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VPN

Olga Torstensson

IDE

Halmstad University

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What is a VPN?

A Virtual Private Network (VPN) isdefined as network connectivity deployed

on a shared infrastructure with the samepolicies and security as a private network.

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VPN Taxonomy

Overlay VPNsService providers provide virtual point-to-point links.

Peer-to-peer VPNsService providers participate in thecustomer routing.

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Virtual Private Networks (VPNs)

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Why Have VPNs?

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Tunneling and Encryption

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Plaintext, Encryption,Ciphertext, and Decryption

Network

Plaintext

Hello

Encryption

Method &

Key

Ciphertext 11011101

Encryption

Key

Ciphertext 11011101 Plaintext

Hello

Decryption

Method &

Key

Decryption

Key

Interceptor

Party A

Party B

Note:

Interceptor Cannot Read

Ciphertext Without the

Decryption Key

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Use VPNs with a Variety of Devices

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VPN Types

Remote Access VPN Solutions

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Site-to-Site VPN Solutions

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VPN Types and ApplicationsVPN Types and ApplicationsType Application As Alternative To

SiteSite--toto--SiteSite

VPNVPN

ExtranetExtranet

VPNVPN

Benefits

SiteSite--toto--SiteSite

InternalInternal

ConnectivityConnectivity

Extend ConnectivityExtend Connectivity

Increased BandwidthIncreased Bandwidth

Lower CostLower Cost

Leased LineLeased Line

Frame RelayFrame Relay

ATMATM

RemoteRemote

AccessAccess

VPNVPN

Remote DialRemote Dial


DedicatedDedicated

DialDial

ISDNISDN

Ubiquitous AccessUbiquitous Access

Lower CostLower Cost

ExternalExternal


FaxFax

MailMailFacilitatesFacilitates

EE--CommerceCommerce

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VPN Requirements Vary ByVPN Requirements Vary By

ApplicationApplication

Central Site

Site-to-SiteRemote Office

ExtranetBusiness Partner

POP

DSLCable

Mobile User

Home Telecommuter

VPNInternet

Extension of classic WAN

Compatibility with diversenetwork traffic types

Integration with routing

Deployment scalability

Evolution away from dial

Per-user manageability

Multi-OS (desktop) support

Deployment scalability

Site-to-Site VPNRemote Access VPN

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Tunneling Protocols

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VPN Protocols

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Selecting Layer 3 VPN Tunnel Options

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Identifying VPN and IPSec Terms

Tunnel

Encryption/Decryption

Cryptosystem

Hashing

Authentication Authorization

Key Management

Certificate of Authority Service

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Identifying VPN and IPSec Terms

IPSec main protocols are used to provideprotection for user data:

Authentication Header AH

Encapsulating Security Payload ESP

Internet Key Exchange IKE

Internet Security Association KeyManagement Protocol ISAKMP

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Cryptographic System

ConfidentialityAuthentication

Message Integrity

Anti-Replay ProtectionClient PC withCryptographic

SystemSoftware

Server withCryptographic

SystemSoftware

Secure Communication

ProvidedAutomatically

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Cryptosystem Overview

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Symmetric Encryption

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Asymmetric Encryption

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RSA

Rivest, Shamir, Adelman (1977)

patented, royalty

public key cryptosystem

variable key length (usually 512-2048 bit)

based on the (current) difficulty of factoringvery large numbers

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RSA

each entity has two keys

public key (can be published)

private key (must be kept secret)

it is not feasible to determine the privatekey from the public key

one key encrypts, the other key decrypts amessage

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RSA

100-1000 times slower than DES

used for two services

privacywith encryption (usually smallamounts such as session keys)

authenticationand non-repudiationwithelectronic signing

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RSA encryption (privacy)

Alice gets Bobs public key

Alice encrypts message with Bobs public key

Bob decrypts message using his private key

BobAlice

ClearEncrypted

Bobs Private KeyBobs Public Key

DecryptionDecryptionEncryptionEncryption

PriPriPub

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RSA signatures (authentication)

PriPri Pub

Clear ClearEncrypted

Alice encrypts message with her private key

Bob gets Alices public key

Bob decrypts message using Alices publickey

Alices Public KeyAlices Private Key

EncryptionEncryption DecryptionDecryption

BobAlice

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Key Exchange Diffie-Hellman

Algorithm

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The Diffie-Hellman algorithm

algorithm for secure key exchange overinsecure channels

based on the difficulty of finding discrete

logarithms used to establish a shared secret between

parties (usually the key for symmetricencryption or HMACs)

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Modular Exponentiation

Generator, gg

Modulus (prime), pp

YY = ggXX mod pp

22 237276162930753723237276162930753723mod7992739798459792657265179927397984597926572651

