Transcript No Slide Title

Distributed Security Issues 2
CP3397
Design of Networks and Security
Monday, July 20,
2015
1
Objectives
To investigate



Authentication techniques
Authorisation techniques.
Administration issues
Monday, July 20,
2015
2
Authentication
The process of establishing proof of identity
at least two levels required:

One - Client connects to Server
 includes both machine and user

Two - for every message communicated
Common mechanisms

Password protection
 may need encryption over network

Message authentication
 often uses a checksum
Monday, July 20,
2015
3
Symmetric encryption - DES
DES is a block cipher that operates on 64 bit data
fragments using 56 bit key
In the Chain Block Cipher (CBC) mode, each block
of plaintext is exclusive-ORed with the ciphertext
output from the previous encryption operation.
Thus, the next block of ciphertext is a function of its
corresponding plaintext, the 56-bit key and the
previous block of ciphertext.
Identical blocks of plaintext no longer generate
identical ciphertext, which makes this system much
more difficult to break.
Monday, July 20,
2015
4
The CBC mode of DES is the normal
technique used for encryption in modern
business data communications.
A variation on CBC is used where the
message may not be a multiple of 64 bits, or
where interactive (character at a time)
encryption and decryption is desired. This is
called Cypher Feedback Mode (CBM), and
uses shift registers to permit one byte at a
time to be encrypted or decrypted.
Monday, July 20,
2015
5
Network Authorisation
Firewalls
A Firewall is a particular type of network
authorisation mechanism
It can operate at the boundary of a network,
i.e. between a LAN and the Internet or at the
boundary of a security domain i.e two
separate security domains on one LAN
A firewall filters messages destined for
specific objects
Monday, July 20,
2015
6
Firewalls – in concept
It is a installation that regulates the flow of
traffic between network segments.
Based its decisions on the idea that some of
the packets may be coming from a nonsecure segment into a secure one
Decisions based on:- protocols used,
destination address, source address etc.
Monday, July 20,
2015
7
FireWalls
The Outside
World
Monday, July 20,
2015
The secure network ???
T
h
e
s
e
c
u
r
eA Firewall
N
e
8
Firewalls - technically
Can be either software or hardware –
just impellent the rules/policy
Inside
network
Outside
network
Router
Packet filtering
Monday, July 20,
2015
Router
Application
gateway
Packet filtering
9
Routers
These can make decisions on whether to forward a packet
based on its destination or source Address, and port no –
stop users telneting in etc.
Packets failing tests are just dropped without informing
sender
Router on internet side firewall decides if packet can enter
LAN
Router on LAN side decides if packet can leave LAN – bit
more tricky as port numbers are assigned according to
convention, also some services (e.g. FTP) port numbers are
assigned dynamically - security could be bypassed
Monday, July 20,
2015
10
Application Gateway
Operates at the application level – inspects
contents of packets.
Can do virus scanning on packets, or check
emails for improper content etc. In practice
may be too time consuming – so full service
may not be attempted
May also do NAT at this point – leads to
internal addressing which can make hacking
more difficult
Monday, July 20,
2015
11
Firewalls
Denial Of Service – firewalls cannot combat these attacks as
they mask a genuine request
Hacker sends a TCP/SYN packet to establish connection –
web server allocates a table slot for the connection and
sends SYN + ACK packet back. If hacker does not respond
table slot tied up for few seconds until timed out.
Hacker sends 1000’s of these and table slots all tied up with
these – no legitimate connections can be made – DOS.
Usually request packets have false IP address in to mask
hacker
Worst variant is a Distributed Denial of service attack –
hacker already broken into hundreds of computers around
world and get them to attack your website – difficult to
defend
against
even if detected the CPU is tied up
Monday, July
20,
2015
12
processing
and discarding these bogus requests
Non-Repudiation
The process of establishing beyond doubt


The identity of the sender - proof of origin
That delivery has taken place - proof of delivery
 e.g. Electronic Data Interchange (EDI)


ensuring that an order is genuine
ensuring that an invoice has been received
Cryptography can be used for Digital
Signature

if an originator’s public key will decrypt a
message, the originator must have sent it
Monday, July 20,
2015
13
Possible attacks
Impostor

Ian logs onto the server but then pretends to be
Andy
Eavesdropper

Ian logs onto the server but sits and listens to
Andy’s conversation with the server and gathers
some very sensitive data
Replay

