4040-840 Security and Trust

all-inOne, section 6.

6. Authentication Application

6.1. Kerberos: Info

Info:

6.2. Basics

how do client/server identify each other: Mac Address IP address DNS
All individual information can be compromised
two identical IP adresses can not be used on the same network
Example: Mount of a file system:

--: export of a file system to: host/group/client
--: mount is based on IP address

How can you mount a remote FS?
Pretend you have a different (friendly) IP address

6.3. Example: Sun Network File System

The NFS is an implementation and a specification of a softwaresystem for accessing remote files
Goal: allow some degree of sharing file systems in a transparent manner.
Sharing is based on a server/client relationship.

Mount Protocol

Export. See share(1m).

% hostname
ilon
% cat /etc/dfs/dfstab

#       Place share(1M) commands here for automatic execution
 ...
#       share  -F nfs  -o rw=engineering  -d "home dirs"  /export/home2
share -F nfs -o ro=altair -d "ilon's home dir" /
share -F nfs -o rw=altair -d "ilon's home dir" /export/home

Use:

ilon% ssh  cs
cs% su -
cs# mount ilon:/export/home/bischof /mnt
cs#

The NFS Protocol

The NFS protocol provides a set of RPCs for remote file operations.

Searching for a file within a directory.
Reading a set of directory entries
Manipulation of links and directories
Accessing file attributes
Reading and writing directories

Use of NFS

% man automount
 ...
The automount utility installs autofs mount points and asso-
     ciates  an  automount map with each mount point.  The autofs
     file system monitors attempts to access  directories  within
     it  and notifies the automountd(1M) daemon.  The daemon uses
     the map to locate a file system, which it then mounts at the
     point  of  reference within the autofs file system.
 ...

See also: automount(1m) and automountd(1m).

% cat /etc/auto_master
# Master map for automounter
#
+auto_master
/net            -hosts          -nosuid,nobrowse
/home           auto_home       -nobrowse
/xfn            -xfn
% niscat auto_home.org_dir | grep bischof
bischof altair:/export/home/bischof

What is happening if I can 'fake' an existing IP address?

6.4. Services

What kind of services can a system provide?

--: Mounting of FS's
--: Mail
--: ftp
--: Web
--: CPU

What are the consquences if they ressources are 'stoled'?
How can we protect theses ressources?

--: Qood question; and your answer is?

6.5. Problems

How can the service provided be authenticated?
How can the client machine be authenticated?
How can the user be authenticated?
In a network where there is no central sys admin?

6.6. Motivation:

Single Machines/single user are protected by doors etc
Connecteted machines are not protected by default
Approaches to Security:

--: Each node assures identify of users and each server relies on this (Wer garantiert das die machinen nicht ausgetauscht werden?)
--: What does ssh guarantee you?
--: The clients authenticate themself to the server and the user identification is up to the client.
--: The user proofs her/his identity to the server and the server proofs his identity to the client

General problem: How can you/a system proof your/its identity?
Do you have a passport?

6.7. Threats

For example:

DNS attacks http://www.securityfocus.com/brief/483
Person in the middle attack
access to service by pretending to be somebody else
modify IP address to come from a 'friedly' computer
use replay attack to gain access under the use of eavesdropping

6.8. Kerberos

From http://web.mit.edu/kerberos/

Kerberos is a network authentication protocol. It is designed to provide strong authentication for client/server applications by using secret-key cryptography. A free implementation of this protocol is available from the Massachusetts Institute of Technology. Kerberos is available in many commercial products as well.

Look at: http://web.mit.edu/kerberos/advisories/MITKRB5-SA-2011-008.txt
Cerberus: is in Greek and Roman mythology, is a multi-headed hound (usually three-headed)which guards the gates of the Underworld, to prevent those who have crossed the river Styx from ever escaping.
http://www.theoi.com/Gallery/M12.1.html

6.9. Kerberos: Provides

Kerberos is an authentication service
Kerberos can be used over an unprotected network
Kerberos relies on a third part authentification server
Kerberos does not rely on the authentication of the host OS system
Information can not gained by listening into the conversation (Ali Baba's Cage)
Kerberos relies on symetric key encryption
Kerberos allows access to a services to a authorized users only, under the assumption you can not trust the environment.