Both gg and pp Are Shared and Well-Known

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Diffie-Hellman Key ExchangePrivate Value, XXAAPublic Value, YYAA

Private Value, XXBBPublic Value, YYBB

(shared secret)

AliceAlice BobBob

YYBB mod p = g mod p =YYAA mod pXXBBXXAA XXBB

YYAA

YYBB

YYBB = g mod pXXBBYYAA =g mod p

XXAA

XXAA

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Hashing Hashing is a one-way function. It cannot

be reversed

From the hash, you cannot compute theoriginal message

Hashing is repeatable If two parties apply the same hashing method

to the same bit string, they will get the samehash

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Hashing

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Encryption Versus HashingEncryption

Uses a key as aninput to anencryption method

Output is similar inlength to input

Reversible; ciphertextcan be decryptedback to plaintext

Use of Key

Length ofResult

Reversibility

Hashing

Key is usually addedto text; the two arecombined, and thecombination is hashed

Output is of a fixedshort length,regardless of input

One-way function; hashcannot be de-hashed backto the original string

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Tunnel Versus Transport Mode

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IPsec Operation: Tunnel andTransport Modes

Secure Connection

Secure onthe Internet

Transport Mode

SiteNetwork

SiteNetwork

Securityin Site

Network

Securityin Site

Network

ExtraSoftwareRequired

ExtraSoftwareRequired

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TunneledConnection

Secure onthe Internet

Tunnel Mode

SiteNetwork

SiteNetwork

NoSecurityin Site

Network

NoSecurityin Site

Network

NoExtra

Software

NoExtra

Software

IPsecServer

IPsecServer

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Transport Mode

Orig. IPHdr

IPsecHdr

Protected PacketData Field

Destination IP AddressIs Actual Address;

Vulnerable to Scanning

Tunnel Mode

New IPHdr

IPsecHdr

ProtectedOriginal Packet

Destination IP Address isIPsec Gateway Address

Host IP AddressIs not Revealed

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IPsec ESP and AH Protection

IPHeader

ESPHeader

ProtectedESP

Trailer

IPHeader

AuthenticationHeader

Protected

Confidentiality

Authentication and Message Integrity

Authentication and Message IntegrityNo Confidentiality

Protocol = 50

Protocol = 51

EncapsulatingSecurityPayload

AuthenticationHeader

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IPSec Security Protocols

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Modes and Protections

PossiblePossibleTunnel Mode(IPsec Gatewayto Gateway)

PossiblePossibleTransport Mode(End-to-End)

AHAuthentication

Integrity

ESPConfidentiality

AuthenticationIntegrity

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IPSEC Concepts Peers

Transform sets

Security Associations

Transport and Tunnel modes

Authentication Header (AH) &Encapsulating Security Payload (ESP)

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Peer Authentication

Peer authenticationmethods:

Pre-shared keys

RSA signatures

HRservers

Peerauthentication

Remote officeCorporate Office

Internet

A peerof an IPSEC device is another device participating inIPSEC. A peer can be a router, firewall, server or someremote PCwith IPSEC support.

Peeringbetween two IPSEC device is usually a point to pointrelationship

Peers

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Transform Sets A transform set is a list of IPsec protocols and

cryptographic algorithms that a peer can accept.Because IPsec allows for the use of differentprotocols and algorithms, a peer needs to declareand negotiate with other peers what it can support.

Peers communicate the protocols and algorithmsthey support by exchanging transform sets. For twopeers to communicate successfully, they must sharea common transform set, otherwise peering fails.

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A Transform Set

An IPsec security protocol, AHor ESPor both

An integrity/Authentication algorithm

ie MD5 HMAC or SHA-1 HMAC

An encrypting algorithm DES, 3DES.A null encryption algorithm is also supported.

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Security Association

A Security Association (SA) is alogical connection that provides

data flowing from one peer to

another by using a transform set.

Security associations are likelogical tunnels between peers.

Traffic entering an SA is protectedand transported to the other side.

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IPsec Security Associations

IPsec Policy Server

2. Security Association (SA)for Transmissions from A to B

3. Security Association (SA)For Transmission from B to A

(Can Be Different Than

A to B SA)

Party A Party B

1. List ofAllowableSecurity

Associations

1. List ofAllowableSecurity

Associations

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Security Association

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Establishing IPsec SecurityAssociations Using IKE

Internet Key ExchangeSecurity Association

UDP Port 500

Party A Party B

IPsec SAsFirst establish IKE association andprotected session

Then create IPsec SAs within theProtection of the IKE session.

