james joshi associate professor, sis
TRANSCRIPT
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IS 2150 / TEL 2810IS 2150 / TEL 2810Information Security & Privacy
James JoshiAssociate Professor, SIS,
Lecture 7Feb 24 2016Feb 24, 2016
Network SecurityKey Management
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Objectives
Understand/explain the issues related to, and utilize the techniques Key management
Authentication and distribution of keysSession key Key exchange protocols Session key, Key exchange protocols
Mechanisms to bind an identity to a key Generation, maintenance and revoking of keys
Security at different levels of OSI model Privacy Enhanced email
IPSec
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IPSec
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Notation X Y : { Z || W } kX,Y
X sends Y the message produced by t ti Z d W i h d b k kconcatenating Z and W enciphered by key kX,Y,
which is shared by users X and Y A T : { Z } kA || { W } kA T A T : { Z } kA || { W } kA,T
A sends T a message consisting of the concatenation of Z enciphered using kA, A’s key, and W enciphered using k the key shared byand W enciphered using kA,T, the key shared by A and T
r1, r2 nonces (nonrepeating random numbers)
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1, 2 ( p g )
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Interchange vs Session Keys Interchange Key
Tied to the principal of communicationSession key Session key Tied to communication itself
ExampleExample Alice generates a random cryptographic key ks
and uses it to encipher m She enciphers k with Bob’s public key k She enciphers ks with Bob s public key kB Alice sends { m } ks { ks } kB
Which one is session/interchange key?
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Which one is session/interchange key?
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Benefits using session key
In terms of Traffic-analysis by an attacker? Replay attack possible?p y p Prevents some forward search attack
Example: Alice will send Bob message that is p geither “BUY” or “SELL”.
Eve computes possible ciphertexts {“BUY”} kBd {“SELL”} kand {“SELL”} kB.
Eve intercepts enciphered message, compares, and gets plaintext at once
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and gets plaintext at once
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Key Exchange Algorithms Goal:
Alice, Bob to establish a shared key
Criteria Key cannot be sent in clear
l b h d Alice, Bob may trust a third party All cryptosystems, protocols assumed to be
publicly known
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Classical Key Exchange
How do Alice, Bob begin? Alice can’t send it to Bob in the clear! Alice can t send it to Bob in the clear!
Assume trusted third party, CathyAlice and Cathy share secret key k Alice and Cathy share secret key kA
Bob and Cathy share secret key kB
U thi t h h d k k Use this to exchange shared key ks
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Simple Key Exchange Protocol
Alice Cathy{ request for session key to Bob } kA
Alice Cathy{ ks }kA , { ks }kB
Alice Bob{ ks } kB
Alice Bob{m}ks
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Alice Bob
What can an attacker, Eve, do to subvert it?
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Needham-Schroeder
Alice CathyAlice || Bob || r1
Alice Cathy{ Alice || Bob || r1 || ks ||{ Alice || ks } kB } kA
{ Alice || k } kAlice Bob
{ Alice || ks } kB
{ r2 } ksAlice Bob
{ 2 } s
Alice Bob{ r2 – 1 } ks
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Alice Bob
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Questions
How can Alice and Bob be sure they are talking to each other?talking to each other?
Is the previous attack possible?p p
Key assumption of Needham-Schroedery p All keys are secret; What if we remove that assumption?
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What if we remove that assumption?
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Needham-Schroeder with Denning-Sacco Modification
Alice CathyAlice || Bob || r1
Alice Cathy{ Alice || Bob || r1 || ks || { Alice || T || ks } kB } kA
{ Alice || T || k } kAlice Bob
{ Alice || T || ks } kB
{ r2 } ksAlice Bob
{ 2 } s
Alice Bob{ r2 – 1 } ks
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Alice Bob
Use time stamp T to detect replay! Synchronized Clocks needed!
