IS UNIT 3 PPT- PART 2.pptx is very helpful for engineering students of any Electronics and Communication batch

User Authentication
 Something the individual knows : (passwords, pin , tokens, patterns, etc.
 Something the individual have (static biometrics): e.g. fingerprint, retina ,face
 Something the individual does (dynamic biometrics): e.g. voice, handwriting
 These All can be used alone or combined.
 All can provide user authentication.
 All have issues.
 Note that user authentication is different from message authentication.
 Message authentication is the procedure that allows two parties to verify that
contents of received message have not been changed and source is authentic.

Authentication Protocols
 An important application area of authentication is authentication
protocols.
 It may be one-way authentication protocol or mutual
authentication protocol
 Mutual Authentication :
 Mutual Authentication Protocols used to satisfy or convince
communicating parties about each others identity and to exchange
session keys
 Two key issues are :
 Confidentiality – to protect session keys.
 Timeliness – to prevent replay attacks.

Replay Attacks :
– Simple replay (Opponent copy the message and replay it later)
– Repetition that can be logged
– Repetition that cannot be detected
– Backward replay without modification
• The countermeasures include :
– Use of sequence numbers (generally impractical)
– Timestamps (needs synchronized clocks)
– Challenge/Response (using unique nonce)

 One-Way Authentication :
 It is required when sender & receiver are not in communications at
same time
 (e.g. email) because it is not necessary for the sender and receiver
be online at the same time
 The message is forwarded to the receiver ‘s mailbox , where it is
buffered till the receiver read it
 Header will deliver by store and forward email protocol , like SMTP
 It want contents to be protected & sender authenticated .

Kerberos
 Kerberos is authentication protocol or service or key distribution center KDC
 Designed at MIT in 1980. Systems ,including windows use Kerberos.
 The Kerberos is a Greek word that signify a three- headed dog (used to keep
outsiders away)
 It is a trusted key server system.
 Provides centralized third-party authentication in distributed network.
 Kerberos is designed for open distributed environment.
 Means it is a network authentication protocol that provides authentication
for client/server applications.

Kerberos
 Users at workstations want to access the services on servers that are
distributed throughout the network.
 Following threats from Attacker exist & servers should not allow :
 1. Attacker can use particular workstation and act as a valid user working from
that workstation.
 2. Attacker can alter the network address of a workstation.
 So any request sent from altered workstation appears to come from valid
workstation
 3. Attacker use a replay attack to gain entrance to server.
 Note : In the above cases, an attacker may use services and data that he is
not authorized to use.

Kerberos
 To solve these problems , instead of using authentication protocol
at each server, Kerberos provides a centralized authentication
server that authenticate users to servers and servers to users
 Unlike other authentication technique, Kerberos makes use of
symmetric i.e. secret key cryptography
 It does not use public- key encryption
 The two commonly used versions of Kerberos are :
 1. Kerberos version 4 (implementation still exist)
 2. Kerberos version 5 (corrects security deficiencies of version 4 and
issued as internet standard )

Kerberos Requirements
 Motivation of Kerberos come from following requirement:
 Users must prove their identity to the servers for each services.
 Servers also must prove their identity to the clients or users.
 First Report on Kerberos identified following requirements :
 1. Secure : Attacker should not obtain information appear like user.
 Means Kerberos should be strong enough that opponent does not find weak link.
 2. Reliable : Kerberos should be highly reliable and should employ the
distributed server architecture with one system able to back up other
 3. Transparent : Users should not be know that authentication is taking place.
 4. Scalable: Should be capable of supporting large number of clients and servers.

Kerberos Requirements
 To satisfy these requirements, Kerberos is Implemented as a third
party authentication service using an authentication protocol.
 Kerberos is trusted because both client and server trust Kerberos to
mediate their mutual authentication.

Kerberos Version 4
Kerberos version 4 Overview :
 It is a basic third-party authentication scheme.
 Version 4 of Kerberos makes use of DES algorithm.
 In distributed client/server architecture, Kerberos provides user
authentication by using one or more Kerberos server.
 Different approaches for security are as following :
 1. Simple Authentication Dialogue :
 In this approach an Authentication Server (AS) is used that maintains the
username and password of all users and store in a centralized database.
 The AS shares a unique secret key with each server and these keys
distributed in secure manner.

