Computer Security1 Bishop: Chapter 9 Key Management.

Computer Security 1

Bishop: Chapter 9

Key Management

Computer Security 2

Topics

• Key exchange– Session vs interchange keys

– Classical vs public key methods

– Key generation

• Cryptographic key infrastructure– Certificates

• Key storage– Key escrow

– Key revocation

• Digital signatures

Computer Security 3

Notation• X Y : { Z || W } kX,Y

– X sends Y the message produced by concatenating Z and W enciphered by key kX,Y, which is shared by users X and Y

• A T : { Z } kA || { W } kA,T

– A sends T a message consisting of the concatenation of Z enciphered using kA, A’s key, and W enciphered using kA,T, the key shared by A and T

• r1, r2: nonces (nonrepeating random numbers)

Computer Security 4

Session, Interchange Keys• Alice wants to send a message m to Bob

– Assume public key encryption– Alice generates a random cryptographic key ks and uses

it to encipher m• To be used for this message only• Called a session key

– She enciphers ks with Bob’s public key kB

• kB enciphers all session keys Alice uses to communicate with Bob

• Called an interchange key

– Alice sends { m } ks { ks } kB

Computer Security 5

Benefits• Limits amount of traffic enciphered with single

key– Standard practice, to decrease the amount of traffic an

attacker can obtain

• Possible attacks– Example: Alice will send Bob a message that is either

“BUY” or “SELL”. Eve computes possible ciphertexts { “BUY” } kB and { “SELL” } kB. Eve intercepts enciphered message, compares, and gets plaintext at once.

Computer Security 6

Key Exchange Algorithms• Goal: Alice, Bob get shared key

– Key cannot be sent in clear• Attacker can listen in• Key can be sent enciphered, or derived from

exchanged data plus data not known to an eavesdropper

– Alice, Bob may trust third party– All cryptosystems, protocols publicly known

• Only secret data is the keys, ancillary information known only to Alice and Bob needed to derive keys

• Anything transmitted is assumed known to attacker

Computer Security 7

Classical Key Exchange

• Bootstrap problem: how do Alice, Bob begin?– Alice can’t send it to Bob in the clear!

• Assume trusted third party, Cathy– Alice and Cathy share secret key kA

– Bob and Cathy share secret key kB

• Use this to exchange shared key ks

Computer Security 8

Simple Protocol

1. Alice Cathy{ request for session key to Bob } kA

2. Alice Cathy{ ks } kA || { ks } kB

3. Alice Bob{ ks } kB

• Then ks can be used as the secret key between Alice and Bob. e.g., 4. Alice Bob {M} ks

Computer Security 9

Problems ?

• How does Bob know he is talking to Alice?– Replay attack: Eve records message from Alice to Bob

(esp. messages 3 and 4), later replays it; Bob may think he’s talking to Alice, but he isn’t.

e.g., Session key reuse: Eve replays message from Alice to Bob, so Bob re-uses session key.

e.g., Eve replays the message {“Deposit $500 to Jack’s account” }, originally sent from Alice to Bob.

• Protocols must provide authentication and defense against replay.

Computer Security 10

Needham-Schroeder

1. Alice CathyAlice || Bob || r1

2. Alice Cathy{ Alice || Bob || r1 || ks || { Alice || ks } kB } kA

3. Alice Bob{ Alice || ks } kB

4. Alice Bob{ r2 } ks

5. Alice Bob{ r2 – 1 } ks


Argument: Alice talking to Bob

• Second message– Enciphered using key only she and Cathy know

• So Cathy must have enciphered it

– Response to first message• As r1 in it matches r1 in first message

• Third message– Alice knows only Bob can read it

• As only Bob can derive session key from that message

– Any messages enciphered with that key are from Bob


Argument: Bob talking to Alice• Third message

– Enciphered using key only he and Cathy know• So Cathy must have enciphered it

– Names Alice, session key• Cathy provided session key, says Alice is the other party

//identity associated with the session key

• Fourth message– Uses session key to determine if it is replay from Eve

• If not, Alice will respond correctly in fifth message• If so, Eve can’t decipher r2 and so can’t respond, or responds

incorrectly


Denning-Sacco Modification• Assumption of Needham-Schroeder: All keys are

secret.• Question: Suppose Eve can obtain the session key.

How does that affect the protocol?– In what follows, Eve knows ks .

a. Eve Bob{ Alice || ks } kB

b. Alice (intercepted by Eve) Bob{ r2 } ks

c. Eve Bob{ r2 – 1 } ks


Problem & Solution

• In the protocol above, Eve impersonates Alice.• Problem: replay in the third step of Needham-

Schroeder– i.e., Step a in the previous slide

• Solution: use time stamp T to detect replay


Needham-Schroeder with Denning-Sacco Modification

1. Alice CathyAlice || Bob || r1

2. Alice Cathy{ Alice || Bob || r1 || ks || { Alice || T || ks } kB } kA

3. Alice Bob{ Alice || T || ks } kB

4. Alice Bob{ r2 } ks

5. Alice Bob{ r2 – 1 } ks

• Bob will reject the message if T is too old.


Needham-Schroeder with Denning-Sacco Modification

• Weakness: If clocks are not synchronized, Bob may either reject valid messages or accept replays.– [Denning-Sacco] Parties with slow clocks are vulnerable

to replay.

– [Gong] Parties with fast clocks are also vulnerable.

+ Resetting clock does not eliminate vulnerability.


