8.5 AUTHENTICATION AND KEY DISTRIBUTION

1
8.5AUTHENTICATION AND KEY DISTRIBUTION

• Bassam Tork
• Dr. Yanqing Zhang

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Outline

• Concepts
• Authentication Protocols
• Design of Authentication Protocols
• Needham-Schroeder Protocol
• Kerberos Protocol Version V
• Research
• Case Study Key Distribution Framework for a
  Mobile Agent Platform
• References

3
Overview Randy Chow et al,1997

• Authentication is the process of verifying the
  identity of an object entity.
• one-way verificationPassword verification.
• Two way authentication both communicating
  entities verify each others identity.
• This type of mutual authentication is important
  for communication between autonomous principals
  in a client/server or peer-to-peer distributed
  environment.

4
Orvveiew George Coulouris et al,2001

• Cryptography is used for authenticating
  communication between pairs of principals.
• If keys are held in private, a successful
  decryption authenticates the decrypted message as
  coming from a particular sender.

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Authentication Protocols Randy Chow et al,1997

• Authentication protocols are all about
  distribution and management of secret keys.
• Key distribution in a distributed environment is
  an implementation of distributed authentication
  protocols.

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Design of Authentication Protocols Randy Chow
et al,1997

• Many authentication protocols have been proposed
• All protocols assume that some secret information
  is held initially by each principal.
• Authentication is achieved by one principal
  demonstrating the other that it holds that secret
  information.
• All protocols assume that system environment is
  very insecure and is open for attack. So any
  message received by a principal must have its
  origin authenticity, integrity and freshness
  verified.

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Design of Authentication Protocols Randy Chow
et al,1997

• To achieve these goals, most protocols need to
  rely on an authentication server.
• Capable Authentication server delivers
  good-quality session keys and distribute them to
  the requesting principals securely.
• Trustworthy Authentication server maintains a
  table containing a name and a secret key for each
  principal. The secret key is used only to
  authenticate client processes to the
  authentication server and to transmit messages
  securely between client processes and the
  authentication server.

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Design of Authentication Protocols Randy Chow
et al,1997

• Protocols are divided into two categories to
  verify the freshness of a message.
• First category uses nonce and challenge/ response
  handshake to verify freshness.
• Second category uses timestamps and assumes that
  all machines in distributed system are
  clock-synchronized.

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The Evolution of Authentication ProtocolsRandy
Chow et al,1997

• Needham-Schroeder Protocol.
• Kerberos protocol.

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Needham-Schroeder Protocol George Coulouris
et al,2001

• This original work includes secret-key protocol
  and public key protocol
• Public-key protocol does not depend on the
  existence of authentication server and is hence
  more suitable for use in networks with many
  independent management domains.
• Secret-key protocol provides a solution to
  authentication and key distribution based on an
  authentication server.

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Needham-Schroeder Secret-key Protocol
http//en.wikipedia.org/wiki/Needham-Schroeder_p
rotocol, May 2008

• The protocol is based on the generation and
  transmission of ticket by the authentication
  server.
• A ticket is an encrypted message containing a
  secret key for use in communication between A and
  B.
• The protocol consists of 5 steps.

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Needham-Schroeder Secret-key Protocol

• A-gtS A, B, NA
• A requests S to supply a key for communication
  with B
• S-gtA NA, B, KAB, A, KAB KB KA
• S returns a message encrypted in As secret key,
  containing a newly generated key KAB, and a
  ticket encrypted in Bs secret key
• A-gtB A, KAB KB
• A sends the ticket to B
• B-gtA NB KAB
• B decrypts the ticket and uses the new key KAB
  to encrypt another nonce NB
• A-gtB NB - 1 KAB
• A demonstrates to B that it was the sender of
  the previous message by returning an agreed
  transformation of NB

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Weakness

• If session key between A and B is compromised,
  and the ticket to B is recorded, an intruder can
  impersonate A by carrying out last 3 steps.
• The weakness can be remedied by adding a
  timestamp to message3, so that it becomes. A-gtB
  A, t, KAB KB
• B decrypts this message and checks that t is
  recent. This is the solution adopted in Kerberos

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Kerberos Protocol V George Coulouris et
al,2001

• Based on Needham-Schroeder but uses timestamps
• It is included in the Distributed Computing
  Environment (DCE) and in the windows 2000 as the
  default authentication service.
• A Kerberos server is known as a Key Distribution
  Centre (KDC).
• Each KDC has an authentication service (AS) and
  a Ticket Granting service (TGS).