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Otway-Rees Protocol
Alice Bobn || Alice || Bob || { r1 || n || Alice || Bob } kA
Cathy Bobn || Alice || Bob || { r1 || n || Alice || Bob } kA ||{ r2 || n || Alice || Bob } kB
Cathy Bobn || { r1 || ks } kA || { r2 || ks } kB
Alice Bobn || { r1 || ks } kA
12Uses integer n to associate all messages with a particular exchangeUses integer n to associate all messages with a particular exchange
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Replay Attack Eve acquires old ks, message in third step
n || { r1 || ks } kA || { r2 || ks } kBE f d i t t t Ali Eve forwards appropriate part to Alice If Alice has no ongoing key exchange with Bob
Accept/reject the message ? Alice has ongoing key exchange with Bob
Accept/reject the message ? If replay is for the current key exchange and If replay is for the current key exchange, and
Eve sent the relevant part before Bob did, Does replay attack occur?
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Kerberos Authentication system
Based on Needham-Schroeder with Denning-Sacco modificationmodification
Central server plays role of trusted third party (“Cathy”)
Ticket (credential) Ticket (credential) Issuer vouches for identity of requester of service
Authenticator Identifies sender
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Ticket Credential saying issuer has identified ticket
requesterExample ticket issued to user u for service s Example ticket issued to user u for service sTu,s = s || { u || u’s address || valid time || ku,s }kswhere: ku,s is session key for user and service Valid time is interval for which the ticket is valid
u’s address may be IP address or something u s address may be IP address or something else Note: more fields, but not relevant here
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Authenticator Credential containing identity of sender of ticket
Used to confirm sender is entity to which ticket was issuedissued
Example: authenticator user u generates for service sAu,s = { u || generation time || kt } ku,s
hwhere: kt is alternate session key Generation time is when authenticator generated
Note: more fields, not relevant here
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Protocol Authentication server
user ASuser || TGS
{ ku TGS } ku || Tu TGS{ u,TGS } u || u,TGS
user TGSservice || Au,TGS || Tu,TGS
user AS
user TGSuser || { ku,s } ku,TGS || Tu,s
A || Tuser service
Au,s || Tu,s
user service{ t + 1 } ku,s
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user service
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Problems
Relies on synchronized clocks If not synchronized and old tickets, authenticators
not cached, replay is possible
Tickets have some fixed fields Dictionary attacks possible Kerberos 4 session keys weak (had much less
than 56 bits of randomness); researchers atthan 56 bits of randomness); researchers at Purdue found them from tickets in minutes
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Public Key Key Exchange
Here interchange keys known eA, eB Alice and Bob’s public keys known to allA B
dA, dB Alice and Bob’s private keys known only to owner
Si l l Simple protocol ks is desired session key
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Problem and Solution?
Alice Bob{ ks } eB
Any problem ?Any problem ?
Ali B b{ { ks } dA } eBAlice Bob{ { s } A } B
What about this?What about this?
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Public Key Key Exchange
Assumes Bob has Alice’s public key, and vice versavice versa If not, each must get it from public
serverserver If keys not bound to identity of owner,
attacker Eve can launch a man-in-the-attacker Eve can launch a man in themiddle attack
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Man-in-the-Middle Attack
Alice Petersend me Bob’s public key
E P tsend me Bob’s public key
Eve intercepts request
Eve Peterp y
Eve PetereB
eAlice
eE Eve
Ali B b{ ks } eE Eve intercepts messageAlice Bob
Eve Bob{ ks } eB
ve inte cepts message
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Eve Bob
Peter is public server providing public keysPeter is public server providing public keys
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Cryptographic Key Infrastructure Goal:
bind identity to keyClassical Crypto: Classical Crypto: Not possible as all keys are shared
Public key Crypto:Public key Crypto: Bind identity to public key Erroneous binding means no secrecy between
principalsprincipals Assume principal identified by an acceptable
name
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Certificates
Create token (message) containing Identity of principal (here, Alice) Corresponding public key Timestamp (when issued) Other information (identity of signer)
signed by trusted authority (here, Cathy)CA = { eA || Alice || T } dC
C is A’s certificate
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CA is A s certificate
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Use Bob gets Alice’s certificate
If he knows Cathy’s public key, he can decipher the certificatethe certificate
Now Bob has Alice’s public key Problem:
Bob needs Cathy’s public key to validate certificate
Two approaches: pp Merkle’s tree, Signature chains
