Authentication Protocols

1
Authentication Protocols
Celia Li Computer Science and Engineering York
University
2
Outline

• Overview of authentication
• Symmetric key authentication protocols
  (Kerberos)
• Public key authentication protocols (SSL)
• Authentication protocols in Wireless Local Area
  Networks (WLAN)

3
Outline

• Overview of authentication
• Symmetric key authentication protocols
  (Kerberos)
• Public key authentication protocols (SSL)
• Authentication protocols in Wireless Local Area
  Networks (WLAN).

4
Overview of Authentication

• The process of determining whether someone is,
• in fact, who it is declared to be.
• An authentication protocol consists of a sequence
  
• of messages between principals and will be
• described using different notations.
• Principals parties who cooperate by exchanging
• messages over networks.

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Notation

•  
• E(K M) denotes encrypting message plaintext M
• with key K.
• Ka key of principal A 
• Kab shared key of principle A and B
• Principals are generally denoted by capitals such
  as
• A, B and S (server)
• (1) A ? B M1
• (2) B ? S M2
• (3) S ? B M3

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Notation

• Na A number generated by a principal A.
  Generally
• termed as a nonce.
• A nonce could be
• a timestamp a number denoting the current time.
• a sequence number
• a random number
•  
• A ? B A, E(Kab Na)

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Overview of Authentication

• Timeliness in Authentication Protocols
• An authentication protocol is required to
  guarantee that the parties involved in the
  authentication process are present during the
  execution of the protocol.
• Nonces are generally used to assess timeliness
  in authentication protocols.

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Attacks on Authentication Protocol

• Replay attack
• The attacker records the message of an
  authentication protocol and replays this
  information to attempt to falsely authenticate to
  the other principle.
•  
• Counter measures
• Using timestamps, sequence number or random
  numbers.
•  

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Outline

• Overview of authentication
• Symmetric key authentication protocols
  (Kerberos)
• Public key authentication protocols (SSL)
• Authentication protocols in Wireless Local Area
  Networks (WLAN)

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Symmetric Key Authentication Without Trusted
Third Party

• Symmetric key one-pass unilateral authentication
  protocol

B authenticates A
If Na (timestamp) has a recent value, B can prove
As identity.

• Symmetric key two-pass unilateral
  authentication protocol

B authenticates A

• Symmetric key three-pass mutual authentication

B authenticates A
A authenticates B
Na Timestampt , Ra, Rb Random numbers.
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Symmetric Key Authentication With Trusted Third
Party

• NS (Needham-Schroeder) Symmetric Key
  Authentication

A B trust S, Kas is a shared key of A S, Kbs
is a shared key of B S

• A gets a shared key Kab generated by S

• B gets shared key Kab

• A authenticates B

• B authenticates A

S Trusted Third Party
(1)
(2)
(3), (5)
A
B
(4)
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Kerberos Authentication

• A typical example of symmetric key
• authentication with trusted third party
• The trusted third party in Kerberos is the
• authentication server which is in charge of
• distribute shared keys.
• Kerberos ticket is employed for the
• authentication purpose

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Kerberos Authentication

• Kerberos Ticket
• A certificate issued by an authentication
  server, encrypted using the key of the
  authentication server.
• The ticket contains
• the key that will be used for authentication of
  the client to the verifier who will verify the
  correctness of the key
• the name of the principal to whom the key was
  issued
• an expiration time of the ticket

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Kerberos authentication

• Client C wants to access server S. Client C and
  sever S need to authenticate
• each other
• Authentication server is comprised of two
  parts
• A Key Distribution Server
• G Ticket Grant Server

• (1) C sends a random number N1 to A
• (2) A sends back a ticket Tcg to C.
• Kac shared key between A and C
• Kcg shared key between C and G
• C authenticates A if N1 is correct

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Kerberos authentication

• (3) C forwards the ticket Tcg to G
• N2 random number
• T time stamp
• G authenticates C if time stamp is correct
• (4) G sends back a service ticket Tcs to C.
• Kcs shared key between C and S
• C authenticates G if N2 is correct

• (5) C sends ticket Tcs to S
• T time stamp
• S authenticates C if time stamp is correct
• (6) C authenticates S if time stamp is correct

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Outline

• Overview of authentication
• Symmetric key authentication protocols
  (Kerberos)
• Public key authentication protocols (SSL)
• Authentication protocols in Wireless Local Area
  Networks (WLAN)

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Public Key Certificate

• Commonly used for public key authentication
  protocols
• An electronic document which uses a digital
  signature to bind together a public key with an
  identity
• the name of a person or an organization, their
  address
• Issued by a certification authority (CA)
• e.g. VeriSign
• The certificate can be used to verify if a
  public key belongs to an individual.

