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Kerberos and X.509Fourth Edition

by William Stallings

Lecture slides by Lawrie Brown

!hanged by Somesh "ha#

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$uthentication $%%lications

& will consider authentication 'unctions

& de(elo%ed to su%%ort a%%lication)le(el

authentication * digital signatures& will consider Kerberos  + a %ri(ate),eyauthentication ser(ice

& then X.509  directory authenticationser(ice

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Kerberos

& trusted ,ey ser(er system 'rom -/& %ro(ides centralised %ri(ate),ey third)

%arty authentication in a distributednetwor,+ allows users access to ser(ices distributed

through out the networ,

+ without needing to trust all wor,stations

+ rather all trust a central authentication ser(er& two (ersions in use 1 * 5

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Kerberos 2e3uirements

& 'irst %ublished re%ort identi'ied itsre3uirements as+ security

+ reliability

+ trans%arency

+ scalability

& im%lemented using an authentication%rotocol based on 4eedham)Schroeder

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Kerberos 1 (er(iew

& a basic third)%arty authentication scheme& ha(e an $uthentication Ser(er $S#  + users initially negotiate with $S to identi'y

themsel(es

+ $S %ro(ides a non)corru%tible authenticationcredential tic,et granting tic,et /6/#  

& ha(e a /ic,et 6ranting ser(er /6S# 

+ users subse3uently re3uest access to otherser(ices 'rom /6S on basis o' users /6/ 

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$ Sim%le $uthentication 7ialogue

& 8# ! ) $S 7! :: ;! :: 7<+ ! = client+ $S = authentication ser(er+ 7! = identi'ier o' user on !+ ;! = %assword o' user on !

+ 7

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-essage >

& ># $S ) ! /ic,et

& /ic,et = E K

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-essage A

& A# ! )

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;roblems

& Each time a user needs to access adi''erent ser(ice heCshe needs to entertheir %assword+ 2ead email se(eral times

+ ;rintD mailD or 'ile ser(er

+ $ssume that each tic,et can be used only

once otherwise o%en to re%lay attac,s#& ;assword sent in the clear

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$uthentication 7ialogue

& nce %er user logon session

& 8# ! ) $S 7! :: 7/6S

& ># $S ) ! E K!# ?/ic,et/6S@& /ic,et/6S is e3ual to

+ E K/6S# ?7! :: $7! :: 7/6S 

:: /S8 :: Li'etime8 @

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E%laining the 'ields

& /6S = /ic,et)granting ser(er

& 7/6S = denti'ier o' the /6S

& /ic,et/6S = /ic,et)granting tic,et or/6/ 

& /S8 = timestam%

& Li'etime8 = li'etime 'or the /6/ & K !# = ,ey deri(ed 'rom users %assword

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-essages A# and 1#

& nce %er ty%e o' ser(ice

& A# ! ) /6S 7! :: 7

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-essage 5

& nce %er ser(ice session

& 5# ! )

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;roblems

& Li'etime o' the /6/ + Short   user is re%eatedly as,ed 'or their

%assword

+ Long   o%en to re%lay attac,+ scar ca%tures /6/ and waits 'or the user

to logo''

+ Sends message A# with networ, address7! networ, address is easy to 'orge#

& Same %roblem with S6/ 

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What should we doJ

& $ networ, ser(ice /6S or ser(er# should beable to (eri'y that+ %erson using the tic,et is the same as the %erson

that the tic,et was issued to+ 2emedy use an authenticator

& Ser(er should also authenticate to user+ therwise can setu% a G'a,eH ser(er

+ $ G'a,eH tuition %ayment ser(er and ca%ture thestudents credit card

+ 2emedy use a challenge)res%onse %rotocol

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Kerberos 2ealms

& a Kerberos en(ironment consists o'+ a Kerberos ser(er+ a number o' clientsD all registered with ser(er

+ a%%lication ser(ersD sharing ,eys with ser(er

& this is termed a realm + ty%ically a single administrati(e domain

& i' ha(e multi%le realmsD their Kerberosser(ers must share ,eys and trust

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Kerberos

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2eading assignment

& nter)realm authentication in (ersion 1+ ;ages 188)18A

&

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X.509 $uthentication Ser(ice