Kerberos Version 4
 When any client makes a request from its workstation to some
server for service, the following process takes place :
 C = Client , AS = Authentication Server , V = Server , IDc = Identifier of user
on client C.
 IDv = identifier of server V ,Pc = password of user on Client C,
ADc = network address of C.
 Kv = secret encryption key shared by AS and V.
 1. Client C log on to workstation and requests access to server V for service.
 2. C in the client workstation requests for the user’s password and then
sends a message to the AS that contains user’s ID, user’s password and
server’s ID.

Kerberos Version 4
 Message from client to AS = (IDc ,Pc, IDv)
 3. AS now checks the pair IDc and Pc in it’s database to see whether user
has given proper password and whether user is permitted to use server
V.
 4. If both tests are passed , then AS accepts the user as authentic and
then tell the server that user is authentic.
 To do this, AS creates the ticket that contains user’s ID , network address
of user’s and server’s ID.
 5. The ticket is encrypted using secret key shared by AS and this server.
 So, Ticket = E[ IDc , ADc, IDv]
 6. AS now send this encrypted ticket to client C.
 Neither client nor opponent can alter ticket because it is encrypted.

Kerberos Version 4
 7. With this ticket ,C can now apply to server V for using service.
 8 .For this, the Client C sends a message to server V containing ID of
client C and ticket.
 Message = [IDc, Ticket]
 9.The server V decrypts the message and verifies that user’s ID in
ticket is same as the unencrypted user ID in the message.
 10. If these two matches, then the server is assured that user is
authenticated.
 11. Now server allow the requested service to client.

Kerberos Version 4
 The first problem with this scheme is that each time client request
for a new service ,he needs to have new ticket from AS
 Second problem is that this scheme sends the plaintext passwords
hence its security is vulnerable.
 Consider the following dialogues :
 1 . C →AS : IDc, Pc, IDv
 2. AS→C : Ticket
 3. C→V : IDc ,Ticket
 Ticket = E[ IDc , ADc, IDv]

Kerberos Version 4
 More secure Authentication Dialogue :
 This scheme uses the Ticket Granting Server (TGS).
 Function of TGS is to issue tickets to the users who are authenticated by AS.
 The user requests the AS for ticket granting ticket (Tickettgs)
 The client saves this ticket in the user’s works station.
 Whenever the user wants to access a new service , he requests the TGS using
the ticket to authenticate itself.
 The TGS then grants the ticket for the particular service.
 Client saves each service – granting ticket issued by TGS for later use to
authenticate it’s user to server whenever the user requests for particular
service.

Kerberos Version 4
 The new message dialogues are as :
 1. Request from client to AS C → AS : IDc, IDtgs
 2. Response from AS to C AS→ C : Ek (Tickettgs)
 3. Request from client to TGS C→TGS : IDc, IDv, Tickettgs
 4. Response from TGS to client TGS→C : Tickettgs
 5. Message from client to server C→ V : IDc, Ticketv
 Here, Tickettgs = E( Ktgs, [IDc,ADc,IDtgs, TS1,Lifetime1])
 Ticketv = E( Kv, [IDc,ADc,IDv, TS2,Lifetime2])

IS UNIT 3 PPT- PART 2.pptx is very helpful for engineering students of any Electronics and Communication batch

Kerberos Realms
• A full service Kerberos environment consists of a Kerberos server ,a
number of clients, and number of application servers requires the
following :
• Kerberos server must have the user ID
• It must have hashed password for all participating users.
• Kerberos must share secret key with all participant users.
• All users are registered with Kerberos sever.
• All servers are registered with Kerberos server.
• Such an environment which provides all of these is called as
“Kerberos Realm”.
• Typically a single administrative domain.
• If have multiple realms means connection between two realms, their
Kerberos servers must share keys and trust.

