Chettinad College of Engineering and technology, Karur Department DEPARTMENT of Information OF ECETechnology Cryptography and Network Security Two

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1 Chettinad College of Engineering and technology, Karur Department DEPARTMENT of Information OF ECETechnology Cryptography and Network Security Two Mark Questions UNIT I 1. What is OSI security architecture? The OSI security architecture is useful to managers as a way of organizing the task of providing security. The OSI security architecture focuses on security attacks, mechanisms, and services. These can be defined briefly as Security attack: Any action that compromises the security of information owned by an organization. Security mechanism: A process (or a device incorporating such a process) that is designed to detect, prevent, or recover from a security attack. Security service: A processing or communication service that enhances the security of the data processing systems and the information transfers of an organization. The services are intended to counter security attacks, and they make use of one or more security mechanisms to provide the service. 2. Define Threat. 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 vulnerability. 3.Define Attack 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. 4. Specify the four categories of security threats Interruption Interception Modification Fabrication 5. Explain active and passive attack with example? (i) Passive attack: A passive attack attempts to learn or make use of information from the system but does not affect system resources. Eg: release of message contents, traffic analysis (ii) Active attack: An active attack attempts to alter system resources or affect their operation. 1

2 Eg: A masquerade takes place when one entity pretends to be a different entity Replay involves the passive capture of a data unit and its subsequent retransmission to produce an unauthorized effect. Modification of messages simply means that some portion of a legitimate message is altered, or that messages are delayed or reordered, to produce an unauthorized effect. The denial of service prevents or inhibits the normal use or management of communications facilities. 6. Compare active and passive attacks. Passive attacks Active attacks A passive attack attempts to learn or make An active attack attempts to alter system use of information from the system but does resources or affect their operation not affect system resources Eg: release of message contents, traffic Eg: masquerade, Replay, Modification of analysis messages, denial of service Difficult to detect Easy to detect 7. What are the various security services? Authentication Access control Data confidentiality Data integrity Nonrepudiation 8. Define Authentication The assurance that the communicating entity is the one that it claims to be. Types are: Peer Entity Authentication Data-Origin Authentication 2

3 9. Define Access control The prevention of unauthorized use of a resource (i.e., this service controls who can have access to a resource, under what conditions access can occur and what those accessing the resource are allowed to do). 10. Define Data confidentiality The protection of data from unauthorized disclosure. Types are: Connection Confidentiality Connectionless Confidentiality Selective-Field Confidentiality Traffic-Flow Confidentiality 11. Define Data integrity The assurance that data received are exactly as sent by an authorized entity (i.e., contain no modification, insertion, deletion, or replay). Types are: Connection Integrity with Recovery Connection Integrity without Recovery Selective-Field Connection Integrity Connectionless Integrity Selective-Field Connectionless Integrity 12. Define Nonrepudiation It provides protection against denial by one of the entities involved in a communication of having participated in all or part of the communication. Types are: Nonrepudiation, Origin Nonrepudiation, Destination 13. What are the security mechanisms are there? SPECIFIC SECURITY MECHANISMS o Encipherment o Digital Signature o Access Control 3

4 o Data Integrity o Authentication Exchange o Traffic Padding o Routing Control o Notarization PERVASIVE SECURITY MECHANISMS o Trusted Functionality o Security Label o Event Detection o Security Audit Trail o Security Recovery 14. What is the relationship Between Security Services and Mechanisms? 15. Draw a model for network security. 4

5 16. What is cryptology? Cryptology is the study of cryptography and cryptanalysis. 17. Define cryptanalysis? It is a process of attempting to discover the key or plaintext or both. 18. Define cryptography. The many schemes used for encryption constitute the area of study known as cryptography. 19. Define Steganography Steganography is a technique for hiding a secret message within a larger one in such a way that others cannot discern the presence or contents of the hidden message. 20. Define plaintext. An original message is known as the plaintext. 21. Define cipher text. An original message is known as the plaintext, while the coded message is called the cipher text. 22. Define encryption. The process of converting from plaintext to cipher text is known as enciphering or encryption; 23. Define decryption. The process of restoring the plaintext from the cipher text is deciphering or decryption. 24. Differentiate symmetric and asymmetric encryption Symmetric encryption asymmetric encryption It is a form of cryptosystem in which encryption It is a form of cryptosystem in which encryption and decryption performed using the same key. and decryption Performed using two keys. It is It is also known as conventional encryption. also known as two-key, or public-key encryption Eg: DES, AES Eg: RSA, ECC 25. What are the ingredients of symmetric encryption? Plaintext Encryption algorithm Secret key Ciphertext Decryption algorithm 5

