ch14: key distribution (computer and network security)
TRANSCRIPT
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Chapter 14:
Key Distribution
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Agenda
Symmetric Key distribution using symmetric techniques Symmetric Key distribution using Asymmetric techniques
Symmetric Key distribution using hybrid techniques
Asymmetric Key distribution
Certifcate Example
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Defnition
Link encryption (link or physical layer encryption): is an appcommunications security that encrypts and decrypts all trac at eaa communications line
End-to-end encryption (Application encryption) : messages aencrypted by the sender at the point o! origin and only decrypted bintended recei#er$
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%o& to share a Secret Key?
1.A
can select a 'ey and physically deli#er it toB
$ 2. A third party can select the 'ey and physically deli#er it to A
3. (! A and B ha#e pre#iously and recently used a 'ey) one party cathe ne& 'ey to the other) encrypted using the old key$
4. (! A and B each has an encrypted connection to a third partydeli#er a 'ey on the encrypted lin's to A and B $
*hich one+s, are more appropriate !or -in' encryption . *hich onemore appropriate !or end/to/end encryption .
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Symmetric Keydistribution using
symmetric technique
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Defnition
A session key is a temporary encryption 'ey used bet&een t&o prA aster key is a long/lasting 'ey that is used bet&een a 'ey distcenter and a principal !or the purpose o! encoding the transmission'eys$ 0ypically) the master 'eys are distributed by noncryptograpmeans$
A nonce is an arbitrary number used only once in a cryptographic
communication) in the spirit o! a nonce &ord +
23 5678,$ 0he nona timestamp) a counter) or a random number9 the minimum requirethat it diers &ith each request$
*e &ill al&ays tal' about distributing session 'eys or public '
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using Symmetric encryptionexample
%o& &ould A ma'esure that ; is the onereplying.
%o& &ould ; be surethat A is the one
replying.
%o& &ould A
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using Symmetric encryptionexample
%o& &ould A ma'esure that ; is the onereplying.
%o& &ould ; be surethat A is the one
replying.
%o& &ould A
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A 0ransparent Key ControlScheme
0he sender doesn=tha#e to &orry or e#enbe a&are by theencryption
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%ierarchical Key Control
>ne KDC !or all the &orld . Each host is lin'ed to one KDC
KDCs lin' together to !or a tree !or communication
0he tree decrease the o#erhead o#er each KDC) and minimi?e the a !aulty KDC$
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!ession "ey Li#etie
$hen !hould % e&change a ne' session key *hen the old 'ey is exploit$ *hen it is used too much by number o! messages or time$ +to pre#ent a
predictions,$
(n connection oriented approach) it could be used !or e#ery ne& connect
it is ore secure to change session key #re*uently+, $hy duse a ne' session key #or each essage
0he 'ey exchange induce extra o#erhead +delay, on the connection$ *e need to decide &isely bet&een security and o#erhead$
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0ypes o! session 'eys
ata-encrypting 'ey) !or general communication across a net&o /%0-encrypting 'ey) !or personal identifcation numbers +@(s, uselectronic !unds trans!er and point/o!/sale applications
ile-encrypting 'ey) !or encrypting fles stored in publicly accesslocations$
B$ Etc
*hy do &e ha#e dierent types o! 'eys.
%o& do you 'no& &hat is the type o! the 'ey.
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DES control schema
seE!
!or encrypting 'ey a'es use o! the extra bits in each F4/bit DES +or GDES, 'ey$ +re&as HF/bit,
ype o# in#oration in this e&tra its : sed !or Encryption
sed !or Decryption
Session Key< aster Key
Liitation : 0he tag length is limited to bits) limiting its Iexibility and !unctionality
;ecause the tag is not transmitted in clear !orm) it can be used only at tdecryption) limiting the &ays in &hich 'ey use can be controlled$
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Control Jector schema
(t defes the limitation o! the-ast schema:
Si?e o! control #ector is#ariable and Iexible
Control #ector are sent in aclear !orm
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Key distribution usinAsymmetric techniqu
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using Asymmetric encryptionexample
an in the iddle Attac'
Solution?