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Kerberos Protocol V George Coulouris et
al,2001

• Kerberos deals with three kinds of security
  object
• Ticket a token issued to a client by the
  Ticket-Granting Service for presentation to a
  particular server, verifying that the sender has
  recently been authenticated by Kerberos. Tickets
  include an expiry time and a newly generated
  session key for the use by the client and the
  server
• Authenticator a token constructed by a client
  and sent to a server to prove the identity of the
  user.
• It contains clients name and a timestamp and
  is encrypted in the appropriate session key
• Session key a secret key generated by Kerberos
  and issued to a client for use when communicating
  with a particular server.

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Kerberos Protocol V

• A Kerberos ticket has a fixed period of validity
  starting at time t1 and ending at time t2. A
  ticket for a client C to access a serve S takes
  the form
• C,S, t1, t2, Kcs Ks, which we denote as
  ticket(C,S) Ks
• The clients name is included in the ticket to
  avoid possible use by impostors.
• The protocol consists of 4 steps.

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Kerberos Protocol V

• Step A obtain Kerberos session key and TGS
  ticket, once per login session
• C-gtA C,T, n
• Client C requests the Kerberos authentication
  server A to supply a ticket for communication
  with the TGS T
• A-gtC KcT, n, ticket(C, T) KT Kc
• A returns a message containing a ticket
  encrypted in its secret key and a session key for
  C to use with T.

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Kerberos Protocol V

• To obtain a ticket for any server S, C constructs
  an authenticator encrypted in KcT of the form
• C, t KcT, which we denote as auth(C) KcT
• Step B obtain ticket for a server S, once per
  client-server session
• C-gtT auth(C)KcT, ticket(C, T) KT, S, n
• C requests the ticket-granting server T to
  supply a ticket for communication with another
  server S
• T-gtC Kcs, n, ticket(C,S) Ks KcT
• T checks the ticket. If it is valid T generates
  a new session key Kcs and returns it with a
  ticket for S (encrypted in the servers secret
  key Ks).

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Kerberos Protocol V

• Step C issue a server request with a ticket
• C-gtS auth(C) Kcs, ticket(C, S) Ks, request,
  n
• C sends the ticket to S with a generated
  authenticator for C and a request.
• Step D Authenticate server (optional)
• S-gtC n Kcs

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Drawbacks http//en.wikipedia.org/wiki/Kerberos_
(protocol), October 2009

• Single point of failure It requires continuous
  availability of a central server. When the
  Kerberos server is down, no one can log in.
• This can be mitigated by using multiple Kerberos
  servers and fallback authentication mechanisms.
• Kerberos requires the clocks of the involved
  hosts to be synchronized.
• The tickets have a time availability period and
  if the host clock is not synchronized with the
  Kerberos server clock, the authentication will
  fail.
• The default configuration requires that clock
  times are no more than 10 minutes apart.

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• Research

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• Case Study
• Key Distribution Framework for a Mobile Agent
  PlatformLeila Ismail et al, 2008

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OutlineLeila Ismail et al, 2008

1-Definition 2-Security threats in a mobile agent
system. 3-The rationale beyond authentication for
mobile agents. 4-Present the requirements for key
distribution in a mobile agent system. 5-
Describe key distribution mechanisms. 6-Describe
the key distribution protocol for a mobile agent
platform and for the mobile agents. 7-
comparison with related works, conclusion. 8-Futur
e Work
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Definition

• A Mobile Agent is a process with its own code
  and data that can migrate in the network from one
  host (called agent server) to another to perform
  a specific task on behalf of their users.
• Mainly intended to address efficiency problems
  of Distributed applications.
• They reduce communication costs by moving
  computation to or close
• to the host on which the target data reside.
• Can move closer to potential resources for
  access.
• In addition agents communicate with each others
  either locally or remotely.
• In these scenarios, protection becomes an urgent
  need.

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Example

The agent moves from one travel agency to
another. At each travel agency, the agent
compares the prices of the flights got from the
current agency with those got from the previous
agencies, selects the flight with the best price
and moves to the next agency
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Security Threats

• Spying and intercepting the agent (Man in the
  Middle Attacks).
• A competitor travel agency can modify to its
  interest the intermediate results of the cheapest
  price obtained by the mobile agent (agents
  integrity).
• An attacker can also modify the agents data
  causing an error leading to a Denial of Service
  attack (DOS).
• This type of attack can be mounted through
  vulnerable communication channels of the system.
  An attack of this nature can happen during an
  agents migration.

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Security Threats-cont.

