GSRI-ch5

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© From Computer Networking, by Kurose&Ross5: Securing IP 5-1

Managing and SecuringComputer Networks

Guy Leduc

Chapter 5:Network Layer Security

For a summary, see:

Computer Networking: A Top Down Approach ,

6th edition.Jim Kurose, Keith RossAddison-Wesley, March 2012.(section 8.7)

Mainly based on

Network Security - PRIVATE Communication in a PUBLIC World C. Kaufman, R. Pearlman, M. Speciner Pearson Education, 2002.(chapters 17 and 18)


Chapter 5: Network LayerSecurity

Chapter goals:

! security in practice:" Security in the network layer (versus other layers)

" IPsec

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Chapter Roadmap

! Security in the network layer

! IPsec - The big picture

! IPsec protocols: AH and ESP

! IPsec Key Exchange protocol: IKE


Relative Location of SecurityFacilities in the TCP/IP Stack

! Both are general-purpose (i.e. application independent) solutions, but! IPsec is NOT specific to TCP

" Does work with UDP, and any other protocol above IP (e.g., ICMP, OSPF)

! IPsec protects the whole IP payload, including transport headers (e.g. port #)" Traffic analysis is thus more difficult (could be web, email, …)

! IPsec is from network entity to network entity, not from application processto application process" “Blanket coverage”

HTTP FTP SMTP

TCP / UDP

IP / IPsec

HTTP FTP SMTP

SSL / TLS

TCP

IP

Security at network levelSecurity at transport level

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Virtual Private Networks (VPNs)! Institutions often want private networks for

security" Costly! Separate routers, links, DNS infrastructure

! VPN: institution’s inter-office traffic is sentover public Internet instead" Encrypted before entering public Internet"

Logically separate from other traffic


I P h e a d e r

I P s e c h e a d e r

S e c u r e p a y l o a d

I P

h e a d

e r

I P s e c

h e a d

e r

S e c u r e

p a y l o

a d I P

h e a d e r

I P s e c

h e a d e r

S e c u r e

p a y l o

a d

I P

h e a d e r

p a y l o

a d I P h

e a d e r

p a y l o

a d

headquartersbranch office

salesperson

in hotel

laptop

w/ IPsec

router w/

IPv4 and IPsec

router w/

IPv4 and IPsec

public

Internet

Virtual Private Networks (VPNs)

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Three functional areas

! IP-level security encompasses the following3 functional areas:" origin authentication (and data integrity)

• assures that a received packet was, in fact,transmitted by the party identified as the source inthe packet header

• includes replay attack prevention• also assures that the packet has not been altered

" confidentiality• enables communicating nodes to encrypt messages to

prevent eavesdropping by third parties

" key management• secure exchange of keys


IP Security Overview! In 1994, the Internet Architecture Board (IAB) issued a

report entitled "Security in the Internet Architecture"" General consensus that the Internet needs more and better

security" In 1997, 2500 reported security incidents affecting nearly

150,000 sites" Most serious attacks: IP spoofing and packet sniffing" This justified the 2 main functions of IPsec

! The security capabilities were designed for IPv6 butfortunately they were also designed to be usable with thecurrent IPv4

! IPsec can encrypt and/or authenticate all traffic at the IPlevel. Thus IPsec provides the capability to securecommunications across a LAN, across private and publicWANs, and across the Internet" VPN (Virtual Private Networks)" Secure remote access over the Internet" Enhancing Extranet and Intranet connectivity with partners" Enhancing Electronic Commerce

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Benefits of IPsec! When IPsec is implemented in a firewall or router, itprovides strong security that can be applied to alltraffic crossing the perimeter

! IPsec is below the transport layer and so is transparentto applications" No need to change software on a user or server system when

IPsec is implemented in a firewall or router" No need to train users, issue keying material on a per-user basis,

or revoke keying material when users leave the organization

! IPsec can provide security to individual users if needed! IPsec can play a vital role in the routing architecture. It

can ensure that:" router and neighbour advertisements come from authorized

routers" a redirect message comes from the router to which the initial

packet was sent" a routing update is not forged


Chapter Roadmap





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IPsec Transport Mode

!