Kerberos Version 5
• developed in mid 1990’s
• specified as Internet standard RFC 1510
• provides improvements over v4
– addresses environmental shortcomings
• encryption alg, network protocol, byte order, ticket
lifetime, authentication forwarding, interrealm auth
– and technical deficiencies
• double encryption, non-std mode of use, session keys,
password attacks

X.509 Authentication Service
 It is a part of CCITT X.500 directory service standards.
Distributed servers maintaining user information database.
 It defines framework for authentication services by X.500 directory to its
users.
Directory may store public-key certificates.
Each certificates contains public key of user signed by private key of
certification authority.
 Also defines authentication protocols .
 X.509 is an important standard because the certificate structure and
authentication protocol defined in X.509 are used in various contexts
 Example ,the X.509 certificate used in IP security and SSL/TLS.

Has 3 versions. X.509 was initially issued in 1988 and version 2 issued in
1993.
Then third version was issued in 1995 and again revised in 2000.
Version 3 resolves some security concerns and limited flexibility in
version 1 and 2.
It is based on public-key cryptography & digital signatures.
The standard not dictate specific algorithms , but RSA recommended.
The digital signature use hash function which is not specific.
X.509 certificates are widely used.

X.509 Certificates
• CA has created a side effect that each certificate may have different
format .
• So certificates that need to be used universally, must have universal
format.
• Heart of X.509 scheme is public –key certificates associated with user.
• X.509 is the way to describe certificate in structured way.
• These user certificates issued by a trusted Certification Authority
(CA), and placed in a directory by the CA.

X.509 Certificates
• All X.509 certificate has format which contains the following
elements :
– 1. Version V (1, 2, or 3) : This field specifies version of the
certificate format.
– Default version is 1. If issuer unique identifier are present , version
2.
– 2. Serial Number: Integer number that is unique with CA identifies
the certificate
– 3. Signature algorithm identifier AI : This field consists of an
algorithm , that is used by the CA to sign the certificates.

4. Issuer X.500 name (CA) : This field consists of name of the certification
Authority who created and signed the certificate.
5. Period of validity (TA) : This field consists of two dates ,the first and last
on which the certificate is valid.
6. Subject(User) name (Name of owner) : This field consists of the name of
user whose public key certified by this certificate.
7. Subject(User) public-key information : (algorithm, parameters, key)
This field consists the public – key of the user with identifier of the
algorithm for which this key is used with associated parameters.
8. Issuer unique identifier (v2,v3):This field is optional present in version
2 ,3.
This is an unique integer number used to identify an issuer i.e. to identify
CA.

X.509 Certificates
– 9. Subject(User) unique identifier (v2+v3) :
– It is also an optional field present in version 2 and 3.
– It is an unique integer number used to identify a user.
– 10. Extension fields (v3) : This field is present in version 3. A set of
one or more extension fields.
– 11. Signature : It covers all other fields of certificate. It contains
hash code of other fields encrypted with CA’s private key.
– This field includes the signature algorithm identifier.
• The standard use the following notation to define a certificate :
• CA<<A>> = CA [V, SN, AI, CA, UCA, A, UA, Ap, Ta]

Notation : CA<<A>> denotes certificate of user A signed and issued by
Certification authority CA.
CA[I] = the signing of I by CA .
It consists of I with an encrypted hash code appended
V = version of the certificate
SN = serial number of the certificate
AI = identifier of the algorithm used to sign the certificate
CA = name of the certificate of authority
UCA = optional unique identifier of the CA
A = name of the user A
UA = Optional unique identifier of the user A
Ap = public key of user A
Ta = period of validity of the certificate.
The CA signs the certificate with its private key.
If the corresponding public key is known to user , then user can verify that
certificate is signed by the CA is valid.

Certificate Revocation
• In some cases certificates must be revoked before
its expiration.
• Certificates have a period of validity
• May need to revoke before expiry, eg:
1. user's private key is compromised
2. user is no longer certified by this CA
3. CA's certificate is compromised
• CA’s maintain list of revoked certificates.
– the Certificate Revocation List (CRL)
• Users should check certificates with CA’s CRL

IS UNIT 3 PPT- PART 2.pptx is very helpful for engineering students of any Electronics and Communication batch

More Related Content

Similar to IS UNIT 3 PPT- PART 2.pptx is very helpful for engineering students of any Electronics and Communication batch (20)

Recently uploaded (20)

IS UNIT 3 PPT- PART 2.pptx is very helpful for engineering students of any Electronics and Communication batch

Editor's Notes