6 26. What are the requirements of conventional encryption? strong encryption algorithm Sender and receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure. 27. Draw a simplified Model of Symmetric Encryption. 28. Draw a model of Symmetric Cryptosystem. 29. What are the characteristics of cryptographic system? The type of operations used for transforming plaintext to cipher text The number of keys used The way in which the plaintext is processed 6

7 30. What are the types of encryption algorithm? Substitution technique Transposition technique 31. Compare Substitution and Transposition techniques. Substitution techniques Transposition techniques A substitution techniques is one in which It means, different kind of mapping is the letters of plaintext are replaced by achieved by performing some sort of other letter or by number or symbols. permutation on the plaintext letters. Eg: Caser cipher, Monoalphabetic Ciphers, Eg: rail fence, DES, AES. Playfair Cipher, Hill Cipher, Polyalphabetic Ciphers, One-Time Pad 32. Define block cipher. A block cipher processes the input one block of elements at a time, producing an output block for each input block. 33. Define stream cipher. A stream cipher processes the input elements continuously, producing output one element at a time, as it goes along. 34. 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. 35. What are the two approaches to attacking a cipher? Cryptanalysis Brute-force attack 7

8 36. 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 37. 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. 38. 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. 39. What are the two problems with one-time pad? 1.It makes the problem of making large quantities of random keys. 2.It also makes the problem of key distribution and protection. 40. What is a transposition cipher? Transposition cipher is a cipher, which is achieved by performing some sort of permutation on the plaintext letters. 41. 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 42. 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. 43. List the types of cryptanalytic attacks. Cipher text only Known plaintext Chosen plaintext Chosen cipher text Chosen text 44. 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. 8

9 45. 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 46. Define Diffusion & confusion. Diffusion: It means each plaintext digits affect the values of many cipher text digits which is equivalent to each cipher text digit is affected by many plaintext digits. It can be achieved by performing permutation on the data. It is the relationship between the plaintext and cipher text. Confusion: It can be achieved by substitution algorithm. It is the relationship between Cipher text and key. 47. Define Reversible Mapping Each plain text is maps with the unique cipher text. This transformation is called reversible mapping or non-singular mapping. 48. Define Irreversible Mapping Each plain text is maps with the cipher text (not unique). This transformation is called irreversible mapping or singular mapping. 49. What are the features if feistel structure. Block size Key size Number of rounds Subkey generation algorithm Round function Fast software encryption/decryption Ease of analysis 9

10 50. Define Product cipher. It means two or more basic cipher are combined and it produce the resultant cipher is called the product cipher. 51. Define DES. DES (Data Encryption Standard) is one of the encryption algorithm. It exhibits the classic Feistel structure. The algorithm itself is referred to as the Data Encryption Algorithm (DEA). For DES, data are encrypted in 64-bit blocks using a 56-bit key. The algorithm transforms 64-bit input in a series of steps into a 64-bit output. The same steps, with the same key, are used to reverse the encryption. 52. Explain Avalanche effect. A desirable property of any encryption algorithm is that a small change in either the plaintext or the key produce a significant change in the ciphertext. In particular, a change in one bit of the plaintext or one bit of the key should produce a change in many bits of the ciphertext. If the change is small, this might provider a way to reduce the size of the plaintext or key space to be searched. Eg: Double DES, Triple DES 53. 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 Sboxes, each of which accepts 6 bits as input and produces 4 bits as output. 54. What are the features of AES? Symmetric and parallel structure Adapted to modern processors Algorithm can work well with smart cards 55. List the evaluation criteria defined by NIST for AES? The evaluation criteria for AES are as follows: Security Cost Algorithm and implementation characteristics 56. Define Multiple Encryption. Multiple encryption is a technique in which an encryption algorithm is used multiple times. In the first instance, plaintext is converted to ciphertext using the encryption algorithm. This ciphertext is then used as input and the algorithm is applied again. This process may be repeated through any number of stages. 10

11 57. Define double encryption. The simplest form of multiple encryption has two encryption stages and two keys. 58. What is Triple Encryption? How many keys are used in triple encryption? Triple Encryption is a technique in which encryption algorithm is performed three times using three keys. 59. What is the key size for Blowfish? Blowfish makes use of a key that ranges from 32 bits to 448 bits (one to fourteen 32-bit words). That key is used to generate bit subkeys and four 8*32 S-boxes containing a total of bit entries. The total is bit values, or 4168 bytes. 60. Give the five modes of operation of Block cipher. Electronic Codebook(ECB) Cipher Block Chaining(CBC) Cipher Feedback(CFB) 11

