Ias Oral Question

O

ral Questions-IASQ1 What is meant by threat? A potential for violation of security, which exists when there is a circumstances, capability, action or event that could breach security and cause harm. That is, a threat is a possible danger that might exploit vulnerability.

Q2 What is meant by attack? An attack 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

Q3 State some example of security attacks?

Gain unauthorized access to information(ie.violate secrecy or privacy) Disavow responsibility or liability for information the cheater did

originate. Enlarge cheater’s legitimate license (for access, origination, distribution

etc). Pervert the function of software, typically by adding a covert function. Cause others to violate a protocol by means of introducing incorrect

information.

Q4 What are 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 message Denial of service

A masquerade takes place when one entity when one entity pretends to be a different entity. A masquerade attack usually includes one of the other forms of active attack. 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 communication facilities.

Q5 What are passive attacks?

Passive attacks are in the nature of eavesdropping on, or monitoring of, transmissions. Two types of passive attacks are release of message contents and traffic analysis. The release of message contents is easily understood. A telephone conversation, an electronic mail message, and a transferred file may contain sensitive or confidential information.

A second type of passive attack, traffic analysis, is subtler. Suppose that we had a Way of masking the contents of messages or other information traffic so that opponents, even if they captured the message, could not extract the information from the message. Passive attacks are very difficult to detect because they do not involve any alteration of the data.

Q6 What are the essential ingredients of a symmetric cipher? A symmetric encryption scheme has five ingredients:

Plaintext: This is the original intelligible message or data that is fed into the algorithm as input.

Encryption algorithm: The encryption algorithm performs various substitutions and transformations on the plaintext.

Secret Key: The secret key is also input to the encryption algorithm. The key is the value independent of the plaintext. The algorithm will produce a different output depending on the specific key being used at the time. The exact substitutions and transformations performed by the algorithm depend on the key.

Cipher text: This is the scrambled message produced as output. It depends on the plaintext and the key.

Decryption algorithm: This is essentially the encryption algorithm in reverse. It takes the cipher text and the secret key and produces the original plaintext.

Q7 What are the two basic functions used in the encryption algorithm? All the encryption algorithms are based on two general principles:

Substitution: In which each element in the plaintext (bit, letter, group of bits or letters) is mapped into another element.

Transposition: In which elements in the plaintext are rearranged. The fundamental requirement is that no information be lost (that is, that all operations are reversible). Most systems, referred to as product systems, involve multiple stages of substitutions and transpositions.

Q8 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 as symmetric, single-key, secret-key or conventional encryption. If both sender and receiver use a different key, the system is referred as asymmetric, two-key or public key encryption.

Q9 What is the difference between a block cipher and a stream cipher? A block cipher processes the input one block at a time, producing an output block for each input block. A stream cipher processes the input continuously, producing output one element at a time, as it goes alone.

Q10 What are the two general approaches to attacking a cipher? The general two approaches for attacking a cipher

Cryptanalysis: Cryptanalytic attacks rely on the nature of the algorithm plus perhaps some knowledge of the general characteristics of the plaintext or even some samples plaintext-cipher text pairs. This type of attack exploits the characteristics of the algorithm to attempt to deduce a specific plaintext or to deduce the key being used. If the attack succeeds in deducing the key, the effect is catastrophic: All future and past messages encrypted with the key are compromised.

Brute-force attack: The attacker tries every possible key on a piece of cipher text until an intelligible translation into plaintext is obtained. On average, half of all possible keys must be tried to achieve success.

Q11 List and briefly define types of cryptanalytic attacks based on what is known to the attacker? The various types of cryptanalytic attacks, based on the amount of information known to the cryptanalyst

Type of Attack Known to Cryptanalyst

Cipher text Encryption algorithm Cipher text to be decoded

Known plaintext

Encryption algorithm Cipher text to be decoded One or more plaintext-cipher text pairs formed with the secret key

Chosen plaintext

Encryption algorithm Cipher text to be decoded Plaintext message chosen by cryptanalyst, together with its corresponding cipher text generated with the secret key.

Chosen cipher text

Encryption algorithm Cipher text to be decoded Purported cipher text chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the secret key

Chosen text Encryption algorithm Cipher text to be decoded Plaintext message chosen by cryptanalyst, together with its corresponding cipher text generated with the secret key. Purported cipher text chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the

secret key Q12 What is the difference between an unconditionally secure cipher and a computationally secure cipher?

An encryption scheme is unconditionally secure if the cipher text generated by the scheme does not contain enough information to determine uniquely the corresponding plaintext, no matter how much cipher text is available.

An encryption scheme 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 lifetime of the information.

Q13 What is Encipherment? The use of mathematical algorithms to transform data into a form that is not readily intelligible. The transformation and subsequent recovery of the data depend on an algorithm and zero or more encryption keys.

Q14 Define Threats.

Information access threats intercept or modify data on behalf of users who should not have access to that data.

Service threats exploit service flaws in computers to inhibit use by legitimate users.

Q15 What are the four basic tasks in designing a particular security service?

Design an algorithm for performing the security-related transformation. The algorithm should be such that an opponent cannot

defeat its purpose. Generate the secret information to be used with the algorithm. Develop methods for the distribution and sharing of the secret

information Specify a protocol to be used by the two principals that makes use of

the security algorithm and the secret information to achieve a particular security.

