of the source of a data unit. It does not provide protection against the duplication or modification of data units. This type of service supports applications like electronic mail where there are no prior interactions between the communicating entities.

12. What are the key principles of security?

            The classification of security services are as follows:

·         Confidentiality

·         Authentication

·         Integrity

·         Non repudiation

·         Access control

·         Availability

13. Define Data Confidentiality

Confidentiality is the protection of transmitted data from passive attacks. With respect to the content of a data transmission, several levels of protection can be identified. The broadest service protects all user data transmitted between two users over a period of time.

14. Define Data Integrity.

As with confidentiality, integrity can apply to a stream of messages, a single message, or selected fields within a message. Again, the most useful and straightforward approach is total stream protection.

15. What is mean by Availability Service?

A system is available if it provides services according to the system design whenever users request them

16. List the security Mechanisms.

o   Enciperment

o   Digital signature

o   Access Control                       

o   Data integrity

o   Authentication Exchange

o   Traffic Padding          

o   Routing Control         

o   Notarization

17. What are the two basic functions used in encryption algorithms?

 The two basic functions used in encryption algorithms are

• Substitution

            • Transposition

 18. How many keys are required for two people to communicate via a cipher?

             If both sender and receiver use the same key, the system is referred to as symmetric, single key, secret key, or conventional encryption. If the sender and receiver each use a different key, the system is referred to as asymmetric, two-key, or public-key encryption.

19. What is the difference between a block cipher and a stream cipher?

           

Block Cipher	Stream Ciper
A block cipher processes the input one block of elements at a time, producing an output block for each input block.	A stream cipher processes the input elements continuously, producing output one element at a time, as it goes along.

20. What are the two approaches to attacking a cipher?

             The two approaches to attack a cipher are:

                        • Cryptanalysis

                        • Brute-force attack

21. What is the difference between an unconditionally secure cipher and a computationally secure cipher?

             • An unconditionally secure cipher is a scheme such that if the cipher text generated by the scheme does not contain enough information to determine uniquely the corresponding plain text, no matter how much cipher text is available.

            • A computationally secure scheme is such that the cost of breaking the cipher exceeds the value of the encrypted information and the time required to break the cipher exceeds the useful lifetime of the information.

 22. Briefly define the Caesar cipher.

            The Caesar cipher involves replacing each letter of the alphabet with the letter standing three places further down the alphabet. For example:

            Plain: meet me after the toga party

            Cipher: PHHW PH DIWHU WKH WRJD SDUWB

23. Briefly define the monoalphabetic cipher?

             A monoalphabetic cipher maps from a plain alphabet to cipher alphabet. Here a single

cipher alphabet is used per message.

24. Briefly define the playfair cipher.

            The best-known multiple-letter encryption cipher is the playfair, which treats diagrams in the plain text as single units and translates these units into cipher text diagrams. The Playfair algorithm is based on the use of a 5x5 matrix of letters constructed using a keyword. In the case of keyword monarchy, matrix is as follows:

             M O N A R

            C H Y B D

            E F G I/J K

            L P Q S T

            U V W X Z

25. What are the two problems with one-time pad?

             • It makes the problem of making large quantities of random keys.

            • It also makes the problem of key distribution and protection.

26. What is a transposition cipher?

            Transposition cipher is a cipher, which is achieved by performing some sort of permutation on the plaintext letters.

27. Why is it important to study feistel cipher?

             This cipher can be used to approximate the simple substitution cipher by utilizing the concept of a product cipher, which is the performing of two or more basic ciphers in sequence in such a way that the final result or product is cryptographically stronger than any of the component ciphers.

28. Why is it not practical to use an arbitrary reversible substitution cipher?

            An arbitrary reversible cipher for a large block size is not practical, however, from an implementation and performance point of view. Here the mapping itself is the key.

29. What is the difference between diffusion and confusion?

In diffusion, the statistical structure of the plain text is dissipated into long-range statistics of the cipher text. This is achieved by permutation. In confusion, the relationship between the statistics of the cipher text and the value of the encryption key is made complex. It is achieved by substitution.

30. Which parameters and design choices determine the actual algorithm of a feistel cipher?

            • Block size

            • Key size

            • Number of rounds

            • Sub key generation algorithm

            • Round functions

            • Fast software encryption or decryption

            • Ease of analysis

31. What is the purpose of the S-boxes in DES?

 Each row of a S-box defines a general reversible substitution. It consists of a set of eight S-boxes, each of which accepts 6 bits as input and produces 4 bits as output.

32.  Explain the avalanche effect.

 It is that a small change in either the plaintext or the key should produce a significant change in the cipher text. A change in one of the bit of the plaintext or one bit of the key should produce a change in many bits of the cipher text.

33. What is the difference between a mono alphabetic cipher and a poly alphabetic cipher?

 Mono alphabetic cipher: Here a single cipher alphabet is used.Poly alphabetic cipher: Here a set of related mono alphabetic substitution rules is used.

34. List the types of cryptanalytic attacks.

             • Cipher text only

            • Known plaintext

            • Chosen plaintext

            • Chosen cipher text

            • Chosen text

35. When an encryption algorithm is said to be computationally secured?

            The encryption algorithm is said to be computationally secure if

• The cost of breaking the cipher exceeds the value of the encrypted information
• The time required to break the cipher exceeds the useful time of the information.