12 Output Feedback(OFB) Counter (CTR) 61. State advantages of counter mode. Hardware Efficiency Software Efficiency Preprocessing Random Access Provable Security Simplicity. 62. Define covert channel. A communication channel that enables the transfer of information in a away unintended by the designers of the communications facility. It violates a security policy and communication to an outsider is not detected by the management, 63. What are the approaches are used to reduce traffic analysis? Link encryption End to end encryption 64. What is traffic padding? What is its purpose? Traffic padding produces cipher text output continuously, even in the absence of the plain text. A continuous random data stream is generated. When plain text is available, it is encrypted and transmitted. When input plaintext is not present, random data are encrypted and transmitted. This makes it impossible to for an attacker to distinguish between true dataflow and padding and therefore impossible to deduce the amount of traffic. The advantages are: It is impossible for an attacker to distinguish between true data flow and padding data flow It is impossible to deduce amount of traffic UNIT II 1. What are the principle elements of a public key cryptosystem? The principle elements of a cryptosystem are: Plain text Encryption algorithm Public and private key 12

13 Cipher text Decryption algorithm 2. What are roles of public and private key? The two keys used for public-key encryption are referred to as the public key and the private key. Invariably, the private key is kept secret and the public key is known publicly. Usually the public key is used for encryption purpose and the private key is used in the decryption side. 3. Specify the applications of the public key cryptosystem? The applications of the public-key cryptosystem can classified as follows 1. Encryption/Decryption: The sender encrypts a message with the recipient s public key. 2. Digital signature: The sender signs a message with its private key. Signing is achieved by a cryptographic algorithm applied to a message or to a small block of data that is a function of the message. 3. Key Exchange: Two sides cooperate to exchange a session key. Several different approaches are possible, involving the private key(s) of one or both parties. 4. What requirements must a public key cryptosystem to fulfill to a secured algorithm? The requirements of public-key cryptosystem are as follows: 1. It is computationally easy for a party B to generate a pair (Public key KUb, Private key KRb) 2. It is computationally easy for a sender A, knowing the public key and the message to be encrypted, M, to generate the corresponding cipher text: C=EKUb(M) 3. It is computationally easy for the receiver B to decrypt the resulting ciphertext using the private key to recover the original message : M=DKRb(C)=DKRb[EKUb(M)] 4. It is computationally infeasible for an opponent, knowing the public key,kub,to determine the private key,krb. 5. It is computationally infeasible for an opponent, knowing the public key,kub, and a ciphertext, C, to recover the original message,m. 6. The encryption and decryption functions can be applied in either order: M=EKUb[DKRb(M)]=DKUb [EKRb(M)] 5. What is a one way function? One way function is one that map the domain into a range such that every function value has a unique inverse with a condition that the calculation of the function is easy where as the calculations of the inverse is infeasible. 13

14 6. What is a trapdoor one way function? It is function which is easy to calculate in one direction and infeasible to calculate in other direction in the other direction unless certain additional information is known. With the additional information the inverse can be calculated in polynomial time. It can be summarized as: A trapdoor one way function is a family of invertible functions fk, such that Y= fk( X) easy, if k and X are known X=fk-1(Y) easy, if k and y are known X= fk-1(y) infeasible, if Y is known but k is not known 7. Differentiate public key and conventional encryption? conventional encryption public key encryption The same algorithm with the same One algorithm is used for encryption Key is used for encryption and decryption and decryption with a pair of keys, one for encryption and another for Decryption The sender and receiver must use The algorithm and the key must each have one of the Matched the same key pair of keys The key must be secret One of two keys must be kept Secret It must be impossible or atleast It must be impossible or to decipher a message if o other impractial information at least impractical to decipher a is available message if no other information is available. Knowledge of the algorithm plus Knowledge of the algorithm of cipher text must insufficient to samples determine plus one of key plus samples of the key cipher text must be insufficient to determine the other key. 8. Define prime number. A prime number is an integer that can only be divided without remainder by positive and negative values of itself and Prime numbers play a critical role both in number theory and in cryptography. 9. Define fermet s theorem. Fermat s theorem states the following: If p is prime and is a positive integer not divisible by p, then a p-1 =1(mod p) 10. Define Euler s Theorem Euler s theorem states that for every and that are relatively prime: a ᶲ(n) =1(mod n) 14

