Network Security

Today’s Universities CampusPerimeter Security

Anti-virus system Firewalls Remote access VPN, using IPSEC Access controlContent filteringIntrusion 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 *

100 %

96.2 %

78.8 %

78.8 %

55.8 %

57.7 %

25 %

11.5 %

* Other includes packet shapers, proxy servers and smart-card authentication.

Security challenges for remote offices

53.8 % 51.9 %

42.3 %

36.5 %

21.2 %

Lack of personnel/expertise Complexity

Solution costs are too highLack of one-stop shopping from vendors

Management costs are too high

Agenda

NAT – the most common and quite effective zero-mainetnance firewall

PacketFilters and RealFirewalls SSL/TLS: transport layer security

Easy to use CA infrastructure SSH

IPSec: network layer security (VPN) 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 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) Defined in RFC

3022 Standard application

map private IP address range

10.0.0.0 – 10.255.255.255

172.16.0.0 – 172.31.255.255

192.168.0.0-192.168.255.255

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

Source (outgoing) or destination (incoming) IP address

IP header checksum TCP/UDP header

Source (outgoing) or destination (incoming) TCP/UDP port

TCP/UDP checksum

NAT

Fields modified in IP and TCP header:

tos total lengthlenvsnidentification

TTL protocol header checksumsource IP address

destination IP address

flgs fragment offset

options (optional)

data

destination port

options (optional)

data (optional)

source portsequence number

acknowledgement numberwindow size

urgent pointerTCP checksumhlen rsv flags

IP header TCP Header

NAT

Some protocols include IP address in data portion of IP datagram

Example is FTP: FTP uses 2 connections

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

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: 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

FTP ALG must: 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

Ethernet

10.0.0.50www.google.com216.239.57.99

Router Running NATISP Router

Internet

198.198.50.0

Types of NAT I

Static NAT maintains a fixed mapping from private addresses to global

addresses, which must be configured manually. Dynamic NAT

Global IP address is issued for each “session” TCP/IP: NAT router checks for SYN/FIN flags

Types of NAT II

1. Full Cone

2. Restricted Cone

3. Port Restricted Cone

4. 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

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

Packet Sniffing 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

IP Spoofing 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 Denial of service (DOS)

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

Packet filters Standard packet filter Stateful packet filter

Proxy gateways Network Address Translation (NAT) Intrusion Detection Logging

Components of firewall

Firewall Example

Firewall

Internet

HTTP-Server(only port 80 open)

File-Server(not accessiblefrom outside)And NAT

Gateway

Packet Filtering Block or allow packets based on rules. Filtering based on packet headers and interface it

arrives on. Example – Inbound telnet open not allowed.

Filtering Strategies That which is not explicitly permitted is prohibited. That which is not explicitly prohibited is permitted.

Session and protocol tracking 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.

Ethernet

www.google.com216.239.57.99

Internet

Proxy Server

Firewall DualhomedHost

Network Address Translation

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

Message Integrity: Sender and receiver want to ensure that message has not been altered without detection

Taxonomy of Network Security

Secure Communication

SymmetricCryptography(e.g., DES)

AsymmetricCryptography(e.g., RSA)

MessageDigests(e.g., MD5)

Cryptographic Security Technologies En-/Decryption/Signing of E-Mail

e.g. PrettyGoodPrivacy (PGP) En-/Decryption of Shell Communication

e.g. SecureShell (SSH) En-/Decryption on Protocol Level

e.g. SSL (TCP), IPSec (IP)

Basic crypto applications

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) SSL works at transport layer. Provides security to any TCP-

based 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

SSL designed by Netscape TLS IETF standard

compromise between SSL and a Microsoft protocol SSL and TLS provide applications:

Encryption Server authentication (Optional) client authentication

SSL programming libraries are pretty easy to use

SSL Protocol Architecture

TCP

SSL Record Protocol

SSL Handshake

Protocol

SSL Alert

Protocol

HTTP, other apps

SSL Change Cipher Spec

Protocol

SSL Handshake

Pretty complicated why HTTPS websites seem sooooooo slow.

Server (and client) authentication Negotiation of:

Encryption algorithm MAC algorithm Encryption key

Must be done before any data transmission

SSL/TLS and IPSec

How does SSL differ?

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

What is IKE and IPSEC? 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 messages- it’s keeping your keys safe in the long term 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

There are three parts to IPSec: 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

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)

ACK HASH

return HASH

Hash type, SA type (ESP), IP information (encryption domains/selectors)

More details: You don’t really have to use IKE:

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

New IP HeaderESP Head

IP DATAIP Header

IP DATAOld IP Head ESP trailerESP Auth

Authenticated and Encrypted

What does the header look like?