IPsec datagram emitted and received byend-system

! Protects upper level protocols

IPsec IPsec


IPsec – tunneling mode (1)

! End routers are IPsec aware

! Hosts need not be

IPsec IPsec

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IPsec – tunneling mode (2)

! Also tunneling mode

IPsecIPsec


Two Ipsec protocols

! Authentication Header (AH) protocol" provides source authentication & data integrity

but not confidentiality

! Encapsulation Security Protocol (ESP)" provides source authentication, data integrity,

and confidentiality

" more widely used than AH

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Four combinations are possible!

Host modewith AH

Host modewith ESP

Tunnel modewith AH

Tunnel modewith ESP

Most common andmost important


IP Security Overview

!IPsec enables a system to"select security protocols,"determine the algorithm(s) to use, and"put in place any cryptographic keys required

!IPsec services and their support by AH and ESP

AH ESP ESP encryption only encryption+authentication

Access Control x x xConnectionless integrity x xData origin authentication x xRejection of replayed packets x x xConfidentiality x xLimited traffic flow confidentiality x x

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Security associations (SAs)! Before sending data, a virtual connection is

established from sending entity to receivingentity

! Called “security association (SA)”" SAs are simplex: for only one direction

! Both sending and receiving entities maintainstate information about the SA" Recall that TCP endpoints also maintain state

information" IP is connectionless; IPsec is connection-oriented!

! How many SAs in VPN w/ headquarters, branchoffice, and n traveling salespeople?


Security Association (2)

! An SA is uniquely identified by 3 parameters:

" Security Parameters Index (SPI): a bitstring assigned to this SAby the receiver end, and having local significance only. Used toselect the SA under which a received packet will be processed.

" IP Destination Address: can be a router address, can be unicastor multicast.

"

Security Protocol Identifier: indicates whether the association isan AH or ESP SA

! The SPI alone seems to suffice to uniquely identify the SA, but" The same SPI can be assigned to both an ESP SA and an AH SA, so this

security protocol identifier is needed to remove ambiguity

" For multicast, the SPI is chosen by the source, so the destinationaddress field is also needed to remove ambiguity

! Hence, in any IP packet, the SA is uniquely identified by these 3fields

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Example SA from R1 to R2

R1 stores for SA:! 32-bit identifier for SA: Security Parameter Index (SPI) ! origin SA interface (200.168.1.100)!

destination SA interface (193.68.2.23)! type of encryption used (e.g., 3DES with CBC)! encryption key! type of integrity check used (e.g., HMAC with MD5)! authentication key

193.68.2.23200.168.1.100

172.16.1/24172.16.2/24

security association

Internetheadquartersbranch office

R1R2


! endpoint holds SA state in security association database (SAD) , where it can locate themduring processing

! with n salespersons, 2 + 2n SAs in R1’s SAD

! when sending IPsec datagram, R1 accessesSAD to determine how to process datagram

! when IPsec datagram arrives to R2, R2examines SPI in IPsec datagram, indexes SADwith SPI, and processes datagram accordingly

Security Association Database (SAD)

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IPsec datagramFocus for now on tunnel mode with ESP

new IP

header

ESP

hdr

original

IP hdr

Original IP

datagram payload

ESP

trl

ESP

auth

encrypted

“enchilada” authenticated

paddingpad

length

next

header SPI

Seq

#


What happens?

new IP

header

ESP

hdr

original

IP hdr

Original IP

datagram payloadESP

trl

ESP

auth

encrypted


paddingpad

length

next

header SPI

Seq

#

193.68.2.23200.168.1.100

172.16.1/24172.16.2/24

security association

Internetheadquartersbranch office

R1R2

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R1 converts original datagraminto IPsec datagram

! Appends to back of original datagram (which includesoriginal header fields!) an “ESP trailer” field

! Encrypts result using algorithm & key specified by SA

! Appends to front of this encrypted quantity the “ESPheader, creating “enchilada”

! Creates authentication MAC over the whole enchilada ,using algorithm and key specified in SA

! Appends MAC to back of enchilada, forming payload ! Creates brand new IP header, with all the classic IPv4

header fields, which it appends before payload


Inside the enchilada:

! ESP trailer: Padding for block ciphers" Next header contains original packet type" Packet type in new IP header is “ESP”