A packet is copied and then resent later by an
impostor – could be a packet with the encrypted
password in from a legitimate user
Monday, July 20,
2015
14
Man in the middle attack
Ian
Andy
Serve
Andy
rZ
Sever Z
Andy thinks he is talking to server Z, he is actually
talking to Ian, and the sever z thinks his conversation is
with Andy but in fact it is with Ian
And there are more possible attack scenarios
Monday, July 20,
2015
15
Kerberos protocol
Those attacks all pose different problems in
authorisation
Authorisation is compromised and so is data integrity,
with the complication of data being replayed at a
later date.
Kerberos makes use of a trusted third party, termed a
Key Distribution Center (KDC), which consists of two
logically separate parts: an Authentication Server
(AS) and a Ticket Granting Server (TGS). Kerberos
works on the basis of "tickets" which serve to prove
the identity of users.
Monday, July 20,
2015
16
Kerberos overview
Kerberos maintains a database of secret keys;
each entity on the network — whether a
client or a server — shares a secret key
known only to itself and to Kerberos.
Knowledge of this key serves to prove an
entity's identity.
For communication between two entities,
Kerberos generates a session key which they
can use to secure their interactions.
Monday, July 20,
2015
17
Kerberos overview
Who am I?
Andy
Ticket
Authenticator
Server (AS)
Ticket Granting
Server (TGS)
Session
Session Server
Monday, July 20,
2015
18
Kerberos overview
Is based on symmetric cryptography to pass keys
between and authentication server and the client.
Basic idea that the client uses long lived
memorised passwords to get short lived keys,
that aren't worth stealing from these insecure
desktops
Two versions of Kerberos are in widespread use
(4 and 5)
Monday, July 20,
2015
19
Kerberos simplified view
On a "normal" network which uses passwords to
authenticate users, when a user requests a network
service that requires authentication, the user is prompted
to type in their password. The password is transmitted in
plaintext over the network, and access to the network
service is granted.
The central problem solved by Kerberos is how to use
passwords for authentication without sending them over
the network
It’s important to remember that in any Kerberos authentication
dialogue, there are three parties: the Kerberos server, the client
requesting a service, and the server requesting proof of identity. They
all need to have credentials—principals, in Kerberos-terminology—
stored in the Kerberos database.
Monday, July 20,
2015
20
Kerberos overview - LINUX
the Kerberos database contains:

principals and their keys (for users, their keys are derived from
their passwords).
also contains keys for all of the network services.
When a user on the network logs in to their workstation,
their principal is sent to the Key Distribution Center (KDC)
as a request for a Ticket Granting Ticket (TGT). This
request can be sent by the login program (so that it is
transparent to the user) or can be sent by the kinit
program after the user logs in.
The KDC checks for the principal in its database. If the
principal is found, the KDC creates a TGT, encrypts it
using the user's key, and sends it back to the user.
Monday, July 20,
2015
21
Kerberos overview
The login program or kinit decrypts the TGT using the user's key
(which it computes from the user's password). The TGT, which is
set to expire after a certain period of time, is stored in your
credentials cache.
An expiration time is set so that a compromised TGT can only be
used for a certain period of time, usually eight hours (unlike a
compromised password, which could be used until changed). The
user will not have to re-enter their password until the TGT expires
or they logout and login again.
When the user needs access to a network service, the client uses
the TGT to request a ticket for the service from the Ticket Granting
Service (TGS), which runs on the KDC.
The TGS issues a ticket for the desired service, which is used to
authenticate the user.
Monday, July 20,
2015
22
Practicalities
Kerberos needs to consider both efficiency and security


Security means tickets should expire quickly (seconds) to stop replay
Efficiency means hours to reduce the load on the TGS
Even with tickets having long lifetimes the need for a centralised
ticket server would prevent Kerberos being scalable across anything
other than a modest sized network
Overcome by Kerberos split the server into an authentication server
and the TGS.
Allows the nodes in a large network to be partitioned into several
groups- each with own server that knows only about the other
servers and about the nodes in its group
Means if your in own domain you need a ticket from your own TGS
to talk to the TGS in the other domain to get a ticket to talk to other
servers in that group
Monday, July 20,
2015
23
Summary
Using Authentication we can attempt to
eliminate intrusion
We can also attempt to eliminate masquerade
There is no such thing as a perfect
authentication system
You must be aware of the risk when you
allow external access to any network.
Monday, July 20,
2015
24