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Email Security

• email is one of the most widely usedand regarded network services

• currently message contents are notsecure – may be inspected either in transit

 – or by suitably privileged users on

destination system

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Email SecurityEnhancements

• condentiality – protection from disclosure

• authentication – of sender of message

• message integrity – protection from modication

• non-repudiation of origin – protection from denial by sender

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Pretty Good Privacy (PGP)

• widely used de facto secure email

• developed by Phil immermann

• selected best available crypto algs touse

• integrated into a single program

• on !ni"# P$# %acintosh and other

systems• originally free# now also have

commercial versions available

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PGP &peration 'uthentication

* sender creates message+* use S,- to generate .-bit hash

of message

/* signed hash with 0S using sender1sprivate key# and is attached tomessage

2* receiver uses 0S with sender1s

public key to decrypt and recoverhash code

3* receiver veries received messageusing hash of it and compares withdecrypted hash code

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PGP &peration '$ondentiality

* sender generates message and +4-bit random number as session keyfor it

+* encrypt message using $S5-+4 678E 6 /8ES in $9$ mode withsession key

/* session key encrypted using 0Swith recipient1s public key# :attached to msg

2* receiver uses 0S with private key

to decrypt and recover session key

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PGP &peration ' $ondentiality :uthentication

• can use both services on samemessage – create signature : attach to message

 – encrypt both message : signature

 – attach 0S6ElGamal encrypted sessionkey

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PGP &peration '$ompression

• by default PGP compresses messageafter signing but before encrypting – so can store uncompressed message :

signature for later verication

 – : because compression is nondeterministic

• uses 7P compression algorithm

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PGP &peration ' Email$ompatibility

• when using PGP will have binary data tosend (encrypted message etc)

• however email was designed only for te"t

• hence PGP must encode raw binary datainto printable S$77 characters

• uses radi"-2 algorithm –

maps / bytes to 2 printable chars – also appends a $0$

• PGP also segments messages if too big

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PGP &peration ' Summary

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PGP Session ;eys

• need a session key for each message – of varying siS7 ?+*@ mode

• uses random inputs taken fromprevious uses and from keystroke

timing of user

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PGP Public : Private ;eys

• since many public6private keys may be inuse# need to identify which is actually usedto encrypt session key in a message –

could send full public-key with every message – but this is ineAcient

• rather use a key identier based on key – is least signicant 2-bits of the key

 –

will very likely be uniBue• also use key 78 in signatures

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PGP %essage Cormat

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S6%7%E (Secure6%ultipurpose7nternet %ail E"tensions)

• security enhancement to %7%E email – original 7nternet 0C$4++ email was te"t

only

 – %7%E provided support for varyingcontent types and multi-part messages

 – with encoding of binary data to te"tual

form – S6%7%E added security enhancements

• have S6%7%E support in many mail

agents –

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S6%7%E Cunctions

• enveloped data – encrypted content and associated keys

• signed data – encoded message D signed digest

• clear-signed data – clearte"t message D encoded signed

digest

• signed : enveloped data – nesting of signed : encrypted entities

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S6%7%E $ryptographiclgorithms

• digital signatures= 8SS : 0S

• hash functions= S,- : %83

•

session key encryption= ElGamal :0S

• message encryption= ES# 5riple-8ES#0$+62. and others

• %$= ,%$ with S,-

• have process to decide which algs touse

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S6%7%E %essages

• S6%7%E secures a %7%E entity with asignature# encryption# or both

•

forming a %7%E wrapped P;$S obect• have a range of content-types=

 – enveloped data

 – signed data

 – clear-signed data

 – registration reBuest

 – certicate only message

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S6%7%E $erticateProcessing

• S6%7%E uses ?*3.F v/ certicates

• managed using a hybrid of a strict?*3.F $ hierarchy : PGPs web oftrust

• each client has a list of trusted $scerts

• and own public6private key pairs :certs

• certicates must be signed bytrusted $s

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7P Security

• have a range of application specicsecurity mechanisms – eg* S6%7%E# PGP# ;erberos# SSH6,55PS

