Download Security & Cryptography

Network Security
Today’s Universities Campus
Perimeter Security
Anti-virus system
100 %
Firewalls
Remote access VPN,
using IPSEC
96.2 %
78.8 %
78.8 %
Access control
55.8 %
Content filtering
57.7 %
Intrusion Detection System
Remote access VPN
using SSL
Other *
Anti-virus system
Firewalls
Remote access VPN, using IPSEC
Access control
Content filtering
Intrusion Detection System
Remote access VPN using SSL
Other
25 %
11.5 %
* Other includes packet shapers,
proxy servers and smart-card
authentication.
Security challenges for remote offices
53.8 %
Lack of personnel/expertise
Complexity
Management costs are too high
Solution costs are too high
Lack of one-stop shopping from vendors
51.9 %
42.3 %
36.5 %
21.2 %
Agenda
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NAT – the most common and quite effective zeromainetnance firewall
PacketFilters and RealFirewalls
SSL/TLS: transport layer security
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Easy to use
CA infrastructure
SSH
IPSec: network layer security (VPN)

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Difficult to deploy
Transport or Tunnel mode
Use of Private Addresses

Routers in the public Internet will not route
packets whose destination are private addresses
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10.0.0.0/8,
172.16.0.0/12,
192.168.0.0/16
However, it is possible for routers in a private
network to route packets with private addresses
The same private addresses can be reused in
different private networks
NAT Basics

Network Address
Translator (NAT)
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Defined in RFC 3022
Standard application

map private IP
address range
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10.0.0.0 –
10.255.255.255
172.16.0.0 –
172.31.255.255
192.168.0.0192.168.255.255
to public IP address
range
Network Address Port Translation
(NAPT or Masquerading)
NAPT Basics


Network
Address Port
Translator
Can map
multiple
private IP
addresses
and ports to
one public IP
address and
ports
NAT Internals


NAT modifies headers in IP and TCP/UDP
IP header

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
Source (outgoing) or destination (incoming) IP
address
IP header checksum
TCP/UDP header

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Source (outgoing) or destination (incoming)
TCP/UDP port
TCP/UDP checksum
NAT
 Fields modified in IP and TCP header:
IP header
TCP Header
vsn len
tos
total length
source port
destination port
identification
flgs fragment offset
sequence number
TTL
protocol header checksum
acknowledgement number
source IP address
hlen rsv
flags
window size
destination IP address
TCP checksum
urgent pointer
options (optional)
data
options (optional)
data (optional)
NAT
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Some protocols include IP address in data portion of
IP datagram
Example is FTP:

FTP uses 2 connections
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Control connection for login, commands
Data connection for data transfer
FTP client tells FTP server how to open the data
connection -- supplies IP address and port
These are in data section of IP datagram; not protocol
headers, so NAT translation becomes application-specific
NAT - ALG’s
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Application Layer Gateways (or ALG’s) sit on
NAT gateway to translate IP and port
information in data
Must have separate ALG for each application
to be translated
Common applications which need ALG:
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FTP, DNS, SNMP, H.323 (Voice over IP)
USNET-NAT has an FTP ALG
Further complications possible besides
IP/Port translation
NAT ALG for FTP
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FTP ALG must:
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Translate IP address in data portion
Set up NAT router to accept incoming connection
Modify TCP (or UDP) checksum
Check for data length changes - if even one
segment length changes, modify TCP sequence
and ACK numbers for remainder of session
RFC 3022
Example NAT Configuration
Router Running NAT
ISP Router
198.198.50.0
Internet
Ethernet
www.google.com
216.239.57.99
10.0.0.50
Types of NAT I

Static NAT
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maintains a fixed mapping from private addresses to global
addresses, which must be configured manually.
Dynamic NAT

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Global IP address is issued for each “session”
TCP/IP: NAT router checks for SYN/FIN flags
Types of NAT II
1.
2.
3.
4.
Full Cone
Restricted Cone
Port Restricted Cone
Symmetric
Network Address Translation