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Certificate Signature Chains Create certificate
Generate hash of certificateEncipher hash with issuer’s private key Encipher hash with issuer’s private key
Validate Obtain issuer’s public keyp y Decipher enciphered hash Re-compute hash from certificate and compare
P oblem Problem: Validating the certificate of the issuer and
getting issuer’s public key
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X.509 Chains Key certificate fields in X.509v3:
Version Serial number (unique)Serial number (unique) Signature algorithm identifier Issuer’s name; uniquely identifies issuer Interval of validityy Subject’s name; uniquely identifies subject Subject’s public key info
… Signature:
Identifies algorithm used to sign the certificate Signature (enciphered hash)
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X.509 Certificate Validation
Obtain issuer’s public key The one for the particular signature algorithmp g g
Decipher signature Gives hash of certificate
Re-compute hash from certificate and compare If they differ, there’s a problem
Check interval of validity
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This confirms that certificate is current
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Issuers
Certification Authority (CA): entity that issues certificatesissues certificates Multiple issuers pose validation problem Alice’s CA is Cathy; Bob’s CA is Dan; how Alice s CA is Cathy; Bob s CA is Dan; how
can Alice validate Bob’s certificate? Have Cathy and Dan cross-certify Have Cathy and Dan cross certify
Each issues certificate for the other
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Validation and Cross-Certifying Certificates:
Cathy<<Alice>> represents the certificate that C has generated for Ap g
Dan<<Bob> ; Cathy<<Dan>>; Dan<<Cathy>>
Alice validates Bob’s certificate Alice obtains Cathy<<Dan>> Can Alice validate Cathy<<Dan>> ? (how?) Can Alice use Cathy<<Dan>> to validate y
Dan<<Bob>> ? (how?) Signature chain : ??
Show how Bob can validate Alice’s certificate?
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Show how Bob can validate Alice s certificate?
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PGP Chains Pretty Good Privacy:
Widely used to provide privacy for electronic mail and signing files digitally
O PGP tifi t t t d i t k t OpenPGP certificates structured into packets One public key packet Zero or more signature packets
Public key packet: Version (3 or 4; 3 compatible with all versions of PGP, 4 not
compatible with older versions of PGP)p ) Creation time Validity period (not present in version 3) Public key algorithm, associated parameters
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Public key
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OpenPGP Signature Packet Version 3 signature packet
Version (3)Signature type (level of trust) Signature type (level of trust)
Creation time (when next fields hashed) Signer’s key identifier (identifies key to encipher g y ( y p
hash) Public key algorithm (used to encipher hash) Hash algorithm Hash algorithm Part of signed hash (used for quick check) Signature (enciphered hash using signer’s
private key)
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private key)
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Validating Certificates Alice needs to validate
Bob’s OpenPGP cert Does not know Fred
Arrows show signaturesSelf signatures not shown
Does not know Fred, Giselle, or Ellen
Alice gets Giselle’s certK H li htl Ell
Henry
Jack
Knows Henry slightly, but his signature is at “casual” level of trust
Ali t Ell ’ tGiselle
EllenIrene
y
Alice gets Ellen’s cert Knows Jack, so uses
his cert to validate ll ’ h h
Bob
Fred
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Ellen’s, then hers to validate Bob’s
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Digital Signature Construct that authenticates origin, contents
of message in a manner provable to a di i t t d thi d t (“j d ”)disinterested third party (“judge”)
Sender cannot deny having sent messageLimited to technical proofs Limited to technical proofs Inability to deny one’s cryptographic key was used
to signO ld l i h hi k One could claim the cryptographic key was stolen or compromised Legal proofs, etc., probably required;
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g p , , p y q ;
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Signature
Classical: Alice, Bob share key k Alice sends m || { m }k to Bob Alice sends m || { m }k to Bob
Does this satisfy the requirement for y qmessage authentication? How?
Does this satisfy the requirement for a digital signature?
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Classical Digital Signatures Require trusted third party
Alice, Bob share keys with trusted party CathyThe judge must trust Cathy The judge must trust Cathy
Alice Bob{ m }kAlice
Bob Cathy
C th B b
{ m }kAlice
{ m }kBobCathy Bob{ } Bob
How can the judge resolve any dispute where one claims that the How can the judge resolve any dispute where one claims that the
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j g y pcontract was not signed?
j g y pcontract was not signed?