15 11. Find gcd (1970, 1066) using Euclid s algorithm? gcd (1970,1066) = gcd(1066,1970 mod 1066) = gcd(1066,904) = Write an algorithm for testing for primality. TEST ( n) 1. Find integers k,q, with k>0, q odd, so that (n-1=2 k q); 2. Select a random integer a, 1<a<n-1; 3. if a q mod n=1 then return( inconclusive ); 4. for j=0 to k-1 do 5. if a 2jq mod n =n-1 then return( inconclusive ); 6. return("composite") 13. What is the primitive root of a number? We can define a primitive root of a number p as one whose powers generate all the integers from 1 to p-1. That is p, if a is a primitive root of the prime number p then the numbers. 14. Define Diffie-Hellman key exchange. A simple public-key algorithm is Diffie-Hellman key exchange. This protocol enables two users to establish a secret key using a public-key scheme based on discrete logarithms. The protocol is secure only if the authenticity of the two participants can be established. 15. Define elliptic curve arithmetic. Elliptic curve arithmetic can be used to develop a variety of elliptic curve cryptography (ECC) schemes, including key exchange, encryption, and digital signature. For purposes of ECC, elliptic curve arithmetic involves the use of an elliptic curve equation defined over a finite field.the coefficients and variables in the equation are elements of a finite field. 16. What is an elliptic curve? An elliptic curve is defined by an equation in two variables with coefficients. y 2 = x 3 + ax + b 17. What is key distribution center? A key distribution center is responsible for distributing keys to pairs of users such as hosts, processes, applications. Each user must share a unique key with the key distribution center for purposes of key distribution. 15

16 18. 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. 19. Define session key. Communication between end systems is encrypted using a temporary key, often referred to as a session key. 20. Define master key. Session keys are transmitted in encrypted form, using a master key that is shared by the key distribution center and an end system or user. 21. 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 22. List four general characteristics of schema for the distribution of the public key? The four general characteristics for the distribution of the public key are Public announcement Publicly available directory Public-key authority Public-key certificate 23. What is a public key certificate? The public key certificate is that used by participants to exchange keys without contacting a public key authority, in a way that is as reliable as if the keys were obtained directly from the public-key authority. Each certificate contains a public key and other information, is created by a certificate authority, and is given to a participant with the matching private key. 24. What are essential ingredient of the public key directory? The essential ingredient of the public key are as follows: 1. The authority maintains a directory with a {name, public key} entry for each participant 16

17 2. Each participant registers a public key with the directory authority. 3. A participant may replace the existing key with a new one at a time 4. Periodically, the authority publishes the entire directory or updates to the directory. 5. Participants could also access the directory electronically. For this purpose, secure, authenticated communication from the authority to the participant is mandatory. UNIT III 1. What is message authentication? It is a procedure that verifies whether the received message comes from assigned source has not been altered. It uses message authentication codes, hash algorithms to authenticate the message. 2. Define the classes of message authentication function. Message encryption: The entire cipher text would be used for authentication. Message Authentication Code: It is a function of message and secret key produce a fixed length value. Hash function: Some function that map a message of any length to fixed length which serves as authentication. 3. What are the requirements for message authentication? The requirements for message authentication are Disclosure: Traffic Analysis Masquerade Content modification Sequence modification Timing modification Source repudiation Destination repudiation 4. What you meant by hash function? Hash function accept a variable size message M as input and produces a fixed size hash code H(M) called as message digest as output. It is the variation on the message authentication code. 5. What is birthday attack? It means that the opponent would have to try about 2 (hash code size-1) messages to find one that matches the hash code of the intercepted message. To avoid birthday attack, use the large MACs. 17

18 6. What are the properties of hash functions? One way hash function Weak collision resistance Strong collision resistance 7. Differentiate MAC and Hash function? MAC: In Message Authentication Code, the secret key shared by sender and receiver. The MAC is appended to the message at the source at a time which the message is assumed or known to be correct. Hash Function: The hash value is appended to the message at the source at time when the message is assumed or known to be correct. The hash function itself not considered to be secret. 8. Mention any three hash algorithm. MD5 (Message Digest version 5) algorithm. SHA_1 (Secure Hash Algorithm). RIPEMD_160 algorithm. 9. What are the requirements of the hash function? H can be applied to a block of data of any size. H produces a fixed length output. H(x) is relatively easy to compute for any given x, making both hardware and software implementations practical. 10. What you meant by MAC? MAC is Message Authentication Code. It is a function of message and secret key which produce a fixed length value called as MAC. MAC = Ck(M) Where M = variable length message K = secret key shared by sender and receiver. CK(M) = fixed length authenticator. 11. Define HMAC HMAC stands for Hash-based Message Authentication Code. It has been chosen as a mandatory to implement MAC for IP security and also used in Secure Socket Layer(SSL) protocol widely used on the Internet. 12. What are the design objectives of HMAC? To reuse the existing message digest algorithm such as MD5 and SHA-1 18

