Introduction Aja

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Internet KeyExchange



IPSec – Reminder

SPI

SA1

SA2

SA3

……

SAD



IPSec – Reminder

SA• Security Association Database (SAD) holds

SA’s • Security Associations (SA) is a one way ,

cryptographically protected connection between a sender and a receiver that affords

security services to traffic



SA’sspi

12

…

Alice to BobBob’s spi: 17

13

… Bob to AliceBob’s spi: 2

21

SA’sspi

1Bob to Alice

Alice’s spi: 212

…

…

Alice to BobAlice’s spi: 13

17

Alice Bob



IPSec – Reminder

SASA contains the fields:• protocol identifier (ESP or AH)

• mode (tunnel or transport)• algorithms for encryption/ decryption/

authentication and their respective keys

• lifetime• SPI’s • sequence number



IPSec – Reminder

Where does IKE fit in?SA’s building and managing is either: • Static (manual) – keys and other attributes

of SA are manually configured by systemadministrator. Practical for small, relativelystatic environments.

• Dynamic (automated) – On-demandcreation of keys. Handled by IKE protocol



IKE

• IKE is a protocol that builds and manages IPSecSA’s between two computers that implement

IPSec.• IKE is the only standard protocol for building

IPSec SA’s (Standard IPSec implementation mustalso implement IKE)

• IKE (like IPSec) is carried out either between a pair of hosts, a pair of security gateways or a hostand a security gateway



IKE

• IKE is a protocol that builds and manages IPSecSA’s between two computers that implement

IPSec.• IKE is the only standard protocol for building

IPSec SA’s (Standard IPSec implementation mustalso implement IKE)

• IKE (like IPSec) is carried out either between a pair of hosts, a pair of security gateways or a hostand a security gateway



Endpoint to Endpoint Transport

• Both endpoints of the IP connection implementIPsec

• Used with no inner IP header• One of the protected points can be behind a

NAT node

Protected

Endpoint

Protected

EndpointIPsec Tunnel



Gateway to Gateway Tunnel

• Neither point of the IP connection implements IPsec, but network nodes between them protect traffic for part of the way

• Protection is transparent to the endpoints• The inner IP header contains the IP addresses of the

actual endpoints

gateway gatewayProtected

Subnet

Protected

Subnet

IPsecTunnel



Endpoint to Gateway Transport• A protected endpoint (typically a portable roaming

computer) connects back to its corporate networkthrough an IPsec protected tunnel

• The protected endpoint will want an IP addressassociated with the gateway so that packets returned toit will go to the gateway and be tunneled back

• The protected endpoint may be behind a NAT

Protected

Endpointgateway

Protected

Subnet

and/or

Internet

IPsec Tunnel





Key Exchange Protocols

• Key exchange protocols goal is to agree on

a shared key for the two participant• Should implement

- authenticity

- secrecy



Long and Short Term Keys

• To support authenticity parties shouldknow a mutual secret key. This key is

called long term key.• The keys negotiated in the protocol are

called short term keys.

• There are two types of long term keys:1. Pre-shared secret2. Public/private keys



Long and Short Term Keys

Why the need for short term keys?• It is not advisable to encrypt a lot of data with the

same key• It is advisable to separate between encryption keysand authentication keys

Why not sending the new key encrypted using thelong term key?

• PFS



PFS

Perfect Forward Secrecy• Exposure of long term keys will not entail

exposure of short term keys that are created

in the current execution of the protocol

• PFS is optionally provided in IKE (detailedlater)



IKE version 1

• IKE version 1 is a hybrid of three protocols(actually a framework and two protocols)

• Version 1 grew out of ISAKMP frameworkand OAKLEY and SKEME protocols thatwork within that framework.



ISAKMP (IKE version 1)

• Stands for “Internet Security Associationand Key Management” Protocol

• Created by NSA (National SecurityAgency)

• Framework (not really a protocol) forauthentication and key exchange.

• This framework decides on the SA’sattributes the parties will use.