Here’s a picture:

Security Parameter IndexSequence NumberInitialization VectorEncrypted IP Header

UDP header (or whatever)DATA

DataTrailer: padding, pad ln

ESP Authentication

NEW IP HEADER

Padding

ESP Header

Encap. 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

Authentication Header (AH) Authenticates the sender

Encapsulating Security Payload (ESP) Data encryption Can be done in two ways:

Transport mode: only the transport layer segment is encrypted

Tunnel mode encrypt the entire IP datagram put it inside another IP datagram

IPsec (7)

• Some implications:• 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”

IPsecgateway

IPdest

IPsource

IPsecgateway

Secure Tunnel overInsecure IP routing

Secure Intranet Secure Intranet

SSH = Secure Shell. 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.

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

SSH Overview

SSH Communications Security (SCS). www.ssh.com. Founded by Tatu Ylonen, writer of SSH-1. SSH is a trademark of SCS.

Open source version from OpenSSH. IETF Secure Shell (SECSH) working group.

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.

Initial connection. Server authentication (almost always). Sets up secure channel between client and server.

SSH Authentication Protocol Client authentication over secure transport layer channel.

SSH Connection Protocol Supports multiple connections over a single transport layer

protocol secure channel. Efficiency (session re-use).

SSH-2 Architecture

SSH Transport Layer Protocol

SSH Authentication Protocol

TCP

SSH Connection Protocol

Applications

Server (nearly) always authenticated in transport layer protocol.

Client (nearly) always authenticated in authentication protocol. By public key (DSS, RSA, SPKI, OpenPGP). Or simple password for particular application over secure

channel. Establishment of a fresh, shared secret.

Shared secret used to derive further keys, similar to SSL/IPSec. For confidentiality and authentication in SSH transport layer

protocol. Secure ciphersuite negotiation.

Encryption, MAC, and compression algorithms. Server authentication and key exchange methods.

SSH-2 Security Goals

Key establishment through Diffie-Hellman key exchange. 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-2 Algorithms

SSH-1 versus SSH-2

Many vulnerabilities have been found in SSH-1 . 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.

Overload server with connection requests. Buffer overflows.

But SSH-1 widely deployed. And SSH-1 supports:

Wider range of client authentication methods (.rhosts and Kerberos).

Wider range of platforms.

SSH Port Forwarding

Without SSH or port forwarding.

UM User’smachine

LS Loginserver

MO Mail outserver

MI Mail inserver

Src: UM Dest: LS Port: 23

Src: UM Dest: MI Port: 113

Src: UM Dest: MO Port: 25

SSH Port Forwarding

Recall: TCP port number ‘identifies’ application. SSH on local machine:

Intercepts traffic bound for server. Translates standard TCP port numbers.

E.g. port 113 port 5113. Sends packets to SSH-enabled server through SSH secure

channel. SSH-enabled server:

Receives traffic. Re-translates port numbers.

E.g. port 5113 port 113. Forwards traffic to appropriate server using internal network.

SSH Port Forwarding

UM User’smachine

LSSSH-enabled

loginserver

MO Mail outserver

MI Mail inserver

Src: UM Dest: LS Port: 23

With SSH and port forwarding.

Src: UM Dest: MI Port: 113Src: UM Dest: LS Port: 5113Src: LS Dest: MI Port: 113

Src: UM Dest: MO Port: 25Src: UM Dest: LS Port: 5025Src: LS Dest: MO Port: 25

Anonymous ftp for software updates, patches... No client authentication needed, but clients want to be sure of

origin and integrity of software. Secure ftp.

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.

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. E.g. use SSH + port forwarding to secure e-mail

communications.

SSH Applications

6.3 Comparing IPSec, SSL/TLS, SSH

All three have initial (authenticated) key establishment then key derivation. 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.

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.

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, SSHSecurity of all three undermined by: Implementation weaknesses. Weak server platform security.

Worms, malicious code, rootkits,… Weak user platform security.

Keystroke loggers, malware,… Limited deployment of certificates and infrastructure to support

them. 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 (“phishing”) in

response to official-looking e-mail.

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 Implemented by the end point systems Real address to real address Cannot ‘go through’ other networks

Tunnel 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

10.0.0.1

10.0.0.4

10.0.0.3

10.0.0.2

web servera

b

c

NAT

204.x.1.10

Internet

Outgoing PPTP Client Through NAT

VPN Comparisons