19 To allow easy replaceability of the embedded hash function in case faster or more secure hash functions are found. To preserve the original performance of hash function without degradation To use and handle key in a simple way To use authentication mechanism, which is cryptographically more strong. 13. What are the disadvantages of HMAC? The problem of symmetric key exchange between two parties. HMAC cannot be used if the number of receivers is greater than one. If multiple parties share the symmetric key, the receiver has no way to detect from whom the message is coming. It creates, denial of service attack. 14. Differentiate internal and external error control. Internal error control: In internal error control, an error detecting code also known as frame check sequence or checksum. External error control: In external error control, error detecting codes are appended after encryption. 15. What is the meet in the middle attack? This is the cryptanalytic attack that attempts to find the value in each of the range and domain of the composition of two functions such that the forward mapping of one through the first function is the same as the inverse image of the other through the second function-quite literally meeting in the middle of the composed function. 16. What is the role of compression function in hash function? The hash algorithm involves repeated use of a compression function f, that takes two inputs and produce a n-bit output. At the start of hashing the chaining variable has an initial value that is specified as part of the algorithm. The final value of the chaining variable is the hash value usually b>n; hence the term compression. 17. What are the design goals of MD4? Security Speed Simplicity and compactness Favour little endian architecture 19

20 18. Compare MD4 and MD5. Points MD4 MD5 Number of rounds of steps each Use of additive constant t No additive constant for first round. The A different additive same additive constant is sued for each of constant is used for each the steps of second round; Another of the 64 steps. additive constant is used for each of the steps if third round Use of primitive logical Three, one for each round. Four, one for each round functions Inclusion of result of the MD4 did not include this final addition Each step adds in the previous step in each result of the preceding round. step. 19. Compare MD5 and SHA-1 Points MD5 SHA-1 Message digest length in bits Attack to try and find the original message for given message digest Requires operations to break in Requires operations to break in Attack to try to find two messages producing the same message digest Requires 2 64 operations to break in. Requires 2 80 operations to break in. Successful attacks so far Similar message digest for No such claims so far different messages and pseudo collision Speed Faster(64 iterations, 128 bit buffer) slower(80 iterations, 160 bit buffer) Software implementation Simple, does not need any Simple, does not need any large programs or complex large programs or tables. complex tables. 20

21 20. Compare MD5, SHA-1, RIPEMD-160 Points MD5 SHA-1 RIPEMD-160 Digest length 128 bits 160 bits 160 bits Basic unit of processing 512 bits 512 bits 512 bits Number of steps Maximum message size Primitive logical functions Additive constants used Endianness Little endian Big endian Little endian 21. Define authentication protocols. It is sued to convince parties of each other s and to exchange session key. It may be one way authentication or mutual authentication. 22. Define replay attack A replay attack is one in which an attacker obtains a copy of an authenticated packet and later transmits it to the intended destination. The receipt of duplicate, authenticated IP packets may disrupt service in some way or may have some other undesired consequence. 23. Give examples of replay attack. Simple replay attack Repetition that can be logged Repetition than cannot be detected Backward replay without modification 24. What are the countermeasures for replay attack? Use of sequence number Use of timestamps Challenge/response 25. Define digital signature. A digital signature is an authentication mechanism that enables the creator of a message to attach a code that acts as a signature. Typically the signature is formed by taking the hash of the message and encrypting the message with the creator s private key. The signature guarantees the source and integrity of the message. 21

22 26. What are the requirements of digital signature? The signature must be a bit pattern that depends on the message being signed. The signature must use some information unique to the sender to prevent both forgery and denial. It must be relatively easy to produce the digital signature. It must be relatively easy to recognize and verify the digital signature. It must be computationally infeasible to forge a digital signature, either by constructing a new message for an existing digital signature or by constructing a fraudulent digital signature for a given message. It must be practical to retain a copy of the digital signature in storage. 27. What are the properties a digital signature should have? It must verify the author and the data and time of signature. It must authenticate the contents at the time of signature. It must be verifiable by third parties to resolve disputes. 28. What are the approaches of digital signature? Direct digital signature Arbitrated digital signature 29. Distinguish between direct and arbitrated digital signature? Direct digital signature Arbitrated Digital Signature The direct digital signature involves only the communicating parties. The arbiter plays a sensitive and crucial role in this digital signature. This may be formed by encrypting the entire message with the sender s private key. Every signed message from a sender x to a receiver y goes first to an arbiter A, who subjects the message and its signature to a number of tests to check its origin and content. 30. What requirements should a digital signature scheme should satisfy? The signature must be bit pattern that depends on the message being signed. The signature must use some information unique to the sender, to prevent both forgery and denial. It must be relatively easy to produce the digital signature. It must be relatively easy to recognize and verify the digital signature. It must be computationally infeasible to forge a digital signature, either by constructing a new message for an existing digital signature or by constructing a fraudulent digital signature for a given message. It must be practical to retain a copy of the digital signature in storage. 22