NAT is a major problem for media
communications
NAT:
Full Cone

Any computer can send back data to an open
port.
Restricted Cone

Any computer can send back data to an open
port AFTER we send data to their IP.
Port Restricted Cone
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Same as restricted cone but we need to first
send data to their IP AND the port that will be
allowed to send back.
Symmetric
Internet Security Threats I
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Packet Sniffing
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Broadcast media e.g. Ethernet, wireless comms
Promiscuous NIC reads all packets passing by
Can read all unencrypted data (e.g. passwords)
E.g. C sniffs B’s packets
Internet Security Threats II
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IP Spoofing
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Can generate “raw” IP packets directly from application, putting
any value into IP source address field
Receiver can’t tell if source is spoofed
E.g.: C pretends to be (trusted host) B
Internet Security Threats III
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Denial of service (DOS)
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Flood of maliciously generated packets “swamp” to receiver
Distributed DOS (DDOS): multiple coordinated sources swamp
one receiver
E.g.: C and remote host SYN-attack A
No real defense against this attack!!
Types of firewalls
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Packet filters
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Standard packet filter
Stateful packet filter
Proxy gateways
Network Address Translation (NAT)
Intrusion Detection
Logging
Components of firewall
Firewall Example
HTTP-Server
(only port 80 open)
Internet
Firewall
And NAT
Gateway
File-Server
(not accessible
from outside)
Packet Filtering
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Block or allow packets based on rules.
Filtering based on packet headers and interface it
arrives on.
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Filtering Strategies
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Example – Inbound telnet open not allowed.
That which is not explicitly permitted is prohibited.
That which is not explicitly prohibited is permitted.
Session and protocol tracking
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Fragmented IP packets
Packets violating the L4-L7 protocol
Proxy Servers

Proxy services sit between user on the inside and
server on the outside. Instead of talking directly,
user and server talk through proxy.
Firewall
Dual
homed
Host
Internet
www.google.com
216.239.57.99
Proxy Server
Ethernet
Network Address Translation
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Network Address Translation (NAT) allows a network
to use one set of addresses internally and a different
set when dealing with external networks.
It helps conceal internal network and force
connections to go through choke point.
Router does the extra work required for address
translation.
Threat
Alice
Bob
Eve
•Alice and Bob want to communicate
•Eve is eavesdropping (intercept, delete, add messages)
What is Network Security?

Secrecy: Only sender and intended receiver
should be able to “understand” message

Authentication: Sender and receiver want to
confirm identity of each other
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Message Integrity: Sender and receiver want
to ensure that message has not been altered
without detection
Taxonomy of Network Security
Secure Communication
Symmetric
Cryptography
(e.g., DES)
Asymmetric
Cryptography
(e.g., RSA)
Message
Digests
(e.g., MD5)
Cryptographic Security Technologies