Q15 What is the use of digital signature? Data appended to, or a data unit that allows a recipient of the data unit to prove the source and integrity if the data unit and protect against forgery.

Q16 Define security recovery. Security recovery deals with requests from mechanisms, such as event handling and management functions, and takes recovery actions.

Q17 What are the aspects of information security? There are three aspects of the information security.

Security attack Security mechanism Security Service

Q18 List some common information integrity functions?

Identification Authorization Concurrence Liability Endorsement Validation Time of occurrence Registration

Q19 Briefly define the Caesar cipher?

The Caesar cipher involves replacing each letter of the alphabet with the letter standing three places down the alphabet .The alphabet is wrapped around, so that the letter following Z is A.

C = E (p) = (p + 3) mod (26)

The general Caesar cipher algorithm is

C = E (p) = (p + k) mod (26)

where k takes the value in the range 1 to 25

The decryption algorithm is

p = D(C) = (C - k) mod (26)

Q20 Briefly define the monoalphabetic cipher?

A dramatic increase in the key space is achieved by allowing an arbitrary substitution. There are 26! Possible keys. It is referred to as monoalphabetic

substitution cipher, because a single cipher alphabet is used per message.

Q21 What is the difference between a monoalphabetic cipher and a polyalphabetic cipher?

In monoalphabetic cipher single cipher alphabet is used per message. But in polyalphabetic cipher there are multiple ciphertext letters for each plaintext letter, one for each unique letter of keyword.

Q22 What are two problems with the one-time pad? The one- time pad has the following two fundamental difficulties:

There is the practical problem of making large quantities of random keys. Supplying truly random characters in this volume is a significant task.

For every message to be sent, a key of equal length is needed by both sender and receiver. Thus a mammoth key distribution problem exists.

Unit IIQ24 Why is it important to study the Feistel Cipher?

Feistel cipher using 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 then any of the component ciphers.

Feistel proposed the use of a cipher that alternates substitutions and permutations. So Feistel cipher is considered to be an important one.

Q25 What is the difference between a block cipher and a stream cipher?

A block cipher process the input one block of elements at a time producing

an output block for each input block.

A stream cipher process the input elements continuously, producing output one element at a time, as it goes along.

Q26 Why is it not practical to use an arbitrary reversible substitution cipher of the kind shown in Table?

Encryption and decryptions tables for substitution cipher (any example)

In general, for an n-bit general substitution block cipher, the size of the

key is n*2n . For a 64-bit block, which is a desirable length to thwart statistical attacks, the key size is 64*264 = 270 ~ 1021 bits. So it is not practical to use an arbitrarily reversible substitution cipher.

Q27 What is product cipher? Product cipher has the performance of two or more basic ciphers in sequence is such a way that the final result or product is cryptographically stronger than any of the component ciphers.

Q28 What is the difference between Diffusion and Confusion?

In Diffusion the statistical structure of the plaintext is dissipated into long range statistics of the cipher text. This is achieved by having each plaintext digit affect the value of many cipher text digits. Which is equivalent to saying that each cipher text digit is affected by many plaintext digits.

Confusion seeks to make a relationship between the statistics of the cipher text and the value of the encryption key as complex as possible. Thus even if the attacker can get some handle on the statistics of the cipher text, the way in which the key was used to produce that cipher text is so complex as to make it difficult to deduce the key. Q29 Which parameters and design choices determine the actual algorithm of a Feistel cipher?

Block size: Larger block sizes mean greater security but reduced encryption/decryption speed. A block size of 64 bits is a reasonable tradeoff and has been nearly universal in block cipher design. However, the new AES uses a 128-bit block size.

Key size: Larger key size means greater security but may decrease encryption/decryption speed. Key sizes of 64 bits or less are now

widely considered to be inadequate, and 128 bits has become a common size.

Number of rounds: The essence of the Feistel cipher is that a single round offers inadequate security but that multiple rounds offer increasing security.A typical size is 16 rounds.

Subkey generation algorithm: Greater complexity in this algorithm should lead to greater difficulty of cryptanalysis.

Round function: Again, greater complexity generally means greater resistance to cryptanalysis.

Q30 What is the purpose of the S-boxes in DES? The role of the S-boxes in the function F is that the substitution consists of a set of eight S-boxes ,each of which accepts 6 bits as input and produces 4 bits as follows: The first and last bits of the input to box Si form a 2-bit binary number to select one of four substitutions defined by the four rows in the table for Si. The middle four bits select one of the sixteen columns. The

decimal value in the cell selected by the row and column is then converted to its 4-bit representation to produce the output. For example, in S1,for input 011001,the row is 01 and the column is 1100.The value in row 1,column 12 is 9,so the output is 1001.

Q31 Explain the avalanche effect?

A desirable property of any encryption algorithm is that a small change in either the plaintext or the key should produce a significant change in the ciphertext.In particular, a change I one of the plaintext or one bit of the key should produce a change in many bits of the ciphertext.

Q32 What is the difference between differential and linear cyptanalysis?

Linear cryptanalysis based on finding linear approximations to describe the transformations performed in DES

Q33 What are the two different uses of public-key cryptography related to key distribution?

There are two aspects to the use of public-key cryptography In this regard:

The distribution of public keys The use of public-key encryption to distribution secret keys

Q34 List four general categories of schemes for the distribution of public keys.

Public announcement Publicly available directory Public-key authority

Public-key certificates

Q35 What are the essential ingredients of a public-key directory?

