Protocol Analysis. CSCE 522 - Farkas 2 Cryptographic Protocols Two or more parties Communication over insecure network Cryptography used to achieve goal

Protocol Analysis

CSCE 522 - Farkas 2

Cryptographic ProtocolsTwo or more partiesCommunication over insecure networkCryptography used to achieve goal

Exchange secret keysVerify identity (authentication)Secure transaction processing

CSCE 522 - Farkas 3

Emerging Properties of Protocols

Greater interoperation Negotiation of policy Greater complexity Group-oriented protocols Emerging security threats

CSCE 522 - Farkas 4

ProtocolsGood protocol characteristics:Established in advanceMutually subscribedUnambiguousComplete

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Symmetric-Key Distribution: Symmetric-Key Distribution: Symmetric-Key TechniquesSymmetric-Key Techniques

(repeat from lecture on 05/13/2014)Symmetric-Key without ServerSymmetric-Key with Server

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Symmetric-Key Distribution without ServerChange encryption key E(Knew,K), where Knew is

the session key, K is the master key

Encryption Decryption

New key New keyCiphertext C

Sender RecipientK

Originator

(O,R,IO) E([(IO,R,KOR,E((KOR,O), KR)], KO)

E((KOR,O), KR)

Server

CSCE 522 - FarkasCSCE 522 - Farkas 7

Symmetric-Key Distribution with Server

Recipient

Decrypts with KO

Knows KOR

Does not know E((KOR,O), KR)

Decrypts with KR

Knows KOR

Knows KO and KR

CSCE 522 - Farkas 8

Symmetric-Key Distribution: Symmetric-Key Distribution: Public-Key TechniquesPublic-Key Techniques

Simple secret key distribution – insecure Secret key distribution with confidentiality

and authenticationDiffie-Hellman Key Exchange

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Simple secret key distribution

Sender Recipient

1. KE-S ||ID-S

2. E KE-S(Ksession)

Vulnerable to active attack!HOW?

Public key of S

Secret Session key

CSCE 522 - Farkas 10

With confidentiality and authentication

Sender Recipient

1. E KE-R[N1||ID-S]

2. E KE-S[N1||N2]

3. E KE-R[N2]

4. E KE-R E KD-S(Ksession)

Assume: KE-R and KE-S are known in advanceNonce

Question: Why do we need reliable distribution of public keys?


Diffie-Hellman Key Exchange

Proposed in 1976 First public key algorithm Allows group of users to agree on secret key over

insecure channel Cannot be used to encrypt and decrypt messages



Protocol for A and B want to agree on shared secret key: A and B agree on two large numbers n and g, such that

1<g<n A chooses random x and computes X=gx mod n and sends

X to B B chooses random y and computes Y=gy mod n and sends

Y to A A computes Yx mod n = gyx mod n B computer Xy mod n = gyx mod n Secret key: gyx mod n



Requires no prior communication between A and B

Security depends on difficulty of computing x given X=gx mod n

Choices for g and n are critical: both n and (n-1)/2 should be prime, n should be large

Susceptible to intruder in the middle attack (active intruder)


Intruder in the Middle Attack

Bob AliceEveHi Alice, I’m Bob.

Hi Bob, I’m Alice. Hi Bob, I’m Alice.

Hi Alice, I’m Bob.

Intruder and BobUses Diffie-HellmanTo agree on key K.

Intruder and AliceUses Diffie-HellmanTo agree on key K’.

Question: the attacker may want to have K and K’ be the same, Why?


Public-Key DistributionPublic-Key DistributionWithout server

Broadcasting - insecure Publicly available directory

With trusted serverPublic key distribution centerCertificates


Public announcement

John Smith

KE-J.S.

KE-J.S.

KE-J.S.

KE-J.S.

KE-J.S.

KE-J.S.

Question: What are the vulnerabilities of this approach?


Publicly available directory

PublicKeyDirectory

John Smith Mary Rose

KE-J.S. KE-M.R..

Better but notgood enough Directory could Be compromised


Public-key authority

Public-Key Authority

Sender Recipient

1. Request || Time1

2. EKD-Auth[KE-R||Request||Time1]

3. EKE-R(ID-S||N1)

4. Request || Time2

5. EKD-Auth[KE-S||Request||Time2]

6. EKE-S(N1||N2)

7. EKE-R(N2)

Question1: What should the Authority, the Sender and the Recipient know before communication?