En-/Decryption/Signing of E-Mail
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En-/Decryption of Shell Communication
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e.g. PrettyGoodPrivacy (PGP)
e.g. SecureShell (SSH)
En-/Decryption on Protocol Level
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e.g. SSL (TCP), IPSec (IP)
Basic crypto applications
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Algorithms: DES, AES, 3DES
 Used for actual reversible encryption
 “non-entropic”, reversible operations
 Requires a unique “secret key” for the encryptor and
decryptor
Hashes: SHA-1, MD5
 Used to generate a unique mathematical “summary value”
for a given dataset
 “Entropic”, non-reversible operation
 Used to authenticate a data set
 Can be combined with a “secret key” value to create a
custom Hash- ensures that your hash was created by
someone you trust.
Symmetric Key Distribution
• Key distribution
• Public key via trusted Certificate
Authorities
• Symmetric key?
•
•
Diffie-Helman Key Exchange
Public key, then symmetric key (e.g. SSL)
Secure Socket Layer (SSL)
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SSL works at transport layer. Provides security to any TCPbased app using SSL services.
SSL: used between WWW browsers, servers for E-commerce
(shttp, scp).
SSL security services:
 server authentication
 data encryption
 client authentication (optional)
Server authentication:
 SSL enabled browser includes public keys of trusted CAs.
 Browser requests servercertificate, issued by trusted CA.
 Browser uses CA’s public key to extract server’s public key from
certificate.
Visit your browser’s security menu to see its trusted CAs.
SSL and TLS
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SSL designed by Netscape
TLS IETF standard
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SSL and TLS provide applications:
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compromise between SSL and a Microsoft protocol
Encryption
Server authentication
(Optional) client authentication
SSL programming libraries are pretty easy to
use
SSL Protocol Architecture
SSL
Handshake
Protocol
SSL
Change
Cipher
Spec
Protocol
SSL Alert
HTTP, other
Protocol
apps
SSL Record
Protocol
TCP
SSL Handshake
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Pretty complicated
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Server (and client) authentication
Negotiation of:
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why HTTPS websites seem sooooooo slow.
Encryption algorithm
MAC algorithm
Encryption key
Must be done before any data transmission
SSL/TLS and IPSec
How does SSL differ?
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SSL is based on PKI, which uses public/private
key pairs- using entirely different math
Designed to enable secure transfer of data (like a
temporary crypto key) to someone you don’t
necessarily trust
IKE/IPSec does not use PKI, as it is inherently
less safe- and designed for e-commerce use
Actually, PKI-like key exchange is used in some
limited ways in IKE, but the core of IPSec is not
based on public/private key exchange
IPSec Overview
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What is IKE and IPSEC?
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Generally speaking, IKE is a method for securely
exchanging encryption ciphers that will be used in a later
encrypted session
IPSec is an overall term used to describe encrypted data
communication over IP, using the keys exchanged with IKE
Remember, the problem is not just encrypting the messagesit’s keeping your keys safe in the long term
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This is accomplished by renegotiating keys often in IPSec- this
compartmentalizes the encryption and data exchange
This means that secret keys must be exchanged often
IPSec Architecture
IPSec
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There are three parts to IPSec:
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AH- authentication header- provides session security at a
“sophisticated” level by checking data integrity and
protecting against “replay” attacks (protocol 51)
ESP- encapsulating security payload- provides the bulk
data encryption method (protocol 50)
IKE- handles the exchange of secret keys used in the prior
two categories (udp port 500)

NOTE: IKE generally cannot be NATted, as the IP addresses
used by each participating gateway are tracked, and NAT
looks like a replay attack
The guts of key exchange
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Sending Gateway determines a packet needs to be encrypted
Sending Gateway opens an IKE session with the Receiving
gateway- this step defines the IKE SA
Diffie-Hellman key exchange uses hashing of a certificate or
shared secret to authenticate each gateway, and sets up a
public/private data exchange channel
Sending and Receiving Gateways exchange protocol settings,
algorithm settings, and secret keys using PKI
A new IPSec SA is defined for the ESP tunnel, and data begins
to be transferred
New term: Selector- a logical construct similar to a route, that
allows the gateway to determine if an inbound packet is to be
encrypted and passed over a particular SA
Quick Mode IKE
Hash type, SA type (ESP), IP information (encryption domains/selectors)
Hash type, SA type (ESP), IP information (encryption domains/selectors)
ACK HASH
return HASH
More details:
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You don’t really have to use IKE:
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Enter many large ugly numbers
Keep track of them and keep them secret
Pass them from site to site
Change them secretly
Have fun!
IPSec in Tunnel Mode
IP Header
New IP Header ESP Head Old IP Head
IP DATA
IP DATA
ESP trailer ESP Auth
Authenticated and Encrypted
What does the header look like?