The authority maintains a directory with an entry for each participant. Each participant registers a public key with the directory authority. A participant may replace the existing key with a new one at any time. Periodically, the authority publishes the entire directory or updates to

the directory.

Participants could also access the directory electronically.

Q36 What is public-key certificate?

The public-key authority could be a bottleneck in the system, for a user must appeal to the authority for a public key for every other user that it wishes to contact. As before the directory of names and public keys maintained by the authority is vulnerable to tempering.

Q37 What are the requirements for the use of a public-key certificate scheme?

Any participant can read a certificate to determine the name and public key of the certificate’s owner.

Any participant can verify that the certificate originated from the certificate authority and is not counterfeit.

Only the certificate authority can create and update certificates. Any participant can verify the currency of the certificate

Q38 Briefly explain Diffie-Hellman key exchange.

The purpose for this algorithm is to enable two users to exchange a key securely that can then be used for subsequent encryption of messages. It depends for its effectiveness on the difficulty of computing discrete logarithms.

Q39 What types of attacks are addressed by message authentication?

Content modification - Changes to the contents of the message Sequence

modification - Any modification to a sequence of messages between parties, including insertion, deletion, and reordering. Timing

modification - Delay or replay of messages.

Q40 What two levels of functionality comprise a message authentication or digital signature mechanism?

Low-level authentication Higher-level authentication

At the lower level there must be some sort of function that produces an authenticator: a value to be used to authenticate a message. This lower level function is then used as primitive in a higher-level authentication

protocol that enables a receiver to verify the authenticity of message.

Q41 What are some approaches to producing message authentication? Message encryption The cipher text of the entire image serves as its

authenticator. Message

authentication code (MAC)

A public function of the message and a secret key that produces a fixed length value that serves as an authenticator.

Hash function A public function that maps a message of any length into a fixed– length hash value, which serves as the authenticator.

Q 42 When combination of symmetric encryption and an error control code is used for message authentication, in what order must the two functions be performed?

The message is encrypted first, and then the MAC is calculated using the resulting cipher text to form the transmitted block.

Q43 What is a message authentication code?

An alternative authentication technique involves the use of a small fixed size block of data, known as a cryptographic checksum or MAC that is appended to the message.

Q44 What is the difference between a message authentication code and a one-way hash function?

The difference between an MAC and a one-way hash function is that unlike an MAC, a hash code does not use a key but is a function only of the input message.

Q45 In what ways can a hash value be secured so as to provide message authentication? The ways in which a hash code can be used to provide message authentication are:

The message plus concatenated hash code is encrypted using symmetric encryption. The hash code provides the structure required for authentication.

Only the hash code is encrypted using symmetric encryption. This reduces the processing burden.

Only the hash code is encrypted using public key encryption and the sender’s private key. This provides digital signature.

The message plus the public key-encrypted hash code may be encrypted using a symmetric secret key.

A hash function may be used without encryption for message authentication. It assumes that two communicating parties (A and B)

share a common key (s). ’A’ computes the hash value over the concatenation of M and S.B knows S and therefore can re-compute M.

The entire message plus the hash code may be encrypted.

Q46 Is it necessary to recover the secret key in order to attack a MAC algorithm?

A number of keys will produce the correct MAC and the opponent has no way of knowing which the correct key is. On an average 2(n-k) keys produce a match. Therefore attacks do not require the discovery of the key.

Q47 What characteristics are needed in a secure hash function? Requirements of a hash function(H):

H can be applied to a block of data of any size. H produces a fixed length output. H(x) is easy to compute for any given x For any given value h it is computationally infeasible to find y/x with

H(y) =H(x). It is computationally infeasible to find any pair (a, b) such that H(x)

=H(y).

Q48 What is the difference between a strong and a weak collision resistance?

For any given value h it is computationally infeasible to find y/x with H(y) =H(x).This is “weak collision resistance”. It is a one-way property. It is easy to generate a code given a message, but almost impossible to do the reverse.

It is computationally infeasible to find any pair (a, b) such that H(x) =H(y).This is “strong collision resistance”. This guarantees that an alternative message hashing to the same value as a given message cannot be found. This prevents forgery.

Q49 What is the function of a compression function in a hash function?

The hash function involves repeated use of a compression function. The motivation is that if the compression function is collision resistant, then the hash function is also collision resistant function. So a secure hash function can be produced.

Q50 Explain Message Authentication Code

This technique assumes that two communicating parties A and B share a common key K.When A sends a message to B it calculates the MAC as a function of the

message and the key: MAC=CK{M), Where,

M=input message C=MAC function K=shared secret key MAC=message authentication code

The message plus the MAC are transmitted to the recipient. The recipient performs the same calculation on the received message to generate a new MAC. The received MAC is compared to the calculated MAC. If only the sender and receiver know the secret key, if the received MAC matches the calculated MAC , then

The receiver is assured that the message has not been altered. The receiver is assured that the message is from the alleged sender. If the message includes a sequence number then the receiver is

assured of the proper sequence. The MAC function need not be reversible. Usually, it is a many-to-one function. If there are N possible messages then an n bit MAC is used where N>>2n and there are 2k possible keys where the key has k bits.