6.10. The Situation

Past:

Without Kerberos:

With Kerberos:

6.11. Note: Symetric Key Encryption

A single, common key that is used to encrypt and decrypt the message
Trivial Example: shift/stick etc
Examples: Twofish, Serpent, AES (aka Rijndael), DES, Blowfish, CAST5, RC4, TDES, and IDEA.

6.12. Kerberos: Is (as the paper describes)

Secure
Reliable
Transparent
Scalable

What does each requirement entail?

Kerberos uses a third party protocol (chapter 7/spr)
client/server trust Kerberos to mediate their mutual authentication

6.13. Kerberos: Needs

Database of private keys known in a database

--: private keys are known to kerberos and the client

Kerberos can

--: derive keys from passwords
--: generate temporary private keyes
--: generate Session Keys

The AS

--: shares a secret key with each server.
--: can be are distributed

6.14. Credentials

Ticket

--: good for single server and a single client
--: {s, c, addr, timestamp, life, K{s,c} ) K{s}

Authenticator

--: can only be used once
--: can be created by the client
--: { c, addr, timestamp ) K{s,c}

6.15. Kerberos: How it Works

[equation]

Abbreviations (from: Kerberos: An Authentication Service for Open Network Systems, Jennifer G. Steiner, Clifford Neuman, Jeffrey I. Schiller)

tickets:

--: based on private key encryption
--: are used to securely pass the idenity of the person to whom the ticket was issued between the authentication server and the end server
--: a ticket is good for single server and a single client
--: A ticket is used to securly pass the identity of a user
--: A ticket is good for a single server and a user

Kerberos ticket

[equation]

authenticator:

--: authenticator contains information which, when compared against that in the ticket, proves that the client presenting the ticket is the same one to which the ticket was issued.
--: based on private key encryption
--: can only be used once (time stamp)
--: can be generated by the client

Kerberos Authenticator

[equation]

6.16. Problem

to authenticate user/machine
Servers/services

6.17. Kerberos: Getting the Initial Ticket

user enters name
request is sent to authentication server containing the user's name and the name of a special service known as the ticket-granting service
authentication server checks that it knows the client and generates a random session key
creates creates a ticket for the ticket-granting server encypted with the derived password.

--: which contains the client's name,
--: the name of the ticket-granting server,
--: the current time,
--: a lifetime for the ticket,.
--: the client's IP address,
--: and the random session key just created.
--

all encrypted in a key known only to the ticketgranting server and the authentication server.

Kerberos ticket
Questions:

Is it possible to learn something by listening to the network?

: Nein, denn das Packet ist mit verschluesselt, mit den Schluessel the Klienten, welches nur Kerberos vom password ableitet und dem Klienten bekannt ist.

user enters password

password converted to DES key
this key is used to decrypt response from authentification server

From:
http://www.scs.stanford.edu/nyu/05sp/sched/readings/kerberos.pdf

The ticket can now be used to get access to multiple services

6.18. IS Kerberos?

Secure
Reliable
Transparent
Scalable

6.19. Kerberos: Getting a Server Ticket

Request to Ticket granting server
TGS builds, if valid

--: new random session key used between client and server
--: ticket for the new server
--

6.20. Kerberos: Requesting a Service

Once per service and lifetime
assumption: client has server ticket
application builds an authenticator containing the client's name and IP address, and the current time
client then sends the authenticator along with the ticket to the server
server decrypts the ticket, uses the session key included in the ticket to decrypt the authenticator
Note: It is assumed that clocks are synchronized to within several minute.

