Lecture 9 key distribution and user authentication

Key Distribution and
UserAuthentication

Overview

• Symmetric Key Distribution using Symmetric Encryption
 Kerberos

• Key Distribution using Asymmetric Encryption
 X.509 Certificates

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Raja Khurram Shahzad

Symmetric Key Distribution

• Two parties must share same key
 Protected from the access of others
 Frequent key exchange to limit amount of data
compromised

• Key can be exchanged
1. Physical delivery to B
2. A third party physically deliver it to A and B
3. Re-usage of old key to exchange new key
4. A & B both communicate securely with C, C delivers the key

 For option 1 & 2 require manual delivery

 For option 3 link encryption or end-to-end encryption, What if old key
is compromised
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Symmetric Key Distribution

 For option 4 two kinds of keys are used
– Session Key: One time key
– Permanent Key: for distributing session key.

– Necessary element, Key Distribution Center (KDC):
determines which systems are allowed to communicate with each
other.

– Operation of KDC
• A wish to communicate B, transmits request to KDC.
Communication is encrypted using a master key
• KDC approves connection and creates one time session key.
Session key is encrypted with permanent keys of A & B and
delivered to A & B.
• A & B set up logical connection and uses session key.

 Most widely application to use this approach is KERBEROS
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Security Concerns

• Key concerns are confidentiality and timeliness

• To provide confidentiality must encrypt identification and session
key info which requires the use of previously shared private or
public keys

• Need timeliness to prevent replay attacks

• Provided by using sequence numbers or timestamps or
challenge/response

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ET2437 - Network Security

KERBEROS

In Greek mythology, a many headed dog,
the guardian of the entrance of Hades
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KERBEROS

• Users wish to access services on servers.

• Three threats exist:
 User pretend to be another user.
 User alter the network address of a workstation.
 User eavesdrop on exchanges and use a replay attack.

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KERBEROS

• Assumes a distributed client/server architecture

• Provides a centralized authentication server to authenticate
users to servers and servers to users.

• Relies on conventional encryption, making no use of public-
key encryption

• Two versions: version 4 and 5

• Version 4 makes use of DES

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Requirements for Kerberos

• Secure:
 Eavesdropper should not be able to obtain the necessary
information to impersonate a user

• Reliable:
 Kerberos should employ a distributed server architecture, systems
backing up each other

• Transparent:
 User should not be aware that authentication is taking place

• Scalable:
 The system should be capable of supporting large number of clients
and servers

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Overview of Kerberos

• AS = Authentication Server
• SS = Service Server
• TGS = Ticket-Granting Server
• TGT = Ticket Granting Ticket

• User Client-based Logon
 A user enters a username and password on the client machine.
 The client performs a one-way function (hash usually) on the entered
password, and this becomes the secret key of the client/user.

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• Client Authentication
 The client sends a clear text message of the user ID to the AS
requesting services on behalf of the user. (Note: Neither the secret
key nor the password is sent to the AS.)
– The AS generates the secret key by hashing the password of the
user found at the database.

 The AS checks client rights in its database. If valid, the AS sends
back the following two messages to the client:
– Message A: Client/TGS Session Key encrypted using the secret key
of the client/user.
– Message B: Ticket-to Get-Ticket (which includes the client ID, client
network address, ticket validity period, and the client/TGS session
key) encrypted using the secret key of the TGS.

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 Client receives messages A and B, and decrypt message A to obtain
the Client/TGS Session Key.
– The session key is used for further communications with the TGS.
(Note: The client cannot decrypt Message B, as it is encrypted using
TGS's secret key.)

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• Client Service Authorization
 When requesting services, the client sends the following two
messages to the TGS:
– Message C: Composed of the TGT from message B and the ID of the
requested service.
– Message D: Authenticator (which is composed of the client ID and the
timestamp), encrypted using the Client/TGS Session Key.

 TGS retrieves message B out of message C. It decrypts message B
using the TGS secret key to have "client/TGS session key". Using this
key, the TGS decrypts message D (Authenticator) and sends the
following two messages to the client:
– Message E: Client-to-server ticket (which includes the client ID, client
network address, validity period and Client/Server Session Key)
encrypted using the service's secret key.
– Message F: Client/server session key encrypted with the Client/TGS
Session Key.
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• Client Service Authorization
 For requesting services, the client sends the following two messages
to the TGS:
– Message C: Composed of the TGT from message B and the ID of the
requested service.
– Message D: Authenticator (which is composed of the client ID and
the timestamp), encrypted using the Client/TGS Session Key.

 TGS retrieves message B out of message C.
– It decrypts message B using the TGS secret key. This gives it the
"client/TGS session key". Using this key, the TGS decrypts message
D (Authenticator) and sends the following two messages to the client:
• Message E: Client-to-server ticket (which includes the client ID, client
network address, validity period and Client/Server Session Key)
encrypted using the service's secret key.
• Message F: Client/server session key encrypted with the Client/TGS
Session Key.
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• Client Service Request
 Client receives messages E and F from TGS.
 The client connects to the SS and sends the following two messages:
– Message E from the previous step (the client-to-server ticket,
encrypted using service's secret key).
– Message G: a new Authenticator, which includes the client ID,
timestamp and is encrypted using client/server session key.
 The SS decrypts the ticket using its own secret key to retrieve
the Client/Server Session Key. Using the sessions key, SS
decrypts the Authenticator and sends the following message to the
client to confirm its true identity and willingness to serve the client:
– Message H: the timestamp found in client's Authenticator plus 1,
encrypted using the Client/Server Session Key.
 ƒ The client decrypts the confirmation using the Client/Server
Session Key and checks whether the timestamp is correctly updated.
If so, then the client can trust the server and can start issuing service
requests to the server.
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 The server provides the requested services to the client.