For example, if we are using 100 bit messages then there are 2100 different messages and if a 10 bit MAC is used there are 210 different MACs. On an average each MAC value is generated by a total of (2100/210) =290 different messages. If a 5bit key is used there are 25=32 different mappings from a set of messages to a set of MAC values. Usually two separate keys are used each of which is shared by the sender and receiver. The message is calculated with the message as input and is then concatenated to the message. The entire block is then encrypted. (For diagrams refer: Prescribed Text Book)

MAC is used when:

There are a number of applications in which the same message is broadcast to many destinations.

When there exchanges where there is heavy load on one side and there is no time to decrypt.

For authentication of a computer in plain text. When it is not needed to keep messages secret but it is important to

provide authentication. Because separation of authentication and confidentiality provides

architectural flexibility. When users wish to prolong the period of protection beyond the time of

reception and yet allow processing of message contents. MAC does not provide digital signature because both sender and receiver share the same key.

Q51 Explain Hash Function

A variation on the message authentication code is the one-way hash function. As with the message authentication code, a hash function accepts a variable size message M as input and produces a fixed-size output, referred to as hash code H (M).

A variety of ways in which hash code can be used to provide message authentication, as follows:

The message plus concatenated hash code is encrypted using symmetric encryption.

Only the hash code is encrypted using symmetric encryption. Only the hash code is encrypted using the public-key encryption and using

the sender’s private key. If confidentiality as well as a digital signature is desired, then the message

plus the public key encrypted hash code can be encrypted using a symmetric secret key.

This technique uses a hash function but no encryption for message authentication.

Confidentiality can be added to the approach of (e) by encrypting the entire message plus the hash code.

Unit IIIQ52 What problem was Kerberos designed to address? The problem that Kerberos addresses is this:

Assume an open distributed environment in which users at workstations wish to access services on servers distributed throughout the network.

We would like for servers to be able to restrict access to authorized users and to be able to authenticate requests for service.

In this environment a workstation cannot be trusted to identify its users correctly to network services.

Q53 What are the three threats associated with user authentication over a network or Internet? The three threats are:

A user may gain access to a particular workstation and pretend to another user operating from that workstation.

A user may alter the network address of a workstation so that the requests sent from the altered workstation appear to come from the impersonated workstation.

A user may eavesdrop on exchanges and use a replay attack to gain entrance to a server or to disrupt operations.

Q54 List three approaches to secure user authentication in a distributed

environment? Three approaches to secure user authentication in a distributed environment are:

Rely on each individual client workstation to assure the identity of its user or users and rely on each server to enforce a security policy based on user identification (ID).

Require that client systems authenticate themselves to servers, but trust the client system concerning the identity of its user.

Require the user to prove identity for each service invoked. Also require that servers prove their identity to clients.

Q55 What four requirements were defined for Kerberos? The four requirements defined for Kerberos are:

Secure A network eavesdropper should not be able to obtain the necessary information to impersonate a user. More generally Kerberos should be strong enough that a potential opponent does not find it to be the weak link.

Reliable For all services that relay on Kerberos for access control, lack of availability of the supported services. Hence, Kerberos should be highly reliable and should employ distributed server architecture, with one system able to back up another.

Transparent Ideally, the user should not be aware that authentication is taking place, beyond the requirement to enter a password.

Scalable The system should be capable of supporting large numbers of clients and servers. This suggests a modular, distributed architecture

Q56 What entities constitute a full-service Kerberos environment?

A full service environment consists of a Kerberos server, a number of clients and a number of application servers.

Q57 In the context of Kerberos, what is a realm?

The Kerberos server must have the user ID (UID) and hashed password of all participating users in its database. All users are

registered with the Kerberos server.

The Kerberos server must share a secret with each server. Such an environment is referred to as realm

Q58 What are the principle differences between version 4 and version 5 of Kerberos? The principle differences between version 4 and version 5 of Kerberos are:

Encryption system dependence Internet Protocol Dependence Message byte ordering Ticket Life Time Authentication Forwarding Interrealm Authentication

U

nit IVQ59 What are the five principle services provided by the PGP?

Function Algorithm used Description

Digital Signature DSS\SHA or RSA\SHA

The hash code of a message is created using SHA1.This message

digest is encrypted using DSS or RSA with the sender’s private key and included with the message

Message Encryption

CAST or IDEA or Three-key Triple DES with Diffie-Hellman or RSA A message is encrypted using

CAST-128 or IDEA or 3DES with a one-time session key generated by the sender. The session key is encrypted using Diffie-Hellman or RSA with the recipient’s public key and included with the message

Compression ZIP A message may be compressed, for storage or transmission using ZIP

Email compatibility Radix 64 conversion

To provide transparency for email applications, an encrypted message may be converted to an ASCII string using radix 64 conversion

Segmentation ------ To accommodate maximum message size limitations, PGP performs segmentation and reassembly

Q60 What is the utility of a detached signature?

A detached signature may be stored and transmitted separately from the message it signs. This is useful in several contexts. A user may wish to maintain a separate signature log of all messages sent or received. A detached signature of an executable program can detect subsequent virus infection. Finally detached signature can be used when more than one party must sign a document, such as legal contract.

Q61 Why does PGP generate a signature before applying compression? The signature is generated before compression due to 2 reasons:

It is preferable to sign an uncompressed message so that one can store only the uncompressed message together with the signature for future verification

Even if one were willing to generate dynamically a recompressed message for verification, PGP’s compression algorithm presents a difficulty

Q62 What is Radix 64 conversion? Radix 64 converts the input stream into radix 64 format. It expands a message by 33%

Q63 Why is R 64 conversion useful for email generation?

