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Network Security

(Key Management)

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Key Management

One of the major roles of public-keyencryption has been to address theproblems of key distribution

!wo distinct aspects of public keycryptography" !he distribution of public keys !he use of public key encryption to

distribute secret keys

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#istribution of $ublic keys

$ublic announcements$ublicly a%ailable directory$ublic key authority

$ublic key certi&cates

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$ublic 'nnouncements

'ny participant can send his public keyto any other participant or broadcast thekey to the community at large

ample " $*$ that uses +S' has adopted

the practice of appending their public key tomessages that they send to public forumssuch as newsgroups and internet mailing lists

Seems to be con%enient, has major

weaknesses 'nyone can forge, !hat issome user pretend to be user ' andbroadcast its public key ntil noticed ,forger can able to read encrypted

messages intended for ' and can use keys

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$ublic 'nnouncements

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$ublicly '%ailable#irectory

Some sort of security can be achie%edby maintaining a publicly a%ailabledynamic directory of public keys

Maintenance and distribution will bethe responsibility of some trusted entityor organi.ation

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$ublicly '%ailable#irectory

!he authority maintains a directorywith a /name, public key0 entry foreach participant

ach participant register a public keywith the directory authority +egistrationwould ha%e to be in person or by someform of secure authenticatedcommunication

$articipant may replace the e istingkey

$articipants could also access the

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u c y %a a e#irectoryStill its %ulnerable , if an ad%ersary

succeed in obtaining the pri%ate key of thedirectory authority, it can authoritati%elypass out counterfeit public keys andsubse2uently impersonate and ea%esdrop

any participant'nother way is to temper records kept by

the authority

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$ublic Key 'uthority

Stronger security can be achie%ed bypro%iding tighter control o%erdistribution of public keys from directory

'ssumes that central authoritymaintains a dynamic directory of publickeys of all participants

ach participant reliably knows publickey of the authority, with only authorityknows pri%ate key

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Steps in%ol%ed ' sends a time stamped message to the publickey authority containing a re2uest for the current

public key of 3 !he authority responds with the message that is

encrypted using the authority4s pri%ate key , $+ auth !hus ' is able to decrypt the message using theauthority4s public key !herefore ' is assured thatthe message originated with the authority

!he message includes"3 public key, $ b , which ' can use to encrypt

messages destined for 3 !he original re2uest , to enable ' to match this

response with the corresponding earlier re2uest and%erify that original re2uest was not altered beforereception by the authority

!he original timestamp, so ' can determine that this isnot an old message from the authority containing a key4

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5ontinued6

' Stores 34s public key and also usesit to encrypt a message to 3 containingan identi&er of ' (7# ' ) and a nonce (N 8) ,which is used to identify this transactionuni2uely

3 retrie%es '4s public key from theauthority in the same manner as 'retrie%es 34s public key

't this point , public key has beensecurely deli%ered to ' and 3 and theymay begin there protected e change

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!wo 'dditional Steps3 sends a message to ' encrypted

with $ a and containing '4s nonce(N 8)as well as a new nonce generated by 3

(N 9) 3ecause only 3 could ha%edecrypted message (:), the presence ofN8 in message (;) assures ' that the

corresponding is 3' returns N 9 , encrypted using 34s

public key , to assure 3 that is

corresponding is '

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5ontinued !otal are se%en steps, the initial four

steps are used infre2uently becauseboth ' and 3 can sa%e the other4spublic key for future use, known as

caching$eriodically user should re2uest fresh

copies of public key to ensure currency

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$ublic Key 'uthority

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$ublic key certi&cates

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$ublic key 5erti&cates

' user can present his public key to theauthority in a secure manner , and obtain acerti&cate

!he user then publish the certi&cate'ny one needed this user public key can

obtain the certi&cate and %erify that it is%alid by way of the attached trusted

signature' participant can also con%ey its key

information to another by transmitting its

certi&cate

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+e2uirements'ny participant can read a certi&cate

to determine the name and public keyof the certi&cate owner

'ny participant can %erify that thecerti&cate originated from thecerti&cate authority and is notcounterfeit

Only the certi&cate authority cancreate and update certi&cate

'ny participant can %erify the

currency of the certi&cate

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$ublic key 5erti&cates

ach participant applies to thecerti&cate authority, supplying a publickey and re2uest a certi&cate

'pplication must be in person or bysome form of secure authenticatedcommunication

1or participant ', the authoritypro%ides a certi&cate of the form"

=here $+ auth is the pri%ate key used by

the authority and ! is the timestamp

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5ontinued6' may then pass this certi&cate on to any

other participant , who reads and %eri&es thecerti&cate

as follows"

!he recipient uses the authority public key, $auth , to decrypt the certi&cate

3ecause the certi&cate is readable only usingauthority4s public key, this %eri&es authenticityof certi&cate authority

!he elements 7# ' and $ a pro%ides therecipient with the name and the public key of

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!ime Stamp Scenario

' pri%ate key is learned by thead%ersary' generates a new pri%ate>public key

pair and applies to the certi&cateauthority for new certi&cate

Meanwhile , the ad%ersary replays theold certi&cate to 3 if 3 then encryptsmessages using compromised old publickey, the ad%ersary can read thosemessages

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$ublic Key 5erti&cate

f

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#istribution of Secret Keyusing $ublic Key

cryptography$ublic key encryption can be usedfor distribution of Secret keys

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Simple Secret Key#istribution

' generates a public>pri%ate keypair/$ a , $+ a 0 and transmits amessage to 3 consisting of $ a andan identi&er of ', 7# '

3 generates a secret key , K s , andtransmit it to ', encrypted with '4spublic key

' computes #($+ a , ($ a , K s)) toreco%er the secret key 3ecause only

' can decrypt the message, only '

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' and 3 can now securelycommunicate using con%entionalencryption and the session Key K s

't the completion of e change bothdiscard K sNo keys e ist at the start of

communication and none e ist afterthe completion of communication !hus the risk of keys compromise is

minimal

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S t k #i t ib ti ith

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Secret key #istribution withcon&dentiality and

authentication' uses 34s public key to encrypt a message to 3containing an identi&er of ' (7# ' ) and nonce(N 8),which is used to identify this transaction uni2uely

3 sends a message to ' encrypted with $ a and

containing '4s nonce(N 8) as well as a new noncegenerated by 3 (N 9) 3ecause only 3 could ha%edecrypted message(8), the presence of N 8 inmessage(9) assures ' that the correspondent is 3

' returns N9, encrypted using 34s public key, to

assure 3 that its correspondent is '' selects a secret key K s and sends M? ($ b , ($+

a , K s)) to 3ncryption of this message with 34s public key

ensures that only 3 can read it@ encryption with ' As

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#iBe-Cellman Keychange

'lice and 3ob want to share a secret key using#iBe-Cellman'lice chooses a large prime number p and a largenumber called the generator g which is less than

p @ these two %alues ( g and p ) are sent to 3ob'lice chooses a number a and calculates A ? g a (mod p )@ the %alue A is sent to 3ob3ob chooses a number b and calculates B ? g b

(mod p )@ the %alue B is sent to 'lice'lice calculates the shared secret K ? Ba (mod p )3ob calculates the shared secret K ? A b (mod p )

Documents

Lec 12 and 13