Exercise: After each message, show what the recipient of the message can do and what the Recipient know.


Public-key certificates

Certificate Authority

Sender Recipient

KE-S

C-S=EKD-CAuth[Time1,ID-S,KE-S]

1. C-S

2. C-R

KE-R

CR=EKD-CAuth[Time2,ID-R,KE-R]


Certificates

Guarantees the validity of the informationEstablishing trustPublic key and user identity are bound

together, then signed by someone trustedNeed: digital signature


Digital SignatureNeed the same effect as a real signature

Un-forgeableAuthenticNon-alterableNot reusable


Digital signature

Direct digital signature: public-key cryptography based

Arbitrated digital signature:Conventional encryption:

Arbiter sees messageArbiter does not see message

Public-key basedArbiter does not see message


Digital Signatures in RSA

Sender Recipient

Insecure channel

Plaintext PlaintextSigned plaintext

Encryption Alg.

Decryption Alg.

S’s public keyS’s private key(need reliable channel)

Sign Verify

Protocol Analysis Exercise 1.

Assume that Jane and Paul want to efficiently send very large files to each other. They also want to provide integrity verification, third-party message authentication (i.e., a third party can verify who the originator of the message is), and limit the scope of a compromise (i.e., providing forward secrecy). You can assume that Jane and Paul have public and secret key encryption capabilities, can generate a hash function, and they have a shared secret key K0

established before the communication. They do not have access to a mutually trusted server, and no other keys but K0 are known at the beginning of the communication. Propose a security protocol to establish necessary keys and show how Jane can send a file to Paul.

CSCE 522 - FarkasLecture 8-9

Exercise 2.

Message authentication and key agreement

Alice wants to establish a secure communication with Bob. They agree to user the Yahalom protocol for mutual authentication and key agreement. The protocol uses symmetric key encryption only. Alice has a secret key shared with a trusted third party Server, KA and, similarly, Bob has a secret-key shared with Server, KB. NA and NB are nonces generated by Alice and Bob, respectively. E(M, K) indicates encryption of message M with key K, “||” means concatenation of messages. Explain after each protocol step what the recipient of the message knows based on the message and the properties of the encryption and what he/she is capable of doing. For example,


Exercise 2.

Message1: Alice Server:IDA || E(“request for session key to Bob”, KA)

Server:• The server sees that that claimed sender of the message is Alice.

• The server can decrypt the message using KA that is shared between Alice and the Server. The message must have been sent by Alice because KA is only known by Alice and the server.

• The server knows that Alice is requesting a session key to be used by Alice and Bob.

• The server can generate a session key KS to be used by Alice and Bob and send the key to …


Exercise 2.

Message1: Alice Bob: IDA || NA

Bob knows/can do

Message2: Bob Server: IDB || E[(IDB || NA || NB), KB]

Server knows/can do

Message3: Server Alice: E[(IDB || KS || NA || NB), KA] || E[(IDA || KS), KB]

Alice knows/can do

Message4: Alice Bob: E[(IDA || KS), KB] || E(NB, KS)]

Bob knows/can do


Exercise 3.

Secure communication

Consider the following protocol. Ann wants to send a message M securely to Bob but there is no shared secret key between Ann and Bob, Ann does not even know Bob’s public key. However, using the properties of RSA (in particular the commutative property), Ann proposes the following protocol, where E(M, K) indicates encryption/decryption of message M with key K, “||” means concatenation of messages, Kpub

A means the public key of A, KprivA

means private key of A.


Exercise 3.

Message1: Ann Bob: IDA || E(M, KpubA)

Message 2: Bob Ann: IDB || E[(E(M, KpubA)), Kpub

B)

Message3: Ann Bob: IDA || E(M, KpubB)

Show a man-in-the-middle attack against the above protocol.


CSCE 522 - Farkas 30Lecture 8-9

Next class

Review for Test 1

Documents

Protocol Analysis. CSCE 522 - Farkas 2 Cryptographic Protocols Two or more parties Communication over insecure network Cryptography used to achieve goal