The Radix 64 conversion is performed before the segmentation of the messages take place The use of radix 64 is that it converts he input stream to 33%. The

radix 64 converts the input stream to a radix 64 format

Q64 What is MIME?

Multipurpose Internet Mail Extensions (MIME) is an extension to the RFC 822 framework that is intended to address some of the problems and limitations of these use of SMTP.

Some of limitations: -

It cannot transmit executable files or folders. SMTP servers may reject, mail message over a certain size.

Q65 What is S/MIME?

Secure/Multipurpose Internet Mail Extension is a security enhancement to the MIME Internet e-mail format standard, based on technology from RSA Data Security. It is ability to sign and/or encrypt messages.

Q66 What is RFC 822?

RFC 822 defines a format for text messages that are sent using electronic mail. It has been the standard for Internet-based text message and remains in common use. In the RFC822 context, messages are viewed as having an

envelope and contents. The envelope contains whatever information needed to accomplish transmission and delivery. The contents compose the object to be delivered to the recipient.

Q67 How does PGP use the concept of trust?

PGP provide a convenient means of using trust, associating trust with public keys, and exploiting trust information. Each entry in the public-key ring is a public key certificate.

Associated with each such entry is a key legitimacy field that indicates the extent to which PGP will trust that this is a valid public key for this user; the higher the level of trust, the stronger is the binding of this user ID to this key.

Q68 Write short notes on S/MIME?

Secure/Multipurpose Internet Mail Extension is a security enhancement to the MIME Internet e-mail format standard, based on technology from RSA Data Security. It is ability to sign and/or encrypt messages.

RFC 822 defines a format for text messages that are sent using electronic mail. It has been the standard for Internet-based text message and remains in common use. In the RFC822 context, messages are viewed as having an envelope and contents. The envelope contains whatever information needed to accomplish transmission and delivery. The contents compose the object to be delivered to the recipient.

Multipurpose Internet Mail Extensions (MIME) is an extension to the RFC 822 framework that is intended to address some of the problems and limitations of the use of SMTP.

The MIME specification includes the following elements:

1. Five message header fields are defined which may be included in and RFC 822 header.

2. A numbers of content formats are defined, thus standardizing representations that support multimedia electronic mail.

3. Transfer encodings are defined that enable the conversion of any content format into a form that is protected from alteration by the mail system.

Q69 What are the limitations of SMTP? Some of limitations of SMTP are: -

1. It cannot transmit executable files or folders. 2. SMTP servers may reject, mail message over a certain size. 3. Some SMTP implementations do not adhere completely to the

SMTP standards defined in RFC 821.

Common problems include

Deletion, addition, or reordering of carriage return and linefeed Truncating or wrapping lines longer than 76 characters Removal of trailing white space Padding of lines in a message to the same length Conversion of tab characters into multiple space characters

Q69 Explain S/MIME Functionality It offers ability to sign and/or encrypt messages.

Functions

Enveloped Data: This consists of encrypted content of any type and encrypted content encryption keys for one or more recipients.

Signed Data: A digital signature is formed by taking the message digest of the content to be signed and then encrypting that with the private key of signer. The content plus signature are the encoded using base64 encoding.

Clear-signed data: As with assigned data, a digital signature of the content is formed. In this case only the digital signature is encoded using base64.

Signed and enveloped data: Signed-only and encrypted-only entities may be nested, so that encrypted data may be signed and signed data or clear-signed data may be signed and signed data or clear-signed data may be encrypted.

Cryptographic algorithms

Function Requirement Create a message digest to be used in Forming a digital signature

MUST support SHA-1. Receiver SHOULD support md5 for backward compatibility

Encrypt session key for transmission with message

Sending and receiving agents MUST support Diffie-Hellman. Sending agents Should support RSA encryption with key sizes 51 bits to 1024 bits.

MUST: The definition is an absolute requirement of the specification. An implementation must include this feature or function to be in conformance with the specification.

SHOULD: There may exist valid reasons in particular circumstances to ignore this feature or function, but it is recommended that an implementation include this feature of function.

S/MIME incorporates three public-key algorithms. The Digital Signature Standard (DSS) is the preferred algorithm for digital signature. S/MIME uses a variant of Diffie-Hellman that does provide encryption/decryption.

The S/MIME specification includes a discussion of the procedure for deciding which content encryption algorithm to use.

A sending agent should follow the following rules, in the following order:

1. The sending agent SHOULD choose the first capability on the list that it is capable of using.

2. If the sending agent has no such list of capabilities from an intended recipient but has received one or more messages from the recipient, then the outgoing message SHOULD use the same encryption algorithm as was used on the last signed and encrypted message received from that intended recipient.

3. If the sending agent has no acknowledge about the decryption capabilities of the intended recipient and is willing to risk that the recipient may not be able to decrypt the message, then the sending agent SHOULD use tripleDES.

4. If the sending agent has no knowledge about the decryption capabilities of the intended recipient and is not willing to risk that the recipient may not be able to decrypt the message, then the sending agent MUST use RC2/40.

Q70 How to Secure a MIME Entity

S/MIME secures a MIME entity with a signature, encryption, or both. A MIME entity may be an entire message, or if the MIME content type is multipart, then a MIME entity is one or more of the subparts of the message. Then the MIME entity plus some security related date, such as algorithm identifies and certificated, are processed by S/MIME to produce what is known as a PKCS object. A PKCS object is then treated as message content and wrapped in MIME.