! ESP header:" SPI, so receiving entity knows what to do" Sequence number, to thwart replay attacks

! MAC in ESP auth field is created with shared secret key

new IP

header

ESP

hdr

original

IP hdr

Original IP

datagram payloadESP

trl

ESP

auth

encrypted


paddingpad

length

next

header

SPISeq

#

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IPsec sequence numbers! For new SA, sender initializes seq. # to 0! Each time datagram is sent on SA:

" Sender increments seq # counter" Places value in seq # field

! Goal:" Prevent attacker from sniffing and replaying a packet" Receipt of duplicate, authenticated IP packets may disrupt

service

! Method:" Destination checks for duplicates" But doesn’t keep track of ALL received packets; instead uses

a window


IPsec Anti-Replay in Action

#1#2#3#4

#1#2#4#2#2

#2#2 Packet #3 lost,no problem

Packets #2 are outof sequence and/or

duplicates

R1

R2

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Packet reordering and IPsecAnti-Replay Window

#1#2#3#4

Packet #1out of sequence.If in window: OK,

otherwise: drop & log

#2#3#4 #1

Networkmay change thepacket order

R1

R2


SA Database (SAD) - More! When sending IPsec datagram, R1 accesses SAD to determine how

to process datagram! When IPsec datagram arrives to R2, R2 examines SPI in IPsec

datagram, indexes SAD with SPI, and processes datagramaccordingly

! Parameters associated with each SA:" AH information: authentication algorithm, keys, key lifetime, …" ESP information: encryption and authentication algorithm, keys,

initialization values, key lifetimes, …" Sequence number counter: used to generate the sequence number field

in AH and ESP headers" Anti-replay window: used to determine whether an inbound AH or ESP

packet is a replay" Lifetime of the SA" Sequence counter overflow flag: indicates what to do when a counter

overflow occurs (usually close the SA)" IPsec protocol mode: tunnel or transport mode" Path MTU: any observed path maximum transmission unit (to avoid

fragmentation)

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Security Policy Database (SPD)! Policy: For a given datagram, sending entity needs to know if it

should use IPsec" Needs also to know which SA to use

! A nominal Security Policy Database (SPD) defines the means bywhich IP traffic is related to specific SAs (or possibly to no SA)" Info in SPD indicates “what” to do with arriving datagram" Then info in the SAD indicates “how” to do it

! An SPD contains entries, each of which defines a subset of IPtraffic (via some IP and upper-layer protocol field values, calledselectors) and points to an SA for that traffic

! Outbound processing obeys the following general sequence for each

packet:" Compare the values of the appropriate fields in the packet (selector

fields) against the SPD to find a matching SPD entry" Determine the SA associated with that entry (if any) and its associated

SPI" Do the required IPsec processing (i.e. AH or ESP processing)

! Like the packet filter rules in firewalls (see next chapter)


Summary: IPsec services

! Suppose Trudy sits somewhere between R1and R2. She doesn’t know the keys.

! Will Trudy be able to" see contents of original datagram? How about

source, dest IP address, transport protocol,application port?

" flip bits without detection?

" masquerade as R1 using R1’s IP address?

" replay a datagram?

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Chapter Roadmap






Transport and Tunnel ModesBrief overview

! Transport mode" Protection of the IP packet payload only" IP header unchanged

! Tunnel mode" Protection of the entire IP packet" To do this, the entire protected original packet is

treated as the payload of a new "outer" IP packet,with a new outer IP header

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AH - Transport Mode

OriginalIP header

but PT = 51

Auth. headerAH

other headers and payloads

OriginalIP header

other headers and payloads secret key

Digital signature produced by a MAC(Message Authentication Code) algorithm(MD5 or SHA-1)

Original IP datagram

Authenticated IP datagram

Non mutablefields only

Part of the AH header is also authenticated

Partsof


AH - Tunnel Mode

OriginalIP header

Auth. headerAH


OriginalIP header


secret key

Digital signature produced by a MAC(Message Authentication Code) algorithm(MD5 or SHA-1)