• however there are security concernsthat cut across protocol layers

• would like security implemented by

the network for all applications

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7PSec

• general 7P Security mechanisms

• provides – authentication

 – condentiality

 – key management

• applicable to use over H>s# acrosspublic : private I>s# : for the7nternet

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7PSec !ses

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9enets of 7PSec

• in a rewall6router provides strongsecurity to all traAc crossing theperimeter

• in a rewall6router is resistant tobypass

• is below transport layer# hence

transparent to applications• can be transparent to end users

• can provide security for individualusers

• secures routin architecture

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7P Security rchitecture

• specication is Buite comple"

• dened in numerous 0C$s – incl* 0C$ +2.6+2.+6+2.6+2.4

 – many others# grouped by category

• mandatory in 7Pv# optional in 7Pv2

• have two security header e"tensions= – uthentication ,eader (,)

 – Encapsulating Security Payload (ESP)

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7PSec Services

• ccess control

• $onnectionless integrity

• 8ata origin authentication

• 0eection of replayed packets – a form of partial seBuence integrity

• $ondentiality (encryption)

• Himited traAc Jow condentiality

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Security ssociations

• a one-way relationship betweensender : receiver that aKordssecurity for traAc Jow

• dened by / parameters= – Security Parameters 7nde" (SP7)

 – 7P 8estination ddress

 –

Security Protocol 7dentier• has a number of other parameters

 – seB no# , : E, info# lifetime etc

• have a database of Securityssociations

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uthentication ,eader (,)

• provides support for data integrity :authentication of 7P packets – end system6router can authenticate

user6app – prevents address spoong attacks by

tracking seBuence numbers

•based on use of a %$ – ,%$-%83-F or ,%$-S,--F

• parties must share a secret key

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uthentication ,eader

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 5ransport : 5unnel %odes

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Encapsulating Security Payload(ESP)

• provides message content condentiality: limited traAc Jow condentiality

• can optionally provide the same

authentication services as ,• supports range of ciphers# modes# padding

 – incl* 8ES# 5riple-8ES# 0$3# 78E# $S5 etc

 – $9$ : other modes

 –

padding needed to ll blocksi

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Encapsulating SecurityPayload

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 5ransport vs 5unnel %odeESP

• transport mode is used to encrypt :optionally authenticate 7P data – data protected but header left in clear

 – can do traAc analysis but is eAcient

 – good for ESP host to host traAc

• tunnel mode encrypts entire 7P

packet – add new header for ne"t hop

 – good for LP>s# gateway to gateway

security

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Ieb Security

• Ieb now widely used by business#government# individuals

• but 7nternet : Ieb are vulnerable

• have a variety of threats – integrity

 – condentiality

 –

denial of service – authentication

• need added security mechanisms

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SSH (Secure Socket Hayer)

• transport layer security service

• originally developed by >etscape

• version / designed with public input

• subseBuently became 7nternetstandard known as 5HS (5ransportHayer Security)

• uses 5$P to provide a reliable end-to-end service

• SSH has two layers of protocols

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SSH rchitecture

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SSH rchitecture

• SSL connection – a transient# peer-to-peer#

communications link

 – associated with SSH session

• SSL session – an association between client : server

 – created by the ,andshake Protocol – dene a set of cryptographic

parameters

 –

may be shared by multiple SSH

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SSH 0ecord ProtocolServices

• message integrity – using a %$ with shared secret key

 – similar to ,%$ but with diKerent

padding• confdentiality

 – using symmetric encryption with ashared secret key dened by ,andshake

Protocol – ES# 78E# 0$+-2.# 8ES-2.# 8ES# /8ES#

Corte

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SSH 0ecord Protocol&peration

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SSH $hange $ipher SpecProtocol