& %art o' !!// X.500 directory ser(icestandards+ distributed ser(ers maintaining some in'o database

& de'ines 'ramewor, 'or authentication ser(ices

+ directory may store %ublic),ey certi'icates+ with %ublic ,ey o' user

+ signed by certi'ication authority

& also de'ines authentication %rotocols

& uses %ublic),ey cry%to * digital signatures+ algorithms not standardisedD but 2S$ recommended

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X.509 !erti'icates

& issued by a !erti'ication $uthority !$#D containing+ (ersion 8D >D or A#+ serial number uni3ue within !$# identi'ying certi'icate+ signature algorithm identi'ier

+ issuer X.500 name !$#+ %eriod o' (alidity 'rom ) to  dates#+ subect X.500 name name o' owner#+ subect %ublic),ey in'o algorithmD %arametersD ,ey#+ issuer uni3ue identi'ier (>#+ subect uni3ue identi'ier (>#+ etension 'ields (A#+ signature o' hash o' all 'ields in certi'icate#

& notation CA denotes certi'icate 'or $ signed by !$

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btaining a !erti'icate

& any user with access to !$ can get anycerti'icate 'rom it

& only the !$ can modi'y a certi'icate& because cannot be 'orgedD certi'icatescan be %laced in a %ublic directory

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!$ ierarchy

& i' both users share a common !$ then theyare assumed to ,now its %ublic ,ey

& otherwise !$Ms must 'orm a hierarchy

& use certi'icates lin,ing members o' hierarchyto (alidate other !$Ms+ each !$ has certi'icates 'or clients 'orward# and

%arent bac,ward#

& each client trusts %arents certi'icates& enable (eri'ication o' any certi'icate 'rom

one !$ by users o' all other !$s in hierarchy

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!$ ierarchy Nse

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!erti'icate 2e(ocation

& certi'icates ha(e a %eriod o' (alidity

& may need to re(o,e be'ore e%iry8. userMs %ri(ate ,ey is com%romised

>. user is no longer certi'ied by this !$

A. !$Ms certi'icate is com%romised

& !$s maintain list o' re(o,ed certi'icates

+ the !erti'icate 2e(ocation List !2L# 

& users should chec, certs with !$s !2L

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$uthentication ;rocedures

& X.509 includes three alternati(eauthentication %rocedures

&ne)Way $uthentication

& /wo)Way $uthentication

& /hree)Way $uthentication

& all use %ublic),ey signatures

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ne)Way $uthentication

& 8 message $)B# used to establish+ the identity o' $ and that message is 'rom

$

+ message was intended 'or B+ integrity * originality o' message

& message must include timestam%D nonceD

BMs identity and is signed by $

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/wo)Way $uthentication

& > messages $)BD B)$# which alsoestablishes in addition+ the identity o' B and that re%ly is 'rom B

+ that re%ly is intended 'or $

+ integrity * originality o' re%ly

& re%ly includes original nonce 'rom $Dalso timestam% and nonce 'rom B

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/hree)Way $uthentication

& A messages $)BD B)$D $)B# whichenables abo(e authentication withoutsynchroniOed cloc,s

& has re%ly 'rom $ bac, to B containingsigned co%y o' nonce 'rom B

& means that timestam%s need not bechec,ed or relied u%on

& 2eading assignment  %ages 1>1)1>P

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X.509

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!erti'icate Etensions

& ,ey and %olicy in'ormation+ con(ey in'o about subect * issuer ,eysD %lus

indicators o' certi'icate %olicy

& certi'icate subect and issuer attributes+ su%%ort alternati(e namesD in alternati(e

'ormats 'or certi'icate subect andCor issuer

& certi'icate %ath constraints+ allow constraints on use o' certi'icates by

other !$s

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Summary

& ha(e considered+ Kerberos trusted ,ey ser(er system

+ X.509 authentication and certi'icates