S/MIME Certificate Processing

S/MIME uses public-key certificates. The key-management scheme used by S/MIME is in some ways a hybrid between a strict X.509 certification hierarchy and PGP’s web of trust. As with the PGP model, S/MIME managers and/or users must configure each client with a list of trusted keys and with certificate revocation lists. That is the responsibility is local for maintaining the certificated needed to verify incoming

signatures ad to encrypt outgoing messages. On the other hand, the certificates are signed by certification authorities.

An S/MIME user has several key management functions to perform:

1. Key generation: Each key pair MUST be generated from a good source of nondeterministic random input and be protected in a secure fashion.

2. Registration: A user’s public key must be registered with a certification authority in order to receive an X.509 public key certificate.

3. Certificate storage and retrieval: A user requires access to local list of certificated in order to verify incoming signatures and to encrypt outgoing messages.

Q71 Explain briefly PRETTY GOOD PRIVACY

PGP provides a confidentiality and authentication service that can be used for electronic mail and file storage applications. Phil Zimmermann has done the following:

1. Selected the best available cryptographic as building blocks 2. Integrated these algorithms into a general purpose application that is

independent of operating system and processor and that is based on a small set of easy to use commands

3. Made the package and its documentation including the source code, freely available via internet ,bulletin boards ,and commercial networks such as AOL

4. Entered into an agreement with a company to provide a fully compatible, low cost commercial version of PGP.

PGP has grown explosively and now widely used. A number of reasons can be cited for this growth,

1. It is available free worldwide in versions that run on a variety of platforms,including windows ,UNIX,Macintosh,and many more.

2. It is based on algorithms that have survived extensive public review and are considered extremely secure.

3. It has a wide range of applicability from corporations that wish to select and enforce a standardized scheme for encrypting files and messages to individuals who wish to communicate securely with others worldwide over the internet and other networks .

4. It was not developed by ,nor is it controlled by, any governmental or standard organization

5. PGP is now on an internet standards track.

Notation

Ks =Session key used in symmetric encryption scheme

KRa=Private key of user A used in public key encryption scheme.

KUa= Public key of user A, used in public key encryption scheme

EP = Public key encryption

DP = Public key decryption

EC = Symmetric encryption

DC = Symmetric decryption

H = Hash function

|| = Concatenation

Z = compression using ZIP algorithm

R64 = Conversion to radix 64 ASCII format

Q72 Explain PGP services Operational Description PGP provides five services

1. Authentication – DSS/SHA or RSA/SHA 2. Confidentiality – CAST or IDEA or 3DES Diffe or RSA 3. Compression –ZIP 4. Email Compatibility – Radix 64 conversion 5. Segmentation

Authentication

1. The sender creates a message 2. SHA-1 is used to generate a 160 bit hash code of the

message 3. The hash code is encrypted with RSA using the sender’s

private key,and the result is prepended to the message 4. The receiver uses RSA with the sender’s public key to

decrypt and recover the hash code 5. The receiver generates a new hash code for the message

and compares it with the decrypted hash code

Confidentiality

6. The sender generates a message and random 128 bit number to be used as a session key for this message only

7. The message is encrypted ,using CAST -128 with the session key

8. The session key is encrypted with RSA with its private key to decrypt and recover the session key

9. The session key is used to decrypt the message 10.The receiver uses RSA with its private key to decrypt and

recover the session key

Compression

11.The signature is generated before compression for 2 reasons

It is preferable to sign an uncompreesed message so that one can store only the compressed message together with the signature for future verification

Even if one were willing to generate dynamically a recompressed message for verification

12.The message encryption is applied after compression to strengthen cryptographic security

Email CompatibilityWhen PGP is used; at least part of the block to be transmitted is encrypted. If only the signature service is used then the message digest is encrypted. If the confidentiality service is used, the message plus signature are encrypted .Thus part or the entire resulting block consists of a stream of arbitrary 8 bit octets.

Segmentation and reassemblyEmail facilities often are restricted to a maximum message length. To accommodate this restriction PGP automatically subdivides the message that is too large into segments that are small enough to send via email

What are Cryptographic Keys and Key rings? Explain

1. A means of generating unpredictable session keys is needed 2. We would like to allow a user to have multiple public-key/private-

key pairs the user may wish to change his or her key pair from time to time. Also the recipient will know only the old private key until an update reaches them

3. Each PGP entity must maintain a file of its own public/private key pairs as well as a file of public keys of correspondence

Key Rings

Private Key RingsTimestamp: The date/time when this key pair was generatedKey ID: The least significant 64 bits of the public key for the entryPublic key: The public key portion of the pairPrivate key: The private key portion of the pairUser ID: is the user’s email address

Public key RingsTimestamp: The date/time when the entry was generatedKey ID: The least significant 64 bits of the public key for this entryPublic key: the public keys for this entryUserID: Identifies the owner of this key

Unit V Give examples of applications of IPSec?

Secure branch office connectivity over the Internet. Secure remote access over the Internet. Establishing extranet and intranet connectivity with partners. Enhancing electronic commerce security.

What services are provided by IPSec?