Authenticated IP datagram

All fields

New IPheader

New IPheader

built bytunnel end

Non mutablefields only

Part of the AH header is also authenticated

Partsof

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IPsec AH Header 0 1 2

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Payload Len | RESERVED | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Security Parameters Index (SPI) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sequence Number Field | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Authentication Data (variable) | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Total length = 32 bytesNext header identifies protocol type above IP

The sequence number is used to guard against the replay attack


ESP without AuthenticationTransport Mode

OriginalIP header

but PT = 50ESP header other headers and payloads and ESP trailer

OriginalIP header


secret key


IP datagram with transport ESP

Encryption algorithm(e.g. DES with CBC)

ESP trailer(padding)

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ESP without AuthenticationTunnel Mode

new IPheader

ESP header

IP header other headers + payloads

secret key


IP datagram with tunnel ESP

IP header other headers + payloads + ESP trailer

new IPheader

built bytunnel end

Encryption algorithm(e.g. DES with CBC)

ESP trailer(padding)


ESP with AuthenticationTransport Mode

OriginalIP header

ESP header other headers + payloads + ESP trailer

OriginalIP header

other headers + payloads


IP datagram with transport ESP

ESPauthentication

Encrypted part

Authenticated part

ESP trailer

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ESP with AuthenticationTunnel Mode

new IPheader

ESP header

IP header other headers + payloads


IP datagram with tunnel ESP

IP header other headers + payloads + ESP trailer

ESP trailer

ESPauthentication

Encrypted part

Authenticated part


IPsec ESP format 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ----| Security Parameters Index (SPI) | ^Auth.+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov-| Sequence Number | |erage+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ----| Payload Data* (variable) | | ^~ ~ | || | |Conf.

+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov-| | Padding (0-255 bytes) | |erage*+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | || | Pad Length | Next Header | v v+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------| Authentication Data (variable) |~ ~| |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Added length: minimum 8 bytes (+4 bytes IV for DEC-CBC) beforeand minimum 2 bytes after without authentication.

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Combining authentication andconfidentiality! First method: ESP with authentication

" does not authenticate the non mutable parts of the IP header (intransport mode) or new IP header (in tunnel mode)

" applies encryption before authentication• so authentication applies to the cyphertext, rather than the plaintext

! Second method: ESP (without authentication), then AH" does authenticate the non mutable parts of the IP header" has the disadvantage of using two SAs

! Third method: first AH, then ESP (without authentication)" authentication applies to the plaintext

• allows to store the authentication information together with the message(without having to reencrypt the message to verify the authentication)" the authentication header is protected by encryption" still two SAs

! Usage of AH and ESP can be in transport or tunnel modes


Do we need AH?

! We clearly need ESP for encryption, but do we needAH?

! AH protects the IP header itself. But does IP headerprotection matter?"

If it were necessary, ESP in tunnel mode could provide it" Note that intermediate routers cannot check header

integrity. Why?

" So integrity can only be checked at the other end of the SA

! Note also that, even with AH, an untrusted sourcehost could still spoof its own IP address" Only integrity is ensured

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IPsec and NAT ! NAT translates the source IP address and the source

port of the IP packet!" A NAT box actually does IP spoofing

! An IPsec SA cannot go through a NAT box" With AH, the integrity check would fail

" With ESP, the port number is encrypted

" And the NAT box doesn’t have the key

! Need to encapsulate IPsec packets in UDP packets:

IP TCPUser

Data

HASHESP

50IP Encrypted Data

IP PayloadUDP


IPSec Tunnels & QoS

new IPheader

ESP header

IP header IP payload


IP datagram with ESP tunnel

IP header IP payload

New IP header built by tunnel endTOS byte is copiedTOS byte is copied

TOS / DSCP

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Chapter Roadmap






IKE: Internet Key Exchange

! In previous examples, we manually establishedIPsec SAs in IPsec endpoints:Example SA:

SPI: 12345Source IP: 200.168.1.100

Dest IP: 193.68.2.23Protocol: ESPEncryption algorithm: 3DES-cbcHMAC algorithm: MD5Encryption key: 0x7aeaca…

HMAC key:0xc0291f …

! Manual keying is impractical for large VPN with100s of endpoints

! Instead use IPsec IKE (Internet Key Exchange)

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IKE: PSK and PKI! Authentication (proof of who you are) with either

" pre-shared secret (PSK) or

" with PKI (public/private keys and certificates)

! With PSK: both sides start with secret:" run IKE to authenticate each other and to generate IPsec SAs

(one in each direction), including encryption and authenticationkeys

! With PKI: both sides start with public/private key pair

and certificate:" run IKE to authenticate each other and obtain IPsec SAs (onein each direction)

" Similar with handshake in SSL


IKE - 2 phases - overview! IKE has two phases! Phase 1: establish bi-directional IKE SA

" The two peers establish a secure, authenticated channel with which tocommunicate.