23 UNIT IV 1. Define Kerberos. Kerberos is an authentication service developed in which users at work stations wish to access services on servers distributed throughout the network. 2. What are the requirements of Kerberos? Secure Reliable Transparent Scalable 3. Differentiate Kerberos Version 4 and 5(0r) What is the disadvantages of Version 4 Kerberos? Kerberos Version 5 is intended to address the limitations of Kerberos Version 4. Environmental shortcomings Encryption system dependence Internet protocol dependence Message byte ordering Ticket lifetime Authentication forwarding Inter realm authentication Technical deficiencies Double encryption PCBC encryption Session keys Password attacks 4. What is realm? A full service Kerberos environment consisting of a Kerberos server, a no. of clients, no.of application server requires the following: The Kerberos server must have user ID and hashed password of all participating users in its database. The Kerberos server must share a secret key with each server. Such an environment is referred to as Realm. 23

24 5. Draw an X.509 message format. 6. What are the approaches are used in security? PGP(Pretty Good Privacy) S/MIME(Simple/Multipurpose Internet Mail Extension) 7. Define PGP. It provides a confidentiality and authentication service that can be usef for emal and file storage applications. 8. What are the features of PGP? Runs on variety of platforms under different vendors. Based on popular and secure algorithms such as RSA, DSS, Diffie hellman. It has a wide range of applicability and standardized schemes. It was not developed and not controlled by the government or standards organization. It is a Internet Standard. 9. What are the services provided by PGP services? Digital signature Message encryption Compression 24

25 compatibility Segmentation 10. Explain the reasons for using PGP? a) It is available free worldwide in versions that run on a variety of platforms, including DOS/windows, UNIX, Macintosh and many more. b) It is based on algorithms that have survived extensive public review and are considered extremely secure. E.g.) RSA, DSS and Diffie-Hellman for public key encryption, CAST-128, IDEA, 3DES for conventional encryption, SHA-1for hash coding. c) It has a wide range of applicability from corporations that wish to select and enforce a standardized scheme for encrypting files and communication. d) It was not developed by nor is it controlled by any governmental or standards organization. 11. Define compatibility. The process of converting the raw 8-bit binary stream to a stream of printable ASCII characters is called compatibility. 12. Why compatibility function in PGP needed? Electronic mail systems only permit the use of blocks consisting of ASCII text. To accommodate this restriction PGP provides the service converting the row 8- bit binary stream to a stream of printable ASCII characters. The scheme used for this purpose is Radix-64 conversion. 13. Name any cryptographic keys used in PGP? a) One-time session conventional keys. b) Public keys. c) Private keys. d) Pass phrase based conventional keys. 14. Define key Identifier? PGP assigns a key ID to each public key that is very high probability unique with a user ID. It is also required for the PGP digital signature. The key ID associated with each public key consists of its least significant 64bits. 15. What are components of PGP message? Message component Signature component Session key component 25

26 16. Define public key ring. The data structure is used to store public keys of users is referred as public key ring. It has the following fields: Timestamp Key D Public key User ID Owner trust Key legitimacy Signature 17. Define passphrase key. The passphrase is a key generated from the passwords and it has a very short life time. Using this key, 160-bit hash code is generated. After the hash code generation, the passphrase key is discarded. 18. List the limitations of SMTP/RFC 822? a) SMTP cannot transmit executable files or binary objects. b) It cannot transmit text data containing national language characters. c) SMTP servers may reject mail message over certain size. d) SMTP gateways cause problems while transmitting ASCII and EBCDIC. e) SMTP gateways to X.400 network cannot handle non textual data included in X.400 messages. 19. Define S/MIME? Secure/Multipurpose Internet Mail Extension(S/MIME) is a security enhancement to the MIME Internet format standard, based on technology from RSA Data Security. 20. What are the elements of MIME? Five new message header fields are defined which may be included in an RFC 822 header. A number of content formats are defined. Transfer encodings are defined that enable the conversion of any content format into a form that is protected from alteration by the mail system. 21. What are the headers fields define in MME? MIME version. Content type. 26

27 Content transfer encoding. Content id. Content description. 22. What are the various MIME content types? Text Multipart Message Image Video Audio Application 23. What are the various MIME encoding techniques? 7-bit 8-bit Binary Quoted printable Base64 x-token 24. What are the functions of S/MIME? Enveloped data Signed data Clear-signed data Signed and enveloped data 25. What are the key algorithms used in S/MIME? Digital signature standards. Diffi Hellman. RSA algorithm. 26. Give the steps for preparing envelope data MIME? Generate Ks. Encrypt Ks using recipient s public key. 27

28 RSA algorithm used for encryption. Prepare the recipient info block. Encrypt the message using Ks. 27. What are the function areas of IP security? Authentication Confidentiality Key management. 28. Give the application of IP security? Provide secure communication across private & public LAN. Secure remote access over the Internet. Secure communication to other organization. 29. Give the benefits of IP security? It provides strong security that can be applied to all traffic crossing the perimeter. IPsec in a firewall is resistant to bypass if all traffic from the outside must use IP and the firewall is the only means of entrance from the Internet into the organization. IPsec is below the transport layer (TCP, UDP) and so is transparent to applications. IPsec can be transparent to end users. IPsec can provide security for individual users if needed. 30. What are the protocols used to provide IP security? Authentication header (AH) protocol. Encapsulating Security Payload(ESP). 31. Specify the IP security services? Access control. Connectionless interpretty. Data origin authentication Rejection of replayed packet. Confidentiality. Limited traffic for Confidentiality. 28