--: Is this realistic?
--: Time allows to keep track

request received with the same ticket and timestamp as one already received can be discarded

Mutual Authentication:

Kerberos Authenticator

[equation]

6.21. Kerberos: Database

What problems do you see?

6.22. Kerberos: Admin

Is it possible to overcome the kerberos system?
What problem do you see?

Vieles muss functionieren

--: Skalierbar sicher
--: Ein einziger Punkt der alles aum Stillstand bringen kann

6.23. Kerberos: Java API

http://www.cs.rit.edu/~hpb/Jdk5/api/javax/security/auth/kerberos/package-summary.html

6.24. X.509

X.509 is an ITU-T (International Telecommunication Union) defines a framework of authentication services to its users

--: See also rfc320

X.509

--: defines specific formats for Public Key Certificates
--: the algorithm that verifies a given certificate path is valid under a give PKI

6.25. PKI

PKI arrangements enable authenticatian to each other without prior communication
PKI enables the parties

--: to establish confidentiality,
--: message integrity
--: user authentication.
--: without having to exchange any secret information in advance
--: by using a CA

Example

How can we create a secret key without having met before?

We both know a CA
I send you
you calculate
How does this help?

6.26. Digital Signatures

Digital Signatures provide authentication to a message
A digital signature must

--: verify author, date and time of signature
--: authenticate content
--: veriviable by a third party

RSA can be used to create signature keys
How can this be done?

6.27. X.509 Certificate

Certificate

--: Version
--: Serial Number
--: Algorithm ID
--: Issuer
--: Validity
--: Not Before
--: Not After
--: Subject Subject Public Key Info
--: Public Key Algorithm
--: Subject Public Key
--: Issuer Unique Identifier (Optional)
--: Subject Unique Identifier (Optional)
--: Extensions (Optional)
--: ...
--: Certificate Signature Algorithm
--: Certificate Signature

Algorithm:

--: User takes unsigned certificate, adds public key, ID
--: Generates Hash Code
--: added to the certificate
--: CA signs certificate with its private key and stores it

--: This can not be forged
--: Decrypted with public key

--: Can be access by anybody to verify users identity

6.28. Social Networks and Trust

Facebook
Linked In
etc
How to find you...

6.29. How do you know it is your friend?

email other knowledge
What is this good for?

6.30. Trust

how do you know who somebody is in real life?
You do not.
Fraud

6.31. Issues

secret crush
Facebook suicide
Is the network different to real world?

6.32. Outlook Trust Models

Direct Trust

: it is between two users

Hierarchical Trust

: trust extends from the root

A Web of Trust

: It is a graph - trusr lies in the eye of the beholder

Note: Image from Stalling's book, page 456

6.33. Addendum

Phishing (According to Gartner Research/2007)

--: cost: $3.2 Billion
--: 3.6 million Americans defrauded
--: Articles

--: @inproceedings{ 1073009, author = {Rachna Dhamija and J. D. Tygar}, title = {The battle against phishing: Dynamic Security Skins}, booktitle = {SOUPS '05: Proceedings of the 2005 symposium on Usable privacy and security}, year = {2005}, isbn = {1-59593-178-3}, pages = {77--88}, location = {Pittsburgh, Pennsylvania}, doi = {http://doi.acm.org/10.1145/1073001.1073009}, publisher = {ACM Press}, address = {New York, NY, USA}, url = {citeseer.ist.psu.edu/748409.html} } the article
--: @article{1290968, author = {Tom N. Jagatic and Nathaniel A. Johnson and Markus Jakobsson and Filippo Menczer}, title = {Social phishing}, journal = {Commun. ACM}, volume = {50}, number = {10}, year = {2007}, issn = {0001-0782}, pages = {94--100}, doi = {http://doi.acm.org/10.1145/1290958.1290968}, publisher = {ACM}, address = {New York, NY, USA}, } Social phishing