• Spying and intercepting the agents itinerary.
• The itinerary of an agent is the list of agents
  servers to visit. Knowing the agents itinerary,
  a DOS attack can be mounted against the agent
  itself.
• A competitor travel agency for instance can
  modify an agents itinerary so that the agent
  would not visit other competitors.
• The agents itinerary can be also be maliciously
  modified so that to deviate the agent from its
  destination. Thus forcing the agent to move to an
  unknown agent server or to skip certain agents
  server.

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Security Threats-cont.

• Masquerading. This type of attack consists of
  bypassing the authentication mechanisms.
• An agent pretends to be some other uthenticated
  agent to obtain access rights. Gaining access,
  the agent would then compromise the
  confidentiality and the integrity of accessed
  resources.
• For example, an agent can pretend to be an
  administrative agent so that to gain access to
  flights prices and modify the prices to its
  interest.

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Security Threats-cont.

• DOS attack against an agent server by the mobile
  agent.
• An agent executing on an agents server can clone
  itself to an unlimited number of agents. It can
  consume a huge number of CPU cycles, create a
  huge number of processors, etc.

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Security Threats-cont.

• Repudiation The repudiation is the refusal to
  admit responsibility of an action.
• For instance, an agent (representing a user) can
  refuse to admit a banking transaction which
  involves fund transfer.
• In case of collaboration between 2 agents, the
  sender of the message can refuse to admit sending
  the message and the receiver of the message can
  refuse to admit of receiving a message.

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Security Threats-cont.

• Based on the above threats, we need to achieve
  theses security issues
• Protecting an agent server from a mobile agent.
  This consists of protecting the receiving hosts
  resources, such as file system, memory, CPU
  cycles, and I/O channels.
• In particular an agent must not be able to access
  files ,memory for which it does not have access
  rights. Also, an agent must not be able to run
  forever on a receiving host.

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Security Threats-cont.

• Protecting cooperating agents. Agents can
  communicate locally within an agents server or
  remotely. Agents must authenticate each others
  before communication takes place.
• Protecting an agent from an agent server. This
  problem consists of protecting an agent from an
  malicious agents server that is executing it.

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Mobile Agents Authentication

• Authentication is the process of verifying the
  identity of a principal for an eventual access
  control.
• In a mobile agent system, authentication is used
  for 2 main reasons
• Authenticating agent servers. This consists of
  authenticating both the sender and the receiver,
  so that to establish a mutual trust at the
  platform level.

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Mobile Agents Authentication-cont.

• Authenticating mobile agents. It is necessary to
  authenticate mobile agents so that to establish a
  trust at the application level.

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Key Distribution Requirements.

• In designing the key distribution protocol, the
  authors took into consideration the following
  requirements
• Security domain change The Certifiction
  Authority (CA) of the receiving security domain
  must be able to authenticate the agent which
  comes from another security domain.

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Key Distribution Requirements-cont.

• Trust establishment The key distribution process
  should start with a very high trust relationship
  with the Certifiction Authority (CA) .
• Secure key distribution The key distribution
  process should be conducted in a secure manner
  (e.g., trusted path).
• Efficiency The key distribution process should
  not consume a lot of resources, such as machines
  CPUs and network bandwidth.

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Key Distribution Requirements-cont.

• Scalability The key distribution process must be
  scalable enough, so that the mobile agent can
  have the ability to roam widely.
• Transparency The mobile agents should not
  include a code which is proper to key
  distribution.
• This will ease programming as agents programmers
  will concentrate on the programming logic rather
  than the key obtaining issues.

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Key Distribution Requirements-cont.

• Portability The protocol should not be platform
  specific and should be ported to any mobile agent
  platform.
• Ease of administration The key distribution
  protocol should not be a burden on the
  administrator. The protocol is an automated
  infrastructure that should require minimum
  administrators intervention.

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Key Distribution Mechanisms.

• System Components
• A key distribution system for mobile agents
  includes the following components
• Agent An agent is a software component which
  executes on behalf of a particular user who is
  the user of the agent.
• An agent can be mobile and move from one host
  server to another under its own control to
  achieve tasks on these hosts servers.

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Key Distribution Mechanisms.
Architecture of Key Distribution System in a
Mobile Agent Transform
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Key Distribution Mechanisms - cont. -System
Components-

• Agent Server Each host, as part of the mobile
  agent platform, runs an execution environment,
  the agent server.
• Messaging System. A messaging system is part of
  an agent execution environment. It provides
  facilities for agents to communicate both locally
  and remotely

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Key Distribution Mechanisms - cont. -System
Components-

• The CA. It is a trusted third party which
  provides digital certificates for mobile agents,
  users and agent servers.
• All digital certificates are signed by the CA
  for further verification of their authenticity
  and validity.