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Determine IKE (IKE Phase 1) Policy

Determine the following policy details: Key distribution method

Authentication method

IPSec peer IP addresses and hostnames

IKE phase 1 policies for all peers

Encryption algorithm Hash algorithm

IKE SA lifetime

Goal: Minimize misconfiguration

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IKE Phase 1 Policy Parameters

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Determine IPSec (IKE Phase 2) Policy

Determine the following policy details:

IPSec algorithms and parameters for optimal securityand performance

Transforms and, if necessary, transform sets

IPSec peer details

IP address and applications of hosts to be protected

Manual or IKE-initiated SAs

Goal: Minimize misconfiguration

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IPSec Transforms Supported in

Cisco IOS Software

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Authentication Header

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Encapsulating Security Payload

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IPSec Policy Example

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Identify IPSec Peers

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Check Current Configuration

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Ensure the Network Works

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Ensure ACLs are Compatible with IPSec

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Task Configure IKE Task 2 Configure IKE

Step 1 Enable or disable IKE. crypto isakmp enable

Step 2 Create IKE policies. crypto isakmp policy

Step 3 Configure ISAKMP.

crypto isakmp identity

Step 4 Configure pre-shared keys. crypto isakmp key

Step 5 Verify the IKE configuration. show crypto isakmp policy

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Enable IKE

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Create IKE policies

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Create IKE Policies with thecrypto isakmp Command

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IKE Policy Negotiation

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Configure Pre-shared Keys

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Verify IKE Configuration

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Configure IPSec Task 3 Configure IPSec

Step 1 Configure transform set suites. crypto ipsec transform-set

Step 2 Configure global IPSec SA lifetimes. crypto ipsec security-association lifetime

Step 3 Create crypto ACLs using extended access

lists Step 4 Configure IPSec crypto maps.

crypto map

Step 5 Apply crypto maps to interfaces. crypto map map-name

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Configure Transform Set Suites

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Transform Set Negotiation

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Configure Global IPSec Security

Association Lifetimes

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Configure Global IPSec Security

Association Lifetimes

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Purpose of Crypto ACLs

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Create Crypto ACLs Using Extended

Access Lists

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Create Crypto ACLs Using ExtendedAccess Lists

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Configure Symmetrical Peer Crypto

ACLs

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Purpose of Crypto Maps

Crypto maps pull together the various partsconfigured for IPSec, including:

The traffic to be protected by IPSec and a set of SAs

The local address to be used for the IPSec traffic

The destination location of IPSec-protected traffic

The IPSec type to be applied to this traffic

The method of establishing SAs, either manually or

by using RSA

Other parameters needed to define an IPSec SA

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Crypto Map Parameters

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Configure IPSec Crypto Maps

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Example Crypto Map Commands

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Apply Crypto Maps to Interfaces

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IPSec Configuration Examples

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Test and Verify IPSec

Display configured IKE policies.show crypto isakmp policy

(show isakmp policy on a PIX)

Display configured transform sets.show crypto ipsec transform-set

Display phase | security associations.show crypto isakmp sa

(show isakmp sa on a PIX)

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Generic Routing Encapsulation

GRE

GRE is an OSI Layer 3 tunneling protocol:

Encapsulates a wide variety of protocol packet types inside IPtunnels

Creates a virtual point-to-point link to Cisco routers at remotepoints over an IP internetwork

Uses IP for transport

Uses an additional header to support any other OSI Layer 3protocol as payload (for example, IP, IPX, AppleTalk)

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Reasons for using GRE over

IPsec

To pass multicast and broadcast traffic across

the tunnel securely

To pass non-IP traffic securely

To provide resiliency

To assist in saving memory and CPU cyclesin the router, by reducing the number of SAthat need to be set up

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Secure GRE TunnelsIPsec provides what GRE lacks:

Confidentiality through encryption using symmetricalgorithms

Data source authentication using HMACs Dataintegrity verification using HMACs

IPsec is not perfect at tunneling:Older IOS versions do not support IP multicast overIPsec

IPsec was designed to tunnel IP only (nomultiprotocol support)

Using crypto maps to implement IPsec does notallow the use of routing protocols across the tunnel

IPsec does not tunnel IP protocols; GRE does

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GRE over IPsec

GRE over IPsec is typically used to dothe following: Create a logical hub-and-spoke topology of virtual point-

to-point connections

Secure communication over an untrusted transportnetwork (e.g. the Internet)

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GRE over IPsec Encapsulation

GRE encapsulates an arbitrary payload.

IPsec encapsulates unicast IP packet (GRE):

Tunnel mode (default): IPsec creates a newtunnel IP packet

Transport mode: IPsec reuses the IP header ofthe GRE (20 bytes less overhead than tunnelmode)

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