Access control Connectionless integrity Data origin authentication Rejection of replayed packets

What parameters identify an SA and What parameter Characterize the nature of a particular SA? A security association(SA) is uniquely identified by three parameters,

Security Parameter Index(SPI) IP Destination Address Security Protocol Identifier

The parameters that characterize the particular SA is

Sequence number counter Sequence counter overflow Anti-Replay window AH Information ESP Information

Lifetime of this SA IPSec Protocol Mode Path MTU

What is the difference between Transport mode and Tunnel mode? Transport Mode Tunnel Mode

1. It provides protection for upper layer protocols.

2. Used for end-to-end communication between two host

3. AH:Authenticates IP payload and selected portions of IP header and IPv6 extension header

4. It provides protection to the entire IP packet.

5. It is used when one or both ends of an SA is a security gateway, such as firewall or router that implement IPSec.

6. Authenticates enter inner IP packet plus selected portions of outer IP header and outer IPv6 extension headers.

What is replay attack?

A replay attack is one which an attacker obtains a copy of an authenticated packet and later transmits it to the intended destination.

Why does ESP include a padding field?

Padding field is added to the ESP to provide partial traffic flow confidentiality by concealing the actual length of the payload.

What are the basic approaches to bundling SAs?

1. Transport adjacency Refers to applying more than one security protocol to the same

packet, without invoking tunneling. 2. Iterated tunneling

Refers to the application of multiple layers of security protocol affected through IP tunneling.

Write about the AH associated with IPSec

The authentication header provides support for data integrity and authentication of IP packets. The data integrity feature ensures that undetected modification to a packet’s content in transit is not possible. The authentication feature enables an end system or network to authenticate the user or application and filter traffic accordingly.

Authentication is based on the use of a message authentication code (MAC) The authentication header consists of the following fields.

1. Next header (8 bits): Identifies the type of header immediately following this header.

2. Payload length (8 bits): Length of authentication header in 32-bit words, minus 2.

3. Reserved (16 bits): For future use. 4. Security parameters index (32 bits): Identifies a security association. 5. Sequence number (32 bits): A monotonically increasing counter value. 6. Authentication data (variable): A variable-length field that contains the

Integrity Check Value.

Anti-replay serviceA replay attack is one in which an attacker obtains a copy of an authenticated packet and later transmits it to the intended destination. The sequence number field is designed to thwart such attacks.

Integrity check valueThe authentication data field holds a value referred to as the integrity check value. The ICV is a message authentication code or a truncated version of a code produced by a MAC algorithm.

Transport and tunnel modesThese are the two ways in which the IPSec authentication service can be used. In one case authentication is provided directly between a server and client work stations; the work station can be either on the same network as the server or on an external network. As long as the work station and the server share a protected secret key, the authentication process is secure. This case uses a transport mode SA. In the other case a remote work station authenticates itself to the corporate firewall, either for access to the entire internal network or because the requested server does not support the authentication feature. This case uses a tunnel mode SA.

For transport mode AH using IPv4, the AH is inserted after the original IP header and before the IP payload.

In the context of IPv6, the AH is viewed as an end-to-end payload; that is it is not examined or processed by intermediate routers. Therefore the AH appears after the IPv6 base header and the hop-by-hop, routing and fragment extension headers.

For tunnel mode AH the original IP packet is authenticated, and the AH is inserted between the original IP header and a new outer IP header Write about the ESP associated with IPSec ENCAPSULATING SECURITY PAYLOAD

The encapsulating security payload provides confidentiality services,

including confidentiality of message contents and limited traffic flow confidentiality. As an optional feature, ESP can also provide the same authentication services as AH.

ESP FormatESP packet contains the following fields

1. Security parameters index (32 bits): Identifies a security association. 2. Sequence number (32 bits): A monotonically increasing counter value;

this provides an anti-replay function, as discussed for AH. 3. Payload data (variable): This is a transport level segment (transport

mode) or IP packet (tunnel mode) that is protected by encryption. 4. Padding(0-255 bytes):The padding field serves several purposes:

If an encryption algorithm requires the plain text to be a multiple of some number of bytes the padding field is used to expand the plain text to the required length.

The ESP format requires that the cipher text must be an integer multiple of 32 bits. The padding field is used to assure this alignment.

Additional padding may be added to provide partial traffic flow confidentiality by concealing the actual length of the payload.

5. Pad length (8 bits): Indicates the number of pad bytes immediately preceding this field.

6. Next header (8 bits): Identifies the type of data contained in the payload data field.

7. Authentication data (variable): A variable length field that contains the Integrity Check Value computed over the ESP packet minus the authentication data field.

Transport and tunnel modesFigure shows the two ways in which IPSec ESP services can be used. Hosts on the internal networks use the internet for the transport of data but do not interact with the other internet based hosts. By terminating the tunnels at the security gateway to each internal network the configuration allows the hosts to avoid implementing the security capability. The former technique is supported by a transport mode SA, while the latter technique uses a tunnel mode SA.

Transport mode ESPTransport mode ESP is used to encrypt and optionally authenticate the data carried by IP. For this mode using IPv4, the ESP header is inserted into the IP packet immediately prior to the transport layer header and an ESP trailer is placed after the IP packet.

In the context of IPv6, ESP is viewed as an end-to-end payload; that is it is not examined or processed by intermediate routers. Therefore the ESP header appears after the IPv6 base header and the hop-by-hop, routing and fragment extension headers. The destination option extension could appear before or

after the ESP header, depending on the semantics required. For IPv6 encryption covers the entire transport level segment plus the ESP trailer plus the destination options extension header if it occurs after the ESP header.