• one of / SSH specic protocols whichuse the SSH 0ecord protocol

• a single message

• causes pending state to becomecurrent

• hence updating the cipher suite inuse

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SSH lert Protocol

• conveys SSH-related alerts to peer entity• severity

• warning or fatal

•

specic alert• fatal= une"pected message# bad record mac#

decompression failure# handshake failure#illegal parameter

•

warning= close notify# no certicate# badcerticate# unsupported certicate#certicate revoked# certicate e"pired#certicate unknown

•

compressed : encrypted like all SSH data

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SSH ,andshake Protocol

• allows server : client to= – authenticate each other

 – to negotiate encryption : %$

algorithms – to negotiate cryptographic keys to be

used

•

comprises a series of messages inphases

* Establish Security $apabilities

+* Server uthentication and ;ey E"change

/* $lient uthentication and ;e E"chan e

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SSH ,andshake Protocol

5HS (5 H

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 5HS (5ransport HayerSecurity)

• 7E5C standard 0C$ ++2 similar toSSHv/

• with minor diKerences – in record format version number – uses ,%$ for %$

 – a pseudo-random function e"pandssecrets

 – has additional alert codes

 – some changes in supported ciphers

 – changes in certicate types :

negotiations

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Secure Electronic 5ransactions(SE5)

• open encryption : securityspecication

• to protect 7nternet credit cardtransactions

• developed in FF by %astercard#Lisa etc

• not a payment system

• rather a set of security protocols :formats – secure communications amongst parties

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SE5 $omponents

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SE5 5ransaction

* customer opens account+* customer receives a certicate/* merchants have their own certicates

2* customer places an order3* merchant is veried* order and payment are sent@* merchant reBuests payment authori

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8ual Signature

• customer creates dual messages – order information (&7) for merchant

 – payment information (P7) for bank

• neither party needs details of other

• but must know they are linked

• use a dual signature for this – signed concatenated hashes of &7 : P7

DS=E(PRc, [H(H(PI)||H(OI))])

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SE5 Purchase 0eBuest

• SE5 purchase reBuest e"changeconsists of four messages

* 7nitiate 0eBuest - get certicates

+* 7nitiate 0esponse - signed response

/* Purchase 0eBuest - of &7 : P7

2* Purchase 0esponse - ack order

P h 0 t

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Purchase 0eBuest '$ustomer

P h 0 t

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Purchase 0eBuest '%erchant

* veries cardholder certicates using $sigs

+* veries dual signature using customer1spublic signature key to ensure order hasnot been tampered with in transit : thatit was signed using cardholder1s privatesignature key

/* processes order and forwards thepayment information to the paymentgateway for authori

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Purchase 0eBuest '%erchant

P t G t

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Payment Gatewayuthori

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Payment $apture

• merchant sends payment gateway apayment capture reBuest

• gateway checks reBuest

• then causes funds to be transferredto merchants account

• noties merchant using captureresponse

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$hapter +. ' Cirewalls

The function of a strong position is tomake the forces holding it practically

unassailable

—On War, Carl Von Clausewitz

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7ntroduction

• seen evolution of information systems

• now everyone want to be on the 7nternet

• and to interconnect networks

•

has persistent security concerns – cant easily secure every system in org

• typically use a Firewall• to provide perimeter deence• as part of comprehensive security strategy

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Ihat is a CirewallM

• a choke point of control andmonitoring

• interconnects networks with diKering

trust• imposes restrictions on network

services –

only authori5 : usa e monitorin

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Cirewall Himitations

• cannot protect from attacksbypassing it – eg sneaker net# utility modems# trusted

organisations# trusted services (egSSH6SS,)

• cannot protect against internal

threats – eg disgruntled or colluding employees

• cannot protect against transfer of all

virus infected programs or les

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Cirewalls ' Packet Cilters

• simplest# fastest rewall component

• foundation of any rewall system

• e"amine each 7P packet (no conte"t)and permit or deny according to rules

• hence restrict access to services(ports)

