Chapter 4 Data Security (Dr.Atef)

7/31/2019 Chapter 4 Data Security (Dr.Atef)

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public-key encryption helps address key

distribution problems

have two aspects of this:

distribution of public keys use of public-key encryption to distribute secret

keys

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can be considered as using one of:

public announcement

publicly available directory

public-key authority public-key certificates

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users distribute public keys to recipients orbroadcast to community at large E.g., append PGP keys to email messages or post

to news groups or email list

major weakness is forgery anyone can create a key claiming to be someone

else and broadcast it

until forgery is discovered can masquerade as

claimed user

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can obtain greater security by registeringkeys with a public directory

directory must be trusted with properties: contains {name,public-key} entries

participants register securely with directory

participants can replace key at any time

directory is periodically published

directory can be accessed electronically

still vulnerable to tampering or forgery

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improve security by tightening control over

distribution of keys from directory

requires users to know public key for the

directory then users interact with directory to obtain

any desired public key securely

does require real-time access to directory when

keys are needed

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certificates allow key exchange without real-time access to public-key authority

a certificate binds identity to public key-contains other info such as period of validity,

rights of use, etc.

with all contents signed by a trusted Public-Key or Certificate Authority (CA)

can be verified by anyone who knows the

public-key authorities public-key

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use previous methods to obtain public-key

can use for secrecy or authentication

but public-key algorithms are slow

so usually want to use private-key encryptionto protect message contents

hence need a session key

have several alternatives for negotiating a

suitable session key

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proposed by Merkle in 1979

A generates a new temporary public key pair

A sends the public key and its identity to B

B generates a session key K and sends it to Aencrypted using the supplied public key

A decrypts the session key and both use

problem is that an opponent can intercept

and impersonate both halves of protocol

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Simple Use of Public-Key Encryption to Establish a Session Key


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A & B securely exchange public-keys:

-Insures both

(i) confidentiality and (ii) authentication.

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use of private-key KDC

shares secret master key with each user

distributes session key using master key

public-key used to distribute master keys Three-level hierarchy.

rationale

Performance (public key cryptography is used

the least frequently.)

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by Diffie & Hellman in 1976 along with theexposition of public key concepts Williamson secretly proposed the concept in 1970

a practical method for public exchange of asecret key

--even when two parties do not know eachother (they share no secret information).

-does not require a third party (like KDC).

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a public-key distribution scheme establishes a common key

known only to the two participants

value of key depends on the participants(and their private and public keyinformation)

based on exponentiation in a finite field

(modulo a prime or a polynomial)security relies on the difficulty of

computing discrete logarithms (similar tofactoring) hard

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all users agree on global parameters:

large prime integer or polynomial q

number a that isa primitive root mod q

each user (e.g., A) does the following:-chooses a secret key (number): xA < q

-computes: yA = axA mod q

-sends yA to the other party.

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shared session key for users A & B is KAB:

KAB = axA.xB mod q

= yAxB mod q (which B can compute)

= yB

xA

mod q (whichA

can compute) KAB is used as session key in private-key

encryption scheme between Alice and Bob

attacker needs an x, must solve discrete log

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users Alice & Bob who wish to swap keys:agree on prime q=353 and a=3

select random secret keys: A chooses xA=97, B chooses xB=233

compute respective public keys: yA=3

97mod 353 = 40 (Alice)

yB=3233

mod 353 = 248 (Bob)

compute shared session key as: KAB= yBxA mod 353 = 24897 = 160 (Alice)

KAB= yAxB mod 353 = 40

233= 160

(Bob)

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vulnerable to a man-in-the-Middle Attack

authentication of the keys is needed

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Hacker

Alice Bob

YA

Generates Two Private Keys XD1, XD2

Generates Two Public Keys YD1, YD2

YD1

YB

YD2

K1 = (YD1)X

B mod q

K1 = (YB)X

D1 mod q

K2 = (YD2)XA mod q

K2 = (YA)X

D2 mod q


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Darth prepares for the attack by generating two

random private keys XD1 and XD2 and then computing

the corresponding public keys YD1 and YD2.

Alice transmits YA to Bob.

Darth intercepts YA and transmits YD1 to Bob. Darthalso calculates K2 = (YA)

XD2 mod q.

Bob receives YD1 and calculates K1 = (YD1)XB mod q.

Bob transmits YB to Alice.

Darth intercepts YB and transmits YD2 to Alice. Darthcalculates K1 = (YB)

XD1 mod q.

Alice receives YD2 and calculates K2 = (YD2)XA mod q.

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Alice sends an encrypted message M:

E(K2, M).

Darth intercepts the encrypted message

and decrypts it, to recover M.Darth sends Bob E(K1, M) or E(K1, M'),

where M' is any message. In the first case,

Darth simply wants to eavesdrop on the

communication without altering it. In the

second case, Darth wants to modify the

message going to Bob

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have considered:

distribution of public keys

public-key distribution of secret keys

Diffie-Hellman key exchange

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Chapter 4 Data Security (Dr.Atef)