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Public Key Digital Signaturesy g g(RSA)
Alice’s keys are dAlice, eAlice
Alice sends Bob Alice sends Bobm || { m }dAlice
I f di t j d t In case of dispute, judge computes{ { m }dAlice }eAlice
and if it is m, Alice signed message She’s the only one who knows dAlice!
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RSA Digital Signatures
Use private key to encipher message Protocol for use is critical
Key points: Never sign random documents, and when signing,
always sign hash and never document Mathematical properties can be turned against signer
Sign message first then encipher Sign message first, then encipher Changing public keys causes forgery
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Attack #1 Example: Alice, Bob communicating
nA = 95, eA = 59, dA = 11 nB = 77, eB = 53, dB = 17 nB 77, eB 53, dB 17
26 contracts, numbered 00 to 25 Alice has Bob sign 05 and 17:
c = mdB mod nB = 0517 mod 77 = 3 c = mdB mod nB = 1717 mod 77 = 19
Alice computes 0517 mod 77 = 08; correspondingAlice computes 05 17 mod 77 08; corresponding signature is 0319 mod 77 = 57; claims Bob signed 08Note: [(a mod n) × (b mod n)] mod n = (a × b) mod n
J d t d 5753 d 77 08
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Judge computes ceB mod nB = 5753 mod 77 = 08 Signature validated; Bob is toast!
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Attack #2: Bob’s Revenge Bob, Alice agree to sign contract 06 Alice enciphers, then signs:
Enciper: c = meB mod nB = 0653 mod 77Sign: cd mod n (0653 mod 77)11 mod 95 63 Sign: cdA mod nA = (0653 mod 77)11 mod 95 = 63
Bob now changes his public key Bob wants to claim that Alice singed N (13)
C t h th t 13r d 77 6 59 Computes r such that 13r mod 77 = 6; say, r = 59 Computes r.eB mod (nB) = 5953 mod 60 = 7 Replace public key eB with 7, private key dB = 43
B b l i t t 13 J d t Bob claims contract was 13. Judge computes: (6359 mod 95)43 mod 77 = 13 Verified; now Alice is toast
S l ti i fi t d th i h !!
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Solution: sign first and then encipher!!
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ISO/OSI Model
Application Layer
Presentation Layer
Application Layer
Presentation Layer
Peer-to-peer
Presentation Layer
Session Layer
T t L
Presentation Layer
Session Layer
T t LTransport Layer
Network Layer
Transport Layer
Network LayerNetwork Layer
Data Link Layer
Physical Layer
Data Link Layer
Physical Layer
Data Link Layer
Physical Layer
42Flow of bits
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Security at the Transport LayerSecurity at the Transport LayerSecure Socket Layer (SSL)
Developed by Netscape to provide security in WWW browsers and servers
SSL is the basis for the Internet standard protocol – Transport Layer Security (TLS) protocol (compatible with SSLv3)
Key idea: Connections and Sessions A SSL session is an association between two peers An SSL connection is the set of mechanisms used to
transport data in an SSL sessiontransport data in an SSL session
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Secure Socket Layer (SSL) Each party keeps session information
Session identifier (unique) The peer’s X.509(v3) certificate
C i th d d t d l f d t Compression method used to reduce volume of data Cipher specification (parameters for cipher and MAC) Master secret of 48 bits
Connection information Connection information Random data for the server & client Server and client keys (used for encryption) Server and client MAC key
I iti li ti t f th i h if d d Initialization vector for the cipher, if needed Server and client sequence numbers
Provides a set of supported cryptographic mechanisms that are setup during negotiation (handshake protocol)p g g ( p )
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SSL Architecture
Provides a basis for Secure communicationC fid i liConfidentiality + Message authenticity
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SSL R d P t l O tiSSL Record Protocol Operatione.g., HTTP messages
Message type version length of blockMessage type, version, length of block
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Handshake Protocol The most complex part of SSL Allows the server and client to authenticate
each other Based on interchange cryptosystem (e.g., RSA)
Negotiate encryption MAC algorithm and Negotiate encryption, MAC algorithm and cryptographic keys Four roundsFour rounds
Used before any application data are transmitted
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Other protocols
SSL Change Cipher Spec Protocol A single byte is exchanged After new cipher parameters have been
negotiated (renegotiated)
SSL Al P l SSL Alert Protocol Signals an unusual condition
Closure alert : sender will not send anymore Closure alert : sender will not send anymore Error alert: fatal error results in disconnect
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Protocols End-to-end protocol
Example: telnet
End-to-end encryption End to end encryption Example: telnet with messages encrypted/decrypted at the client
and server Attackers on the intermediate hosts cannot read the message
Link protocol Protocol between every directly connected systems
Example: IP – guides messages from a host to one of its immediate hosthost
Link encryption Encipher messages between intermediate host Each host share a cryptographic key with its neighbor Each host share a cryptographic key with its neighbor