Here’s a picture:
NEW IP HEADER
Security Parameter Index
Sequence Number
Initialization Vector
Encrypted IP Header
UDP header (or whatever)
DATA
Data
Padding
Trailer: padding, pad ln
ESP Authentication
Encap. Header
ESP Header
ESP Header
ESP Header
ESP Trailer
Why padding? Some Algorithms (DES) require specific block sizes for “Cipher Block Chaining”,
which speeds encryption.
IPsec Transport mode
• ESP protocol provides network-layer secrecy,
source host authentication and data integrity
• TCP/UDP segment is surrounded by header and
trailer fields
•
•
DES-CBC encryption of TCP/UDP segment + trailer
Trailer lists the Protocol of the segment (TCP, or
UDP, or …). Hidden from observers.
• Normal IP routing using IP header.
Destination sees protocol=50 and decrypts ESP
packet
IPsec – no encryption
• AH protocol provides source authentication and
data integrity, but not secrecy
• Insert an AH header between IP header
(indicated by Protocol = 51)
•
•
Next Header field indicates whether segment is
TCP, UDP, etc.
Authentication Data field contains a digital
signature, or signed message digest calculated over
the original IP datagram
•
•
•
Provides source authentication
Provides datagram integrity tamper check
Digital signature could be DES, MD5, or SHA - negotiated
Tunnel and Transport Mode
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Authentication Header (AH)
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Authenticates the sender
Encapsulating Security Payload (ESP)
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Data encryption
Can be done in two ways:
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Transport mode: only the transport layer segment is
encrypted
Tunnel mode
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encrypt the entire IP datagram
put it inside another IP datagram
IPsec (7)
IP
source
IPsec
gateway
Secure Intranet
Secure Tunnel over
Insecure IP routing
• Some implications:
•
IPsec
gateway
IP
dest
Secure Intranet
Virtual Private Networks (VPN’s) are created and
connected using IPsec
• Create IPsec gateways that tunnel/encapsulate
across the insecure Internet = “Virtual”
• IPsec provides confidentiality = “Private”
SSH Overview
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SSH = Secure Shell.
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Initially designed to replace insecure rsh, telnet utilities.
Secure remote administration (mostly of Unix systems).
Extended to support secure file transfer and e-mail.
Latterly, provide a general secure channel for network
applications.
SSH-1 flawed, SSH-2 better security (and different
architecture).
SSH provides security at Application layer.
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Only covers traffic explicitly protected.
Applications need modification, but port-forwarding eases
some of this (see later).
Built on top of TCP, reliable transport layer protocol.
SSH Overview
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SSH Communications Security (SCS).
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Open source version from OpenSSH.
IETF Secure Shell (SECSH) working group.
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www.ssh.com.
Founded by Tatu Ylonen, writer of SSH-1.
SSH is a trademark of SCS.
Standard for SSH in preparation.
www.ietf.org/html.charters/secsh-charter.html.
Long-running confusion and dispute over naming.
SSH-2 Architecture
SSH-2 adopts a three layer architecture:
 SSH Transport Layer Protocol.
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SSH Authentication Protocol
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Initial connection.
Server authentication (almost always).
Sets up secure channel between client and server.
Client authentication over secure transport layer channel.
SSH Connection Protocol
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Supports multiple connections over a single transport layer
protocol secure channel.
Efficiency (session re-use).
SSH-2 Architecture
Applications
SSH Connection Protocol
SSH Authentication Protocol
SSH Transport Layer Protocol
TCP
SSH-2 Security Goals
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Server (nearly) always authenticated in transport
layer protocol.
Client (nearly) always authenticated in
authentication protocol.
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Establishment of a fresh, shared secret.
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By public key (DSS, RSA, SPKI, OpenPGP).
Or simple password for particular application over secure
channel.
Shared secret used to derive further keys, similar to
SSL/IPSec.
For confidentiality and authentication in SSH transport
layer protocol.
Secure ciphersuite negotiation.

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Encryption, MAC, and compression algorithms.
Server authentication and key exchange methods.
SSH-2 Algorithms

Key establishment through Diffie-Hellman key
exchange.
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Variety of groups supported.
Server authentication via RSA or DSS signatures on
nonces (and other fields).
HMAC-SHA1 or HMAC-MD5 for MAC algorithm.
3DES, RC4, or AES finalists (Rijndael/Serpent).
Pseudo-random function for key derivation.
Small number of ‘official’ algorithms with simple
DNS-based naming of ‘private’ methods.
SSH-1 versus SSH-2