29 32. What do you mean by Security Association? Specify the parameters that identifies the Security Association? An association is a one-way relationship between a sender and receiver that affords security services to the traffic carried on. A key concept that appears in both the authentication and confidentiality mechanism for ip is the security association (SA). A security Association is uniquely identified by 3 parameters: Security Parameter Index (SPI). IP Destination Address. Security Protocol Identifier. 33. What do you mean by Reply Attack? A replay attack is one in which an attacker obtains a copy of an authenticated packet and later transmits it to the intended destination. Each time a packet is send the sequence number is incremented. 34. Explain man in the middle attack? If A and B exchange message, means E intercept the message and receive the B s public key and b s userid,e sends its own message with its own public key and b s userid based on the private key and Y.B compute the secret key and A compute k2 based on private key of A and Y 35. Steps involved in SS L required protocol? 1. SSL record protocol takes application data as input and fragments it. 2. Apply lossless Compression algorithm. 3. Compute MAC for compressed data. 4. MAC and compression message is encrypted using conventional alg. 36. What is mean by SET? What are the features of SET? Secure Electronic Transaction (SET) is an open encryption and security specification designed to protect credit card transaction on the internet. Features are: 1. Confidentiality of information 2. Integrity of data 3. Cardholder account authentication 4. Merchant authentication 37. What are the steps involved in SET Transaction? 1. The customer opens an account 29

30 2. The customer receives a certificate 3. Merchants have their own certificate 4. The customer places an order. 5. The merchant is verified. 6. The order and payment are sent. 7. The merchant requests payment authorization. 8. The merchant confirm the order. 9. The merchant provides the goods or services. 10. The merchant requests payment. 38. What is dual signature? What it is purpose? The purpose of the dual signature is to link two messages that intended for two different recipients. 39. Compare SET and SSL. Issues SSL SET Aim Exchange of data in an encrypted form e-commerce related payment mechanism Certification Two parties xexchange certificates All the involved parties must be certifiesd bya a trusted third party Authentication Mechanism in place, but not very Strong mechanisnm for strong authenticating all the parties inmvolved Risk of merchant fraud Possible: since customer gives Customer gives financial data to financial data to merchant payment gateway. Risk of customer fraud Possible: no mechanisms exist if a Customer has to sign paymet customer refuses to pya later instructions. Action in case of Merchant is liable Payment gateway is liable customer fraud Practical usage High Low at the moment. Expected to grow 30

31 UNIT V 1. Define intruders. Intruder is an individual who gains or attempts to gain, unauthorized access to a computer system o to gain unauthorized privileges on that system. It is also referred to as hacker or cracker. 2. List the 3 classes of intruder? Masquerader Misfeasor Clandestine user 3. Name the intrusion techniques. One way function Access control 4. What are the merits of intrusion detection system(ids)? If an intrusion is detected quickly enough, the intruder can be identified and ejected from the system before any damage is done or any data are compromised. An effective intrusion detection system can serve as a deterrent, so acting to prevent Intrusions. Intrusion detection enables the collection of information about intrusion 5. What are the approaches of Intrusion detection techniques? Statistical anomaly detection o Threshold detection o Profile based Rule based detection o Anomaly detection o Penetration identification 6. Define audit records. What are the types? A fundamental tool for intrusion detection is the audit record. Some record of ongoing activity by users must be maintained as input to an intrusion detection system. The types are: Native audit records Detection-specific audit records 31

32 7. Compare Native audit records, Detection-specific audit records Native audit records Detection-specific audit records Definition Virtually all multiuser operating A collection facility can be implemented systems include accounting software that generates audit records containing that collects information on user only that information required by the activity intrusion detection system Advantage no additional collection software is needed vendor independent and ported to a variety of systems Disadvantage native audit records may not contain the needed information or may not contain it in a convenient form extra overhead 8. What are the field are there in audit records? Subject Action Object Exception-Condition Resource-Usage Time-Stamp 9. What are the components of distributed intrusion detection? Host agent module LAN monitor agent module Central manager module 10. Define honeypot. Honeypots are decoy systems that are designed to lure a potential attacker away from critical systems. Honeypots are designed to divert an attacker from accessing critical systems collect information about the attacker s activity encourage the attacker to stay on the system long enough for administrators to respond 32