Attackers at the intermediate host will be able to read the message
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Electronic Mail k d UA interacts
with the senderUA hands it to a
Attacker can read email on any of the computer with MTA
UA hands it to a MTA
p Forgery possible
UA UA UA User Agent
MTA MTA MTA Message Transfer
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MTA MTA MTA gAgents
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Security at the Application Layer:y pp yPrivacy-enhanced Electronic Mail
Study by Internet Research Task Force on Privacy or Privacy Research Group to develop protocols with following services Confidentiality, by making the message
d bl t t th d d i i tunreadable except to the sender and recipients Origin authentication, by identifying the sender
preciselyprecisely Data integrity, by ensuring that any changes In
the message are easy to detect
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Non-repudiation of the origin (if possible)
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Design Considerations/goalsfor PEM
Not to redesign existing mail system protocols To be compatible with a range of MTAs, UAs p g ,
and other computers To make privacy enhancements available p y
separately so they are not required To enable parties to use the protocol to
communicate without prearrangement
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PEMBasic Design
Defines two keys Data Encipherment Key (DEK) to encipher Data Encipherment Key (DEK) to encipher
the message sent Generated randomlyy Used only once Sent to the recipient
Interchange key: to encipher DEK Must be obtained some other way than through
th
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the message
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Protocols
Confidential message (DEK: ks)
Ali B b{m}ks || {ks}kBob
Authenticated, integrity-checked message
Alice Bob
ut e t cated, teg ty c ec ed essage
Alice Bobm || {h(m)}kAlice
Enciphered, authenticated, integrity checked message
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checked message
Alice Bob?? {m}ks || {h(m)}kAlice{ks}kBob
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ISO/OSI Model /IPSec: Security at Network Layer
Application Layer
Presentation Layer
Application Layer
Presentation Layer
Peer-to-peer
Presentation Layer
Session Layer
T t L
Presentation Layer
Session Layer
T t LTransport Layer
Network Layer
Transport Layer
Network LayerNetwork Layer
Data Link Layer
Physical Layer
Data Link Layer
Physical Layer
Data Link Layer
Physical Layer
55Flow of bits
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IPSec Protocols Authentication header (AH) protocol
Message integrity Origin authentication Anti-replay services
Encapsulating security payload (ESP) protocol Confidentiality
Message integrity Message integrity Origin authentication Anti-replay services
Internet Key Exchange (IKE)y g ( ) Exchanging keys between entities that need to communicate over the
Internet What authentication methods to use, how long to use the keys, etc.
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Cases where IPSec can be used
Internet/Intranet
End-to-end security between two hostsEnd to end security between two hosts
Internet/SG SGInternet/Intranet
SG SG
57End-to-end security between two security gateways
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Cases where IPSec can be used (2)
InternetSG SG
Intranet Intranet
End-to-end security between two hosts + two gateways
Internet SG
End to end security between two hosts + two gateways
Intranet
58End-to-end security between two hosts during dial-up
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Security Association (SA) Unidirectional relationship between peers Specifies the security services provided to the traffic
carried on the SAcarried on the SA Security enhancements to a channel along a path
Identified by three parameters: IP Destination Address Security Protocol Identifier
Specifies whether AH or ESP is being used Specifies whether AH or ESP is being used Security Parameters Index (SPI)
Specifies the security parameters associated with the SA
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the SA
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Security Association (2)
Each SA uses AH or ESP (not both) If both required two SAs are created
Multiple security associations may be used to provide required security services A sequence of security associations is called SA
bundleExample: We can have an AH protocol followed by Example: We can have an AH protocol followed by ESP or vice versa
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Security Association Databases IP needs to know the SAs that exist in order to
provide security servicesSecurity Policy Database (SPD) Security Policy Database (SPD) IPSec uses SPD to handle messages
For each IP packet, it decides whether an IPSec service is provided bypassed or if the packet is to be discardedprovided, bypassed, or if the packet is to be discarded
Security Association Database (SAD) Keeps track of the sequence number
AH i f ti (k l ith lif ti ) AH information (keys, algorithms, lifetimes) ESP information (keys, algorithms, lifetimes, etc.) Lifetime of the SA
Protocol mode
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Protocol mode MTU et.c.