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Kerberos Version 4

• Terms:
 C = Client
 AS = authentication server
 V = server
 IDc = identifier of user on C
 IDv = identifier of V
 Pc = password of user on C
 ADc = network address of C
 Kv = secret encryption key shared by AS and V
 TS = timestamp
 || = concatenation

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A Simple Authentication Dialogue

(1) C  AS: IDc || Pc || IDv
(2) AS  C: Ticket
(3) C  V: IDc || Ticket

• Ticket = EKv[IDc || Pc || IDv]

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Remaining problems

2. Number of times that a user has to enter a password

4. Plain-text transmission of password

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More secure Authentication Dialogue
Once per user logon session:
• C  AS: IDC || IDtgs
• AS  C: EKc [ Tickettgs ]

Once per type of service:
(3) C  TGS: IDC || IDV ||Tickettgs

(4) TGS  C: TicketV

Once per service session:
(5) C  V: IDC || TicketV

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Remaining problems

3. The lifetime associated with the ticket-granting ticket

5. Servers are not able to authenticate themselves

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Version 4 Authentication Dialogue
Authentication Service Exhange: To obtain Ticket-Granting
Ticket
• C  AS: IDc || IDtgs ||TS1
• AS  C: EKc [Kc,tgs|| IDtgs || TS2 || Lifetime2 || Tickettgs]

Ticket-Granting Service Echange: To obtain Service-Granting
Ticket
(3) C  TGS: IDv ||Tickettgs ||Authenticatorc

(4) TGS  C: EKc [Kc,¨v|| IDv || TS4 || Ticketv]

Client/Server Authentication Exhange: To Obtain Service
(5) C  V: Ticketv || Authenticatorc

(6) V  C: EKc,v[TS5 +1]

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Version 4 Authentication Dialogue

• Problems:
 Lifetime associated with the ticket-granting ticket
 If to short  repeatedly asked for password
 If to long  greater opportunity to replay

• The threat is that an opponent will steal the ticket and use it before
it expires

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Request for Service in Another Realm

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Difference Between Version 4 and 5

• Encryption system dependence (V.4 DES)

• IP - Internet protocol dependence

• Message byte ordering

• Ticket lifetime

• Authentication forwarding

• Inter realm authentication

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Kerberos - in practice
• Currently have two Kerberos versions:
 4 : restricted to a single realm
 5 : allows inter-realm authentication, in beta test

• Kerberos v5 is an Internet standard
• Specified in RFC1510, and used by many utilities

• To use Kerberos:
 need to have a Key Distribution Center (KDC) on your network
 need to have Kerberised applications running on all participating
systems
 major problem - US export restrictions
 Kerberos cannot be directly distributed outside the US in source format
(& binary versions must obscure crypto routine entry points and have
no encryption)
 else crypto libraries must be re-implemented locally

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Key Distribution using Asymmetric Encryption

• Problem : The distribution of Public Keys
 What if a fake user imparsionate to be a legitimate user and distribute his
keys
• Solution : Public-Key Certificates
 Consists of a public key + User ID of the key owner with whole block
signed by a trusted third party
 Third party is a Certificate Authority (CA), trusted by user community
 User deliver public key to CA in a secure manner and obtain a certificate
 User publish the certificate
 Anyone needing this user’s public key can obtain the certificate and verify
it by attached trusted signature

 X.509 Certificates

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Key Distribution using Asymmetric Encryption

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Public-Key Certificate Use

X.509 Certificates

• Standard for a Public Key Infrastructure (PKI)
 Set of hardware, software, people, policies and procedures needed to
create, manage, store, distribute and revoke digital certificates based
on asymmetric cryptography

• Distributed set of servers that maintains a database about users.

• Assumes a strict hierarchical system of certificate authorities (CAs)
for issuing the certificates

• Each certificate contains the public key of a user and is signed with
the private key of a CA.

• Is used in S/MIME, IP Security, SSL/TLS and SET.
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• RSA is recommended to use.

X.509 Certificates

• A certification authority issues a certificate binding a public key to
a particular distinguished user
 A certificate authority or certification authority (CA) is an entity which
issues digital certificates for use by other parties. It is an example of
a trusted third party. There are many commercial CAs that charge for
their services. Institutions and governments may have their own
CAs, and there are free CAs.

• An organization's trusted root certificates can be distributed to all
employees so that they can use the company PKI system

• X.509 also includes standards for certificate revocation list (CRL)
implementations

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X.509 Certificates

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X.509 Certificates

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X.509 Formats

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Obtaining a User’s Certificate

• Characteristics of certificates generated by CA:

 Any user with access to the public key of the CA can recover the
user public key that was certified.

 No part other than the CA can modify the certificate without this
being detected.

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Revocation of Certificates

• Reasons for revocation:

 The users secret key is assumed to be compromised.
 The user is no longer certified by this CA.
 The CA’s certificate is assumed to be compromised.

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Lecture 9 key distribution and user authentication

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