" This is called the IKE Security Association (SA), aka ISAKMP SA• Note: IKE SA is different from IPsec SA

" Based on a Diffie-Hellman (DH) exchange• computationally expensive, but done only once

" Result: one shared key used in (possibly many instances of) phase 2• More precisely, 3 keys are derived from this one (one for IKE encryption,

one for IKE authentication, and one for phase 2)" Phase 1 has two modes: aggressive mode and main mode

! Phase 2: IKE SA is used to securely negotiate IPsec pair of SAs" SAs are negotiated on behalf of services such as IPsec (e.g. AH or

ESP) or any other service which needs key material and/or parameternegotiation

" Uses the 3rd shared secret key (of phase 1) and random numbers tocreate IPsec shared secret keys for AH and ESP SAs

" Those IPsec SAs are unidirectional" Quick procedure and keys can be changed often

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IKE Phase 1 - Thwarting Clogging

Attacks (1)! DH is computationally expensive! IKE employs a mechanism, known as

cookies, to thwart clogging attacks! The protocol starts by a cookie

request containing a random value (c1)! The other side will send back a cookie

response containing this value (c1) anda new random number (c2)" The only overhead is to send an

acknowledgement, not to perform aDH calculation

" If the source address was forged, theopponent may not get any answer

" If the responder is too busy, it doesnot send acknowledgements

! The returned value (c2) must berepeated in the first message of theDH key exchange

c1

c2, c1

DHparam, c2

Check c2,if OKstarts DH

Gets itonly ifinitial IPaddresswas notspoofed


Thwarting Clogging Attacks(2)

! So, cookies must depend on (i.e. be a hash of) the specific parties(IP source and destination addresses, UDP source anddestination ports) and a locally generated secret value

c1

c2, c1

DHparam, c2

Improvement: Don’t keep a copy of c2.Possible thanks to the fact that the party canrecognise that c2 is one of its own cookies!

But then the scheme is vulnerable to the following attack:

c1

c2, c1

DHparam, c’2

Spoofed IP address

Don’t know c2, butcan use anotherc’2 recorded in arun with my address OK, c’2 is one of my cookies

I start DH

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DH - Defence against Man-in-the-Middle

(MIM) (1)! If DH parameters (YA and YB) are permanent and public numbers! And if we can be sure that YA and YB are the numbers reliably

associated with A and B respectively" For example, by means of a PKI (Public Key Infrastructure)" That is the pairs (A, YA) and (B, YB) are certified by some trusted

authority" So-called Fixed DH

! Then" The Man-in-the-Middle attack is not possible" And the exchanges of YA and YB can even be eliminated

" B will need to fetch the certified YA only once! But this needs a PKI

" We lose the simplicity of the original Diffie-Hellman scheme

! Also, the fact that YA and YB are permanent makes them morevulnerable to brute-force attacks to find XA and XB


DH - Defence against MIM (2)

! Authenticated (Ephemeral) Diffie-Hellman! If A and B know some sort of secret with which they can

authenticate each other (before running DH)" Knowledge of a (pre-shared) secret key, or" Knowledge of each other’s public key (and their own private key)

! Then they can use this secret to prove it was they who generatedtheir DH values

! Several solutions:" Encrypt the DH exchange with the pre-shared secret key" Sign the transmitted DH value (Y) with own private key" Encrypt the DH value (Y) with the other side’s public key

• Why does it work, knowing that anyone can so encrypt?" Following the DH exchange, transmit a hash of the pre-shared key and

the DH value (Y) you transmitted" Following the DH exchange, transmit a hash of the agreed-upon

shared DH value, your name and the pre-shared key! Again this needs a PKI or a pre-shared key! Note that the DH values can be changed often in this case

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Back to IKE phase 1 -Authentication! The DH exchange should be authenticated to bar the MIM

attack

! Several authentication methods are used" Authentication with a pre-shared key

" Authentication based on public key cryptography• Authentication with signatures

• Authentication with public key encryption

! But, if one needs public key cryptography anyway, why using DH

to generate a shared secret in the first place?" After all, one party could have generated the secret key and sent it

encrypted with the other party’s public key!