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IPSec Modes
Two modes Transport modep
Encapsulates IP packet data area IP Header is not protected
f Protection is provided for the upper layers Usually used in host-to-host communications
Tunnel modeTunnel mode Encapsulates entire IP packet in an IPSec
envelopeH l i t t ffi l i
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Helps against traffic analysis The original IP packet is untouched in the Internet
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Authentication Header (AH) Next header
Identifies what protocol header follows Payload length Next HeaderNext Header
parameters
y g Indicates the number of 32-bit words in
the authentication header Security Parameters Index
S ifi t th i th l ith
Payload lengthPayload length
Specifies to the receiver the algorithms, type of keys, and lifetime of the keys used
Sequence number SequenceSequence
Security ParametersSecurity ParametersIndexIndex
q Counter that increases with each IP
packet sent from the same host to the same destination and SA
Authentication DataAuthentication DataAuthentication Data
NumberNumber
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Authentication Data
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Preventing replay Using 32 bit sequence numbers helps detect
replay of IP packets The sender initializes a sequence number for
every SAReceiver implements a window size of W to Receiver implements a window size of W to keep track of authenticated packets
Receiver checks the MAC to see if the packet Receiver checks the MAC to see if the packet is authentic
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Transport Mode AHInternet/Intranet
Original IP TCPOriginal IP Header
TCPHeader Payload Data Without IPSec
O i i l IP TCPOriginal IP Header
TCPHeader Payload DataAuth
Header
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NextHeader
PayloadLength SPI Seq.
No. MACAuthenticate Entire packet except forMutable fields
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Tunnel Mode AHInternet SG
IntranetIntranet
Original IP TCPOriginal IP Header
TCPHeader Payload Data Without IPSec
O i i l IP TCPA hN IP Original IP Header
TCPHeader Payload DataAuth
HeaderNew IP Header
A th ti t
AuthHeader
New IP Header
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NextHeader
PayloadLength SPI Seq.
No. MACAuthenticateEntire IP Packet
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ESP – Encapsulating Security Payload
Creates a new header in addition to the IP header
Creates a new trailer Encrypts the payload datayp p y Authenticates Prevents replaye e ts ep ay
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ESP – Encapsulating Security Payload
Security Parameters Index (SPI) Specifies to the receiver the algorithms, type
of keys, and lifetime of the keys usedSequence number
Security Parameters Index (SPI) – 32 bits
Sequence number Counter that increases with each IP packet
sent from the same host to the same destination and SA
P l d ( i bl )
Sequence Number 32 bits
Payload (variable) TCP segment (transport mode) or IP packet
(tunnel mode) - encryption Padding (+ Pad length, next Header)
Payload Data
Padding/ Next Headerg ( g , ) 0 to 255 bytes of data to enable encryption
algorithms to operate properly Authentication Data
MAC created over the packet
Padding/ Next Header
Authentication Data
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MAC created over the packet
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Transport mode ESP
Original IP Header
TCPHeader Payload Data Without IPSec
Header Header y
Original IP TCP Payload DataESP ESP ESPESP ESP ESPHeader Header Payload DataHeader Trailer Auth
Encrypted
Header Trailer Auth
Authenticated
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Tunnel mode ESP
Original IP TCP h SOriginal IP Header
TCPHeader Payload Data Without IPSec
O i i l IP TCPESP ESP ESPN IP ESP ESP ESP
Encrypted
Original IP Header
TCPHeader Payload DataESP
HeaderESP
TrailerESPAuth
New IP Header
ESPHeader
ESPTrailer
ESPAuth
Authenticated
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Summary
Session key is better for secret message exchange
Public key good for interchange key, digital signatures – needs certification system
Various replay/MITM attacks are possible in key exchange protocols and care is needed
Security services available at different levels
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