ISAKMP (IKE version 1)

• Designed to be key exchange independent(supports many different key exchanges)

• In IKE version 1 ISAKMP uses part ofOAKLEY and part of SKEME.



SKEME (IKE version 1)

• Describes a versatile key exchangetechnique

Provides:• anonymity• repudiability• quick key refreshment



OAKLEY (IKE version 1)

• Describes a series of key exchanges anddetails the services provided by each

• Based on Diffie-Hellman algorithm but providing added security

• Generic in that it does not dictate specificformats



OAKLEY (IKE version 1)

Characterized by five important features:1. Cookies to prevent clogging attacks2. Negotiation of a group (specifying global

parameters of DH)3. Use of nonces to ensure against replayattacks

4. Exchange of public key values5. Authentication of DH to prevent man-in-

the-middle attacks



Diffie-Hellman Groups

• A group for the DH key exchange specifiesthe global parameters of DH.

• Each group includes the definition of 2global parameters and the identity of thealgorithm

• Three of these groups are classic DHalgorithm using modular exponentiation



Diffie-Hellman – groups id=1,2,5All these three groups (id=1,2,5) have:• Generator = 2For group id=1:

• Prime = 2^768 - 2^704 – 1 + 2^64 * { [2^638 pi]+ 149686}

For group id=2:• Prime = 2^1024 - 2^960 – 1 + 2^64 * { [2^894 pi]

+ 129093}For group id=5:• Prime = 2^1536 - 2^1472 – 1 + 2^64 * { [2^1406

pi] + 741804}



Diffie-Hellman – groups id=3,4

• Over galois fields using elliptic curves.





Main benefits of IKE Version 2

over Version 1IKEv2 preserves most of the features of IKEv1. The

idea behind IKEv2 was to make it as easy as possible for IKEv1 implementations to bemodified for IKEv2.

• Later we will see that IKE is a two-phase protocol.Version 2 greatly simplified IKE by replacing the8 possible phase 1 exchanges with a single exchange.

• This single exchange provides identity hiding in 2 round trips rather than 3 in version 1



Main benefits of IKE Version 2

over Version 1• Version 2 decreased latency by allowing

setup of SA to be piggybacked on the initial

exchange• Version 2 increased security by allowing

responder to be stateless until initiator can

receive at claimed IP address



Side benefits of IKE Version 2

over Version 1• cryptographic syntax replaced with one

simplified syntax

• a few fields were removed (ex: DOI, SIT)

• possible error states reduced



Details and variations

• IKE normally listens on UDP port 500,though may also be received on port 4500

with a slightly different format



ReliabilityIKE is a reliable protocol.• Initiator responsible for retransmission in the

event of timeout, therefore must remember eachrequest until it receives the corresponding

response• Responder retransmits a response only when it

receives retransmission , therefore must remembereach response until it receives a request with a

larger sequence number plus window size• On failure all states associated with SA are

discarded



Reliability

• IKE definition includes recovery fromtransmission error: packet loss, packet

replay, packet forgery



Functionality

• IKE is designed to function so long as:1. at least one of a series of retransmitted packets

reaches its destination before timing out2. channel not full of forged or replayed packets

(exhausting network or CPU)• Even if these two minimum requirements are

absent, IKE fails cleanly as though the networkwas broken



NAT Traversal

• IPsec through a NAT introduces problems.• protocols which include IP addresses of endpoint

within the payload (like IPSec) necessitate that NAT understands the protocol and modify theinternal references and those in the headers

• In transport mode changing IP address will causechecksums to fail. In tunnel mode there arerouting problems.



NAT Traversal

• For that reason, IKE supports UDPencoding that is easier for NATs to process

• It is less efficient but is easier for NAT to process• This is where port 4500 comes in. When

working through a NAT it is better to passIKE packets over port 4500 which runs the

NAT-friendly protocol.



To Sum Up OverviewWe talked about:• IPSec SAs• what roles IKE play• Design issues• Key exchange protocols: long/short terms keys, pfs• version 1: structure and features• version 2

• Reliability• Terms of functionality• NAT friendly protocol

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Introduction Aja