" With DH, both parties contribute to the shared secret/key. So itwill be random if either side has a good random number generator.


IKE phase 1 - main mode

KAB(A, proof I’m A)

Crypto_suite_chosenB

YA

YB

Crypto_suiteA

KAB(B, proof I’m B)

KAB is the calculated DH shared key.Note, both computations in //.

A only reveals her identity here.Moreover, identities are hidden topassive attackers.So, a MIM will only discover A’s id.But could also be hidden. How?

Anonymous DH: no identity revealed,only the IP addresses

Negotiation of the cryptographic methodsused in later exchanges

! Proof of identity: proof that the sender knows the key associated with theidentity, which can be based on" The pre-shared key" The private signature key or encryption key (two pairs of asymmetric keys

are used)! Typically some hash of (1) the key associated with the identity and (2) almost

all fields in previous messages (also provides integrity). With private signaturekeys, the proof can also be a signature on the fields

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IKE phase 1 - aggressive mode

! Note: in both modes (main and aggressive), nonces are added to messages." The DH shared key is then computed from the DH values AND the nonces" For example: KAB = hash (nonces, standard DH key)

! This allows IKE to reuse the same DH values in successive runs and stillgenerate different shared keys" Protection against replay attack

proof I’m A

B, YB, proof I’m B

A, YA, Crypto_proposalA

Identities revealed, even to passiveattackers: no encryption.

How would you change this modeto hide identities to passive attackers(with public keys)?

Take-it-or-leave-it negotiation.In particular, A chooses a (g,p) pair.


IPsec only authenticates the host!

! If the host is stolen, it can still establishIPsec SAs and connect to a VPN!

! IPsec does not authenticate the user

!

Needs an extra level: user authentication" E.g., IPsec client with Smart card

" Or, extra authentication with username andpassword after IKE phase 1

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Automated Public Key Exchange

• Peers choose their private/public key pairs" they keep the secret key

" their public keys must be certified

• Use a notary = Certification Authority = CA

• Peer must prove authenticity in front of CA

• Notary signs certificates

• Peers dynamically exchange certificates

• Scalable: n peers requires n authentications and ncertificates


Certificates

• Certificates are not secret

• Common structure ITU X.509 v3 orPKCS#6 (S/MIME, SSL, …)

peer namepeer public keyexpiration dateother info

signatureby the CA

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How peers work with CA

CA!s own certificate

signed by CA

0. peer generates public/private key pair

1. peer fetchesCA’s certificate

2. peer transmitsits public key

3. peer’s certificatesigned by CA

4. peer fetches

its certificate

Strong or human authenticationStrong or human authenticationneeded for steps 1. and 2.needed for steps 1. and 2.


How to check a certificate?

• Check CA signature" CA’s certificate needed to get CA signature

• Check black list = CRL (Certificate Revocation

List)" connect to CA to get the CRL

• CRL" List of revoked certificates signed by CA

" Stored on CA or directory service

" No requirement on devices to ensure CRL is current

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How to scale CA?A root CA can delegate authentication to lower CA

root CA own certificate signed by root CA

lower CA certificate signed by root CA

router certificate signed by lower CA

certificates chain of routercertificates chain of router

root

lower CA


IPsec: summary

! IKE used to establish sharedsecret keys, algorithms, SPInumbers

! two principal protocols:" authentication header (AH)

protocol

" encapsulation securitypayload (ESP) protocol

! for both AH and ESP, source,destination handshake:" create network-layer logical

channel called a securityassociation (SA)

! Tunnel and transport modes

! shortcomings" IPsec departs from the

pure connectionlessparadigm

" IPsec interferes withNAT boxes

" IPsec only authenticates ahost, not a user

" IPsec is complex: morethan a dozen RFCs

Documents

GSRI-ch5