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Public Key Certificate

• Public key certificates include
• Public key of the user
• DN (name of the user)
• Validity period of the certificate
• Name of the certificate issuer
• Digital signature of the
• certificate issuer

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Public Key Authentication without Trusted Third
Party
Public key one-pass unilateral authentication
protocol
B authenticates A
Public key two-pass unilateral authentication
protocol
B authenticates A
Public key three-Pass mutual authentication
protocol
B authenticates A
A authenticates B
CertA public key certificate of A Ka-1 private
key of A, Kb-1 private key of b
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Public Key Authentication with Trusted Third Party
NS (Needham-Schroeder) Public Key Authentication
Protocol
A asks Bs public key from S
S sends Bs public key Kb to A
B gets As identity
B asks As public key from S
S sends As public key Ka to B
A authenticates B by verifying Na
B authenticates A by verifying Nb
S Trusted Third Party
(5)
(1)
(4)
(2)
(3)
(7)
A
B
(6)
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Secure Socket Layer Protocol (SSL)

• Universally accepted on the Web for
  authenticated and encrypted communication between
  clients and servers.
• Typical example of public key authentication
• Protect sensitive information such as Social
  Security Number (SSN), Credit Card Number, etc.

Web Server
Client
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SSL

• Server Certificate
• The SSL protocol requires a server certificate
  so that client can authenticate the server.
• Client Certificate (Optional)
• You can optionally configure your server to
  request a client certificate so that server can
  authenticate the client.
• Only discuss how client authenticates server in
  SSL

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SSL
Client authenticates server based on certificate
issued by certificate authority (CA)

• Client ? server Hello message
• Server ? client servers public key certificate,
  including servers public key, digital signature
  signed by certificate authority (CA)
• Client uses CAs public key to verify digital
  signature signed by CA, thus verifies servers
  certificate.
• Client ? server a random number
• in order to prove servers identity.
• 5. Server encrypts random number with its
  private key.
• 6. Server ? client the encrypted message
• Clients decrypts the message using servers
  public key to verify identity of the server.

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Compare SSL and Kerberos
SSL Kerberos
Uses public key encryption Uses symmetric key encryption
Certificate based   Relies on a trusted third party  
Ideal for secure communications with a large, variable user base that is not known in advance, such as the WWW.  Ideal for networked environments where all services and users are known in advance.
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Outline

• Overview of authentication
• Symmetric key authentication protocols
  (Kerberos)
• Public key authentication protocols (SSL)
• Authentication protocols in Wireless Local Area
  Networks (WLAN)

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Authentication Protocols in WLAN

• WPA (Wi-Fi Protected Access)
• Certification program created to secure wireless
  computer networks.
• Provides stronger data encryption and user
  authentication
• Implements the IEEE 802.11i standard.
•  
• The three entities of WPA are the client, the
  Authentication Server (AS), and the Access Point
  (AP).

AS

• Only after the client is authenticated by the
  authentication server, the authenticator (Access
  Point) allows the client to access the network

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Authentication Protocols in WLAN

• Symmetric key approach EAP-LEAP
• Public key approach EAP-TLS
•  Symmetric Key Approaches
• Efficient and require little computational power
  
• Resource constrain, e.g., computational power of
  PDAs, mobile VoIP phones, have.
• Drawbacks
• Most protocols derive the shared secret from the
  user's password and some passwords are not
  strong, it is easy for the attacker to extract
  the secret key from them.

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Lightweight Extensible Authentication Protocol
(LEAP)
The server knows the password of the client.
Password is the shared key between the client C
and authentication server S
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Authentication Protocols in WLAN

• Public key Approaches
• Provides strong security
• Cannot authenticate a client without
• public certificate
• Example EAP-TLS

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EAP-Transport Layer Security (EAP-TLS)

• AS verifies clients digital signature using
  clients public key got from clients certificate
  Certclient
• Get random number p by decrypting with its
  private key

• Client calculates H(c,s,p), compares it with the
  value sent by As

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References
1 Kerberos http//web.mit.edu/kerberos/www/ 2
SSL http//docs.sun.com/source/816-6156-1
0/contents.htm 3 EAP-TLS http//en.wikipedia.or
g/wiki/Extensible_Authentication_Protocol