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Key Distribution Mechanisms - cont. -System
Components-

• Keystore Each agent server has a local database
  which is used to store and retrieve its own
  private/public key pair and the digital
  certificate.
• It also stores the digital certificate of the
  trusted CA and other agent severs, mobile agents,
  and CAs with which the agent server has prior
  communication.
• Similarly, each CA has a local keystore .

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Key Distribution Mechanisms - cont. -System
Components-

• Security Domain A security domain consists of a
  group of agent servers which are under one common
  CA. In the security domain, the agent servers
  have the digital certificate of their local CA
  stored in their local keystores.
• When a mobile agent moves, it can move within the
  same security domain or changes a security domain.

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Key Distribution Mechanisms - cont. - Key
Authentication-

• Examples of entity impersonation threat
• Ex1 when a user agent assumes the identity of
  another already authenticated user agent to
  gain benefit from the communication.
• Ex2 when an agent server assumes the identity of
  another very credible agent server in order to
  gain advantage over it.

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Key Distribution Mechanisms - cont. - Key
Authentication-

• To protect against such threat, public keys of
  both user agents and agent servers must be
  authenticated.
• The most widely approaches, for solving the key
  authentication problem
• 1-ones based on the certifying authority model
  (e.g. Secure Socket Layer/Transport layer
  Security(SSL/TLS)) .
• 2-The web-of-trust (e.g. pretty Good
  Privacy(PGP).
• The researchers adopted the certifying authority
  model as a key
• authentication method for trust establishment.
• They believe that this approach has an advantage
  over the web of trust
• because the web of trust relies on one to one
  trust establishment between
• entities, which cannot be achieved in a mobile
  agent environment where
• prior knowledge between communicating entities
  may not exist.

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Key Distribution Mechanisms - cont. - Key
Authentication-

• For key authentication, the authers made the
• following assumptions
• Trust must be established between different CAs.
  
• Every mobile agent server must be pre-configured
  with the digital certificate of the local CA
  within the same security domain.

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Key Distribution protocol
Key Distribution Protocol for a Mobile Agent
Platform
49
Key Distribution protocol

• The proposed key distribution protocol is
  composed of the
• following phases
• Pre-configuration Phase A domain administrator
  reconfigures the local keystore of the agent
  server with the digital certificate of the local
  CA.
• Initialization Phase An administrator starts a
  mobile agent server which leads to the following
  actions
• 1) An agent server generates a private/public
  key pair locally. 2) An agent server sends a
  request which include the public key to the local
  CA for certification.
• 3) The local CA sends a response to the agent
  server including the agent server digital
  certificate, signed by the local CA.

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Key Distribution protocol cont.

• User Registration Phase An agent user can
  register to an agent platform to be able to
  configure and use its agents.
• The following actions occur
• 1) A private/public key pair is generated
  locally for the user.
• 2) The agent server on which the user is
  registered sends a request which includes the
  user public key to the local CA for
  certification.
• 3) The local CA sends a response to the agent
  server including the agent user digital
  certificate, signed by the local CA.

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Key Distribution protocol cont.

• Agent Mobility Phase This phase details the
  actions that the system takes when an agent moves
  according to two scenarios
• mobility within the same security domain
• mobility across domains as follows.

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Key Distribution protocol cont.- Agent
Mobility within Same Security Domain-

• Agent Mobility within Same Security Domain. In
  this scenario, all the agent servers are under
  the authority of the same CA.
• 1) Sender agent server sends a SYNC request which
  includes its identify and certificate to the
  receiver agent server.
• 2) Receiver agent server verifies the certificate
  (VERIFY operation) using the certificate of the
  CA in the local keystore.
• 3) Receiver agent server sends an ACK response to
  the sender agent server which includes the
  receiver agent server identity and certificate.
• 4) Sender agent server verifies the certificate
  (VERIFY operation) using the certificate of the
  CA in the local keystore.
• 5) Sender agent server sends a message which
  includes the agent, the agent identity and
  certificate, signed by the local CA.
• 6) Receiver agent server verifies the certificate
  of the agent using the certificate of the CA in
  the local keystore.
• 7) Receiver agent server accepts or reject the
  agent and informs the sender agent server of the
  acceptance or rejection.

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Key Distribution protocol cont.- Agent
Mobility Across Security Domain-

• Agent Mobility across Security Domains. In this
  scenario, agent servers are under the authorities
  of different CAs
• 1) Sender agent server sends a SYNC request
  which includes its identify and certificate to
  the receiver agent server.
• 2) VERIFY operation (receiver side) works as
  follows
• - If the receiver agent server does not have the
  certificate of the CA of the sender agent server,
  then the receiver agent server sends a request to
  its local CA. That message includes the sender
  agent server identity and certificate.
• -The Local CA of the receiver agent server sends
  a response to the receiver agent server which
  includes the sender certificate signed by the
  receiver CA.
• -If the receiver local CA does not have the
  certificate of the sender local CA, then the
  receiver local CA forwards the VERIFY request to
  a higher authority CA and so on.