Tunnel mode ESPTunnel mode ESP is used to encrypt the entire IP packet. For this mode the ESP header is prefixed to the packet and then the packet plus the ESP trailer is encrypted. This method can be used to counter the traffic analysis. Write about how security associations can be combined. An individual SA can implement either the AH or ESP protocol but not

both. Sometimes a particular traffic flow will call for the services provided by both

AH and ESP. Multiple SA must be employed for the same traffic flow to achieve the desired IP services.

The term security association bundle refers to a sequence of SAs through which traffic must be processed to provide a desired set of IPSec services. The SAs in a bundle may terminate at different endpoints or at the same endpoints.

Security associations may be combined into bundle in two types. Transport adjacency: Refers to applying more than one security

protocol to the same IP packet without invoking tunneling. Iterated tunneling: Refers to the application of multiple layers of

security protocols affected through IP tunneling.

The two approaches can be combined for example by having a transport SA between hosts travel part of the way through a tunnel SA between security gateways

Authentication plus confidentialityEncryption and authentication can be combined in order to transmit an IP packet that has both confidentiality and authentication between hosts.

Esp. with authentication optionIn this approach the user first applies ESP to the data to be protected and the appends the authentication data field. There are two sub cases.

Transport mode ESP: Authentication and encryption apply to the IP payload delivered to the host but the IP header is not protected.

Tunnel mode ESP:

Authentication applies to the entire IP packet delivered to the outer IP destination address and authentication is performed at that destination.

For both the cases authentication applies to cipher text rather than the plain text.

Transport adjacencyAnother way to apply authentication after the encryption is to use two bundle transport SAs with the inner being an ESP SA and the outer being an AH SA. In this case ESP is used without its authentication option. Because the inner SA is a transport SA, encryption is applied to the IP payload. The resulting packet consists of an IP header followed by an ESP. AH is then applied in the transport mode so that authentication covers the ESP plus the original IP header except for mutable fields.

Transport-Tunnel bundleOne approach to applying authentication before encryption between two hosts is to use a bundle consisting of an inner AH transport SA an outer ESP tunnel SA. IN this case authentication is applied to the IP payload plus the IP header except for the mutable fields. The resulting IP packet is then processed in the tunnel mode by the ESP; the result is that the entire authenticated inner packet is encrypted and a new outer IP header is added.

Basic combinations of security associations

In case1 all security is provided between end systems that implement IPSec. For any two end systems to communicate via an SA they must share the appropriate secret keys. The following are among the possible combinations. • AH in transport mode. • Esp. in transport mode. • AH followed by ESP in transport mode. • Any one of a,b, or c inside an AH or ESP in tunnel mode.

For case2 security is provided only between gateways and no hosts implement IPSec.

Case3 builds on case2 by adding end-to-end security. The gateway –to-gateway tunnel provides either authentication or confidentiality or both for all traffic between end systems. When the gateway-to-gateway tunnel is ESP it also provides a limited form of traffic confidentiality.

Case4 provides support for a remote host that uses the internet to reach an organization’s firewall and then to gain access to some server or workstation behind the firewall. Only tunnel mode is required between the remote host and the firewall.

Unit VI What protocols comprise SSL?

The protocols that comprise SSL are:SSL Handshake Protocol,SSL Change Cipher Spec Protocol,SSL Alert Protocol,Hypertext Transfer Protocol(HTTP) and SSL Recprd Protocol.

What is the difference between an SSL connection and an SSL session?

A Connection is a transport that provides a suitable type of service. For SSL, such connections are peer-to-peer relationships. The connections are transient.

An SSL session is an association between a client and a server. Sessions are created by the Handshake Protocol. Sessions define a set of cryptographic security parameters, which can be shared among multiple connections. List and briefly define the parameters that define an SSL session state. A session state is defined by the following parameters:

Session identifier Peer certificate Compression method Cipher spec Master secret Is resemble

List and briefly define the parameters that define an SSL session connection. A connection state is defined by the following parameters:

Server and client random Server write MAC secret Client write MAC secret Server write key Client write key Initialization vectors Sequence numbers

Unit VIII List the three design goals for a firewall.

1. All traffic from inside to outside, and vice versa, must pass through the firewall. This is achieved by physically blocking all access to the local network except via the firewall.

2. Only authorized traffic, as defined by the local security policy,

will be allowed to pass. Various types of firewalls are used, which implement various types of security policies.

3. The firewall itself is immune to penetration. This implies that use of a trusted system with a secure operating system.

List four techniques used by firewalls to control access and enforce a security policy.

Service control Direction control User control

Behavior control

What are the differences among the three configuration of firewall?

In the screened host firewall, single-homed bastion configuration, the firewall consists of two systems: a packet-filtering router and a bastion host.

In the screened host firewall, dual-homed bastion configuration prevents such a security breach.

In the screened subnet firewall configuration is the most secure one. Here two packet filters are used, one between the bastion host and the Internet and one between the bastion host and the internal network. In the context access control, what is the difference between a subject and an object?

Subject is an entity capable of accessing objects. Any user or application actually gains access to an object by means of a process that represents that user or application. Object is one in which anything is controlled.eg.files, programs and segments of memory.

What is the difference between an access control list and a capability ticket?

An access control list lists users and their permitted access rights for each object.

A capability ticket specifies authorized objects and operations for a user. Each user has a no. of tickets and may be authorized to loan or give them to others.