Many vulnerabilities have been found in SSH-1 .
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SSH-1 Insertion attack exploiting weak integrity
mechanism (CRC-32) and unprotected packet length field.
SSHv1.5 session key retrieval attack (theoretical).
Man-in-the-middle attacks (using e.g. dsniff).
DoS attacks.
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Overload server with connection requests.
Buffer overflows.
But SSH-1 widely deployed.
And SSH-1 supports:
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Wider range of client authentication methods (.rhosts and
Kerberos).
Wider range of platforms.
SSH Port Forwarding
Without SSH or port forwarding.
LS Login
server
UM User’s
machine
Src: UM Dest: LS Port: 23
Src: UM Dest: MI Port: 113
Src: UM Dest: MO Port: 25
MI Mail in
server
MO Mail out
server
SSH Port Forwarding


Recall: TCP port number ‘identifies’ application.
SSH on local machine:

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Intercepts traffic bound for server.
Translates standard TCP port numbers.
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E.g. port 113  port 5113.
Sends packets to SSH-enabled server through SSH
secure channel.
SSH-enabled server:

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Receives traffic.
Re-translates port numbers.

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E.g. port 5113  port 113.
Forwards traffic to appropriate server using internal
network.
SSH Port Forwarding
With SSH and port forwarding.
MI Mail in
server
UM User’s
machine
Src: UM Dest: LS Port: 23
LS
SSH-enabled
login
server
MO Mail out
server
Src: UM Dest: MO Port: 25
Src: UM Dest: MI Port: 113
Src: UM Dest: LS Port: 5113 Src: UM Dest: LS Port: 5025
Src: LS Dest: MI Port: 113 Src: LS Dest: MO Port: 25
SSH Applications

Anonymous ftp for software updates, patches...
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Secure ftp.
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E.g.upload of webpages to webserver using sftp.
Server now needs to authenticate clients.
Username and password may be sufficient, transmitted over
secure SSH transport layer protocol.
Secure remote administration.

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
No client authentication needed, but clients want to be sure
of origin and integrity of software.
SysAdmin (client) sets up terminal on remote machine.
SysAdmin password protected by SSH transport layer
protocol.
SysAdmin commands protected by SSH connection
protocol.
Guerilla Virtual Private Network.
6.3 Comparing IPSec, SSL/TLS, SSH

All three have initial (authenticated) key
establishment then key derivation.

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IKE in IPSec
Handshake Protocol in SSL/TLS (can be
unauthenticated!)
Authentication Protocol in SSH
All protect ciphersuite negotiation.
All three use keys established to build a
‘secure channel’.
Comparing IPSec, SSL/TLS, SSH

Operate at different network layers.

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
This brings pros and cons for each protocol suite.
Recall `Where shall we put security?’ discussion.
Naturally support different application types, can all be
used to build VPNs.
All practical, but not simple.

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Complexity leads to vulnerabilities.
Complexity makes configuration and management harder.
Complexity can create computational bottlenecks.
Complexity necessary to give both flexibility and security.
Comparing IPSec, SSL/TLS, SSH
Security of all three undermined by:
 Implementation weaknesses.
 Weak server platform security.


Weak user platform security.


Keystroke loggers, malware,…
Limited deployment of certificates and infrastructure
to support them.


Worms, malicious code, rootkits,…
Especially client certificates.
Lack of user awareness and education.



Users click-through on certificate warnings.
Users fail to check URLs.
Users send sensitive account details to bogus websites
What is a VPN
Public networks are used to move information between trusted network segments using
shared facilities like frame relay or atm
A VIRTUAL Private Network replaces all of the above utilizing the public
Internet Performance and availability depend on your ISP and the Internet
VPN Implementations
VPN as your Intranet
VPN Components
Technologies
Application Layer: SSL
Tunnel vs Transport

Transport

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
Implemented by the end point systems
Real address to real address
Cannot ‘go through’ other networks
Tunnel

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

Encapsulation of the original IP packet in another
packet
Can ‘go through’ other networks
End systems need not support this
Often PC to a box on the ‘inside’
PPTP: Free from Microsoft
PPTP: Security
Outgoing PPTP Client Through NAT
a
Internet
10.0.0.2
NAT
b
c
10.0.0.3
10.0.0.4
10.0.0.1
204.x.1.10
web server
VPN Comparisons