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Key Distribution protocol cont.- Agent
Mobility Across Security Domain-

• 3) Receiver agent server sends an ACK response
  which includes its identity and certificate.
• 4) VERIFY operation (sender side) works as
  follows
• -If the sender agent server does not have the
  certificate of the CA of the receiver agent
  server, then the sender agent server sends a
  request to its local CA. That message includes
  the receiver agent server certificate.
• -The Local CA of the sender agent server sends a
  response to the sender which includes the
  receiver certificate signed by the sender CA.
• -If the sender local CA does not have the
  certificate of the receiver local CA, then the
  sender local CA forwards the VERIFY request to a
  higher authority CA and so on.

55
Key Distribution protocol cont.- Agent
Mobility Across Security Domain-

• 5) Sender agent server sends to the receiver
  agent server a message which includes the user
  agent, the agent identity and certificate, signed
  by the local CA.
• 6) VERIFY operation (receiver side) works as
  follows
• -If the receiver agent server does not have the
  certificate of the CA of the user agent server in
  its local keystore, then the receiver agent
  server sends a request to its local CA. That
  message includes the agent certificate.
• -The local CA of the receiver agent server sends
  a response to it which includes the agent
  certificate signed by the receiver CA.
• -If the receiver local CA does not have the
  certificate of the home agent local CA, then the
  receiver local CA forwards the VERIFY request to
  a higher authority CA and so on.
• 7) Receiver agent server accepts or reject the
  agent and informs the sender agent server of the
  acceptance or rejection.

56
Related Works

• Literature shows that many proposals have been
  made, to handle how keys are named and bound to
  individuals, and how servers manage key
  distributions
• Examples
• 1) Secure Shell(SSH)
• -Transmits the necessary key information during
  connection setup.
• -Key exchange is automatically done between a
  client and a server.
• - The administrator manually generates the
  private/public key pair on the client machine
• -and manually copy the client public key to all
  the server with which the client will communicate.

57
Related Works-cont.

• 2)EX2 Secure Socket Layer/Transport Layer
  Security SSL/TLS)
• -Transmits the necessary key information during
  connection setup.
• -Key exchange is automatically done between a
  client and a server.
• - The SSL/TLS uses the Diffie-Hellman key
  exchange that allows 2 peers to exchange a
  communication session (shared secret key) using
  certificates already distributed to clients and
  servers.

58
Related Works-cont.

• Pretty Good Privacy (PGP)
• Simple Distributed Security Infrastructure/Simple
  Public Key Infrastructure (SDSI/SPKI) .
• These mentioned proposals assume that the clients
  have a prior knowledge of the servers.
• This approach is not applicable to a mobile agent
  system where agents
• Because agent servers have no prior knowledge of
  each others or their certification authorities.
• Therefore, the authors believe that the hierarchy
  CA model is a natural choice for a key
  distribution framework for a mobile agent
  platform.
• This is due to the the scalable nature of the
  hierarchy CA model which makes no assumption
  about the system communicating entities.

59
Conclusion.

• The proposed a framework for key distribution
• in a mobile agent platform, took into account
• the dynamic nature of a mobile agent platform.
• 2-way authentication between the agent, (running
  on behalf of its user) and the host.
• the ease of use for both administrators and
  agents users as it is automatically executed by
  the underlying platform.

60
Future Works.

• Considering the number of mobile agents, agent
  servers, CAs, and the keystore performance based
  on the the number and the sizes of the private
  keys, public keys and certificates in the system,
  for performance metrics.

61
References

• 1 Randy Chow ,Theodore Johnson,Distributed
  Operating Systems and Algorithms,
  addison-wesley, 1997
• 2 George Coulouris, Jean Dollimore, Tim
  Kindberg,DISTRIBUTED SYSTEMS CONCEPTS AND
  DESIGN, addison-wesley, 2001
• 3 http//en.wikipedia.org/wiki/Needham-Schroeder
  _protocol, October 2009
• 4 http//web.mit.edu/kerberos/, 2007/10
• 5 http//en.wikipedia.org/wiki/Kerberos_(protoco
  l), October 2009
• 6 Leila Ismail, Ezedin Barka, Next Generation
  Mobile Applications, Services and Technologies,
  2008. NGMAST '08. The Second International
  Conference on
• 16-19 Sept. 2008 Page(s)281 287 .