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using Asymmetric encryptionexample
Confdentiality Authentication) $$ ;ut ho& it got the @ublic 'ey o! th
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%ybrid Key distributioS(L SME0N(C AD ASME0N(C 0EC%(O
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%ybrid approachDistribute session 'ey using master 'ey +symmetric Encryption,
Distribute aster 'eys using + @ublic Encryption,
National . /er#orance +public/pri#ate encryption is computational costy,
Back'ard copatiility
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Distribution o! @ubl'ey
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Distribution o! public 'eys
@ublic announcement easy to masquerade #ery high o#erhead
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Distribution o! public 'eys
@ublic announcement @ublic A#ailable directory
directory ha#e to be trusted contains Pname)public/'eyQ entries participants register securely &ith directory participants can replace 'ey at any time directory is periodically published directory can be accessed electronically Still easy to masquerade +an/in/the B,
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Distribution o! public 'eys
@ublic announcement @ublic A#ailable directory @ublic/'ey authority
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@ublic/Key Authority
-i'e @ublic directory but &ith Authentication
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Distribution o! public 'eys
@ublic announcement @ublic A#ailable directory @ublic/'ey authority @ublic/'ey Certifcate
certifcates allo& 'ey exchange &ithout real/time access to'ey authority
a certifcate binds identity to public 'ey usually &ith othe
such as period o! #alidity) rights o! use etc &ith all contents signed by a trusted @ublic/Key or Certifca
Authority +CA, can e eri5ed by anyone &ho 'no&s the public/'ey aut
public/'ey
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@ublic/Key Certifcates1$ Any participant can read acertifcate to determine thename and public 'ey o! thecertifcate=s o&ner$
2. Any participant can #eri!ythat the certifcate originated!rom the certifcate authority
and is not counter!eit$
3. >nly the certifcateauthority can create andupdate certifcates$
4. Any participant can #eri!ythe currency o! the
certifcate$
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@ublic/Key Certifcates -imitation:
(t ta'es time) in case o!certifcate change$
sers might use oldcertifcates$
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T$HUV CEN0(W(CA0E
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@ublic/Key Certifcate se
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T$HUV certifcate contents
6eeer7 @ublic Ke
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6eeer7 @ublic/KeyCertifcates Neq.
1$ Any participant can read a certifcate to determine the name and public certifcate=s o&ner$
2. Any participant can #eri!y that the certifcate originated !rom the certifcand is not counter!eit$
3. >nly the certifcate authority can create and update certifcates$
4. Any participant can #eri!y the currency o! the certifcate$
;ecause certifcates are un!orgeable) they can be placed in a directory &it!or the directory to ma'e special eorts to protect them$
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Chain o! CA (s there only >ne CA in the &orld .
%nitially A has certifcate !rom CA T1$ A securely 'no&s T1=s public 'ey$
; has certifcate !rom CA TR$ ; securely 'no&s TR=s public 'ey$
CAs ha#e securely exchanged their o&n public 'eys
A 'ants to eri#y Bs certi5cate signed y 82
A obtains !rom the directory the certifcate o! TR signed by T1$ A can obtain TR=s public 'ey !rom its certifcate and #eri!y it by means o!
signature on the certifcate$
A then goes bac' to the directory and obtains the certifcate o! ; signed
;ecause A no& has a trusted copy o! TR=s public 'ey)A can #eri!y the sig
securely obtain ;=s public 'ey$
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Chain o! CA 0otation 7 T1 has certifcate o! TR T1 XXTRYY
/reious e&aple7 T1 XXTRYY TR XX;YY +&hat doesmean.,
A chain o! CAs T1 XXTRYY TRXXTGYY TG XXT4YY BB
require that each pair Ti ) TiZ1 to secretly share their public 'ey in ad
%o& to determine the chain o! CAs that contains the certifcate.
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Determine the chain o! CAs Each CAs has t&o types o! certifcates or'ard certi5cates: Certifcates o! T
generated by other CAs
6eerse certi5cates7 Certifcatesgenerated by T that are the certifcates o!other CAs$
%o& to fnd the path in the tree is beyondour scope
A acquires ; certifcate using chain:TXX*YY*XXJYYJXXMYYMXX[YY[XX;YY
; acquires A certifcate using chain:[XXMYYMXXJYYJXX*YY*XXTYYTXXAYY
C tif t N ti
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Certifcate Ne#ocation+in#alidating,$hy
0he user=s pri#ate 'ey is assumed to be compromised$ 0he user is no longer certifed by this CA$ Neasons !or this include that the sub\ect=s nam
changed) the certifcate is superseded) or the certifcate &as not issued in con!ormance policies$
0he CA=s certifcate is assumed to be compromised$
Each CA must maintain a list (C6L) consisting o! all re#o'ed but not expired certissued by that CA
Each certifcate re#ocation list +CN-, posted to the directory is signed by the issue
CN- includes the issuer=s name)
the date the list &as created)
the date the next CN- is scheduled to be issued
An entry !or each re#o'ed certifcate$ +Certifcate is identifed by its serial number,
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@ublic Key (n!rastructure
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0han' Mou