33 11. What are the techniques are used in password selection strategies? User education Computer-generated passwords Reactive password checking Proactive password checking 12. Define virus. Specify the types of viruses? A virus is a program that can infect other program by modifying them the modification includes a copy of the virus program, which can then go on to infect other program. 13. Define Malicious software (or) malware Malicious software is software that is intentionally included or inserted in a system for a harmful purpose. 14. Define worm. A worm is a program that can replicate itself and send copies from computer to computer across network connections. Upon arrival, the worm may be activated to replicate and propagate again. In addition to propagation, the worm usually performs some unwanted function. 15. Give an example for worms. Morris worm Code red worm Nimda 16. Define Dos. A denial of service (DoS) attack is an attempt to prevent legitimate users of a service from using that service. 17. Define DDoS. A distributed denial of service attack is launched from multiple coordinated sources. 18. Define Logic bomb A program inserted into software by an intruder. A logic bomb lies dormant until a predefined condition is met; the program then triggers an unauthorized act. 19. Define Trojan horse A computer program that appears to have a useful function, but also has a hidden and potentially malicious function that evades security mechanisms, sometimes by exploiting legitimate authorizations of a system entity that invokes the Trojan horse program. 33

34 20. Define Backdoor (trapdoor) Any mechanism that bypasses a normal security check; it may allow unauthorized access to functionality. 21. Define Zombie Program activated on an infected machine that is activated to launch attacks on other machines. 22. What are the types of virus? 1) Parasitic virus 2) Memory-resident virus 3) Boot sector virus 4) Stealth virus 5) Polymorphic virus 23. What are the parts of virus? Infection mechanism Trigger Payload 24. What are the various phases of virus? Dormant phase Propagation phase Triggering phase Execution phase 25. What are the methods are used in Antivirus Techniques? Generic decryption Digital immune system Behavior-blocking software 26. Define firewall. A firewall forms a barrier through which the traffic going in each direction must pass. A firewall security policy dictates which traffic is authorized to pass in each direction. A firewall may be designed to operate as a filter at the level of IP packets, or may operate at a higher protocol layer. 27. What are the characteristics of firewall? 1. All traffic from inside to outside, and vice versa, must pass through the firewall. 2. Only authorized traffic, as defined by the local security policy, will be allowed to pass. 34

35 3. The firewall itself is immune to penetration. 28. What is the scope of firewalls? 1. A firewall defines a single choke point that keeps unauthorized users out of the protected network, prohibits potentially vulnerable services from entering or leaving the network, and provides protection from various kinds of IP spoofing and routing attacks. 2. A firewall provides a location for monitoring security-related events. Audits and alarms can be implemented on the firewall system. 3. A firewall is a convenient platform for several Internet functions that are not security related. 4. A firewall can serve as the platform for IPsec. 29. What are the limitations of firewalls? 1. The firewall cannot protect against attacks that bypass the firewall. 2. The firewall may not protect fully against internal threats 3. It cannot protect against the transfer of virus infected programs or files 30. What are the types of firewall? Packet filtering firewall Application level gateways Circuit level gateways 31. What is application level gateway? An application level gateway also called a proxy server; act as a relay of application-level traffic. The user contacts the gateway using a TCP\IP application, such as Telnet or FTP, and the gateway asks the user for the name of the remote host to be accessed. 32. Define bastion host. A bastion host is a system identified by the firewall administrator as a critical strong point in the network s security. 33. What are the characteristics of bastion host? The bastion host hardware platform executes a secure version of its operating system, making it a hardened system. Only the services that the network administrator considers essential are installed on the bastion host. These could include proxy applications for DNS, FTP, HTTP, and SMTP. 35

36 The bastion host may require additional authentication before a user is allowed access to the proxy services. In addition, each proxy service may require its own authentication before granting user access. Each proxy is configured to support only a subset of the standard application s command set. Each proxy is configured to allow access only to specific host systems. This means that the limited command/feature set may be applied only to a subset of systems on the protected network Each proxy maintains detailed audit information by logging all traffic, each connection, and the duration of each connection. Each proxy module is a very small software package specifically designed for network security. Each proxy is independent of other proxies on the bastion host. A proxy generally performs no disk access other than to read its initial configuration file. Each proxy runs as a non-privileged user in a private and secured directory on the bastion host. 34. Define trusted system. It is used to improve the ability of s system to protect against intruders and malicious programs or software threats. 35. Define access matrix. It is a general method of access control for both file and database managamenet system. The basic elemenst are: Subject Object Access right 36. Compare access control list & capability list. Access control list: For each object, an access control list, lists subjects and their permitted access rights. Capability list: It specifies authorized objects and operations for a user or object. Each subject has a number of tickets and may be authorized to loan or give them to others. 37. Define reference monitor. It is a controlling element in the hardware and operating system of a computer that regulates the access of subjects to objects on the basis of security policies of the subject and object. It has access to a file known as the security kernel database ALL THE BEST