• possible default policies – that not e"pressly permitted is

prohibited

 – that not e"pressly prohibited is

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Cirewalls ' Packet Cilters

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ttacks on Packet Cilters

• 7P address spoong – fake source address to be trusted

 – add lters on router to block

• source routing attacks – attacker sets a route other than default

 – block source routed packets

•

tiny fragment attacks – split header info over several tiny

packets

 – either discard or reassemble before

check

Cirewalls Stateful Packet

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Cirewalls ' Stateful PacketCilters

• traditional packet lters do note"amine higher layer conte"t – ie matching return packets with

outgoing Jow• stateful packet lters address this

need

• they e"amine each 7P packet inconte"t – keep track of client-server sessions

 – check each packet validly belongs to

one

Ci ll li ti H l

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Cirewalls - pplication HevelGateway (or Pro"y)

• have application specic gateway 6pro"y

• has full access to protocol – user reBuests service from pro"y

 – pro"y validates reBuest as legal

 – then actions reBuest and returns result

to user – can log 6 audit traAc at application level

• need separate pro"ies for each

service

Ci ll li ti H l

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Cirewalls - pplication HevelGateway (or Pro"y)

Cirewalls $ircuit Hevel

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Cirewalls - $ircuit HevelGateway

• relays two 5$P connections

• imposes security by limiting whichsuch connections are allowed

• once created usually relays traAcwithout e"amining contents

• typically used when trust internal

users by allowing general outboundconnections

• S&$;S is commonly used

Cirewalls $ircuit Hevel

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Cirewalls - $ircuit HevelGateway

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9astion ,ost

• highly secure host system

• runs circuit 6 application level gateways

• or provides e"ternally accessible services

• potentially e"posed to NhostileN elements• hence is secured to withstand this

 – hardened &6S# essential services# e"tra auth

 – pro"ies small# secure# independent# non-

privileged• may support + or more net connections

• may be trusted to enforce policy of trustedseparation between these net connections

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Cirewall $ongurations

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Cirewall $ongurations

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Cirewall $ongurations

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ccess $ontrol

•given system has identied a user

• determine what resources they canaccess

•general model is that of accessmatri" with – subject - active entity (user# process)

 –

object - passive entity (le or resource) – access right ' way obect can be

accessed

• can decompose by–

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ccess $ontrol %atri"

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 5rusted $omputer Systems

• information security is increasinglyimportant

• have varying degrees of sensitivity of

information – cf military info classications= condential#

secret etc

• subects (people or programs) have

varying rights of access to obects(information)

• known as multilevel security –

subects have maimum : current security

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9ell HaPadula (9HP) %odel

• one of the most famous security models• implemented as mandatory policies on

system

•

has two key policies=• no read up (simple security property)

 – a subect can only read6write an obect if thecurrent security level of the subect dominates(O) the classication of the obect

• no write down (Q-property) – a subect can only append6write to an obect if

the current security level of the subect isdominated by (R) the classication of the

obect

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0eference %onitor

Evaluated $omputer

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Evaluated $omputerSystems

• governments can evaluate 75systems

• against a range of standards= – 5$SE$# 7PSE$ and now $ommon $riteria

• dene a number of levelsT ofevaluation with increasingly stringent

checking• have published lists of evaluated

products

 – though aimed at government6defense

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$ommon $riteria

• international initiative specifying securityreBuirements : dening evaluation criteria

• incorporates earlier standards –

eg $SE$# 75SE$# $5$PE$ ($anadian)# Cederal(!S)

• species standards for – evaluation criteria

 – methodology for application of criteria

 – administrative procedures for evaluation#certication and accreditation schemes

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$ommon $riteria

• denes set of security reBuirements • have a 5arget &f Evaluation (5&E)

• reBuirements fall in two categories – functional

 – assurance

• both organised in classes of families :

components

$ommon $riteria

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$ommon $riteria0eBuirements

• Cunctional 0eBuirements – security audit# crypto support#

communications# user data protection#

identication : authentication# securitymanagement# privacy# protection oftrusted security functions# resourceutili

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$ommon $riteria

i i

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$ommon $riteria