Otway-Rees Protocol

• Corrects the problem in the Needham-Schroeder– That is, Eve replaying the third message in the protocol

• Does not use timestamps– Not vulnerable to the problems that Denning-Sacco

modification has

• Uses an integer n to associate all messages with particular exchange


The Protocol

1. Alice Bobn || Alice || Bob || { r1 || n || Alice || Bob } kA

2. Cathy Bobn || Alice || Bob || { r1 || n || Alice || Bob } kA ||{ r2 || n || Alice || Bob } kB

3. Cathy Bobn || { r1 || ks } kA || { r2 || ks } kB

4. Alice Bobn || { r1 || ks } kA


Argument: Alice talking to Bob

• Fourth message– If n matches the first message, Alice knows it is

part of this exchange protocol.

– Cathy generated ks because only she and Alice know kA .

– Alice determines that the enciphered part belongs to the exchange as r1 matches r1 in encrypted part of the first message.


Argument: Bob talking to Alice

• Third message– If n matches the second message, Bob knows it

is part of this exchange protocol.

– Cathy generated ks because only she and Bob know kB .

– Bob knows that the enciphered part belongs to the exchange as r2 matches r2 in encrypted part of the second message.


Replay Attack against the Otway-Rees Protocol ?

• Eve acquires a ks and the message in the third step– n || { r1 || ks } kA || { r2 || ks } kB

• Eve forwards appropriate part to Alice– Alice has no ongoing key exchange with Bob: n

matches nothing, so is rejected.– Alice has ongoing key exchange with Bob: n does not

match, so is again rejected.• If replay is for the current key exchange, and Eve sent the

relevant part before Bob did, Eve could simply listen to traffic; no replay is needed for Eve to get the information.


Kerberos

• Authentication system– Based on Needham-Schroeder with Denning-Sacco

modification

– Central server plays role of trusted third party (“Cathy”)

• Ticket– Issuer vouches for identity of requester of service

• Authenticator– Identifies sender


Idea

• User u authenticates to the Kerberos server:– Obtains ticket Tu,TGS for ticket granting service (TGS)

• User u wants to use service s:– User u sends authenticator Au, ticket Tu,TGS to the TGS

asking for ticket for service s.

– TGS sends ticket Tu,s to user u.

– User sends Au, Tu,s to the server as a request to use s.

• Details follow


Ticket• Credential saying the ticket issuer (i.e., the

authentication server) has identified the ticket requester (i.e., user u)

• Example ticket issued to user u for service s

Tu,s = s || { u || u’s address || valid time || ku,s } ks

where:

– ku,s is session key for user u and the ticket granting service s.

– ks is the key shared between s and the authentication server

– Valid time is interval for which the ticket is valid.– u’s address may be IP address or something else

• Note: more fields, but not relevant here


Authenticator• Credential containing identity of the sender of a

ticket– Used to confirm the sender is the entity to which the

ticket was issued.

• Example: an authenticator that user u generates for authenticating himself to service s

Au,s = { u || generation time || kt } ku,s

where:– kt is an alternate session key– Generation time is when authenticator generated

• Note: more fields, not relevant here



Protocol

1. user Cathyuser || TGS

Cathy2. user{ ku,TGS } ku || Tu,TGS

3. user TGSservice || Au,TGS || Tu,TGS

4. user TGSuser || { ku,s } ku,TGS || Tu,s

5. user serviceAu,s || Tu,s

6. user service{ t + 1 } ku,s


Exercises1. In constructing Au,s (see steps 3 and 5), the user u needs to

know his session key with s, i.e., ku,s. How does u get the

session key? Hint: Show details of Au,s and Tu,s .

2. How is the session key between u and the TGS, i.e., ku,TGS ,

used in the protocol?

3. How is the session key between u and the service provider s,

i.e., ku,s , used in the protocol?

c.f., An alternative illustration of the Kerberos protocol: http://sce.cl.uh.edu/yang/teaching/csci5233fall02/Kerberos_Authentication_Steps.html

http://sce.cl.uh.edu/yang/teaching/csci5233fall02/Kerberos_Authentication_Steps.html

http://sce.cl.uh.edu/yang/teaching/csci5233fall02/Kerberos_Authentication_Steps.html


Analysis

• First two steps get user ticket to use TGS

– User u can obtain session key, ku,TGS , only if u

knows key shared with Cathy, Ku .

• Next four steps show how u gets and uses ticket for service s– Service s validates request by checking sender

(using Au,s) is the same as entity ticket issued to

– Step 6 optional; used when u requests confirmation


Problems

• Relies on synchronized clocks– If not synchronized and old tickets, authenticators not

cached, replay is possible.

• Tickets have some fixed fields– Dictionary attacks possible

– Kerberos 4 session keys weak (had much less than 56 bits of randomness); researchers at Purdue found them from tickets in minutes

• Solutions? A potential research or survey project


Key Exchange using Public Key

• Here interchange keys known– eA, eB : Alice and Bob’s public keys known to all

– dA, dB : Alice and Bob’s private keys known only to the owner

• Simple protocol– ks is the desired session key

Alice Bob{ ks } eB


Problem and Solution

• Vulnerable to forgery or replay

– Because eB known to anyone, Bob has no assurance that

it was really Alice that sent the message

• Simple fix uses Alice’s private key

– ks is the desired session key

Alice Bob{ { ks } dA } eB


Notes• Can include message enciphered with ks• Assumes Bob has Alice’s public key, and vice

versa– If not, each must get it from a public server

– If keys not bound to identity of the owner, attacker Eve can launch a man-in-the-middle attack (next slide; Cathy is public server providing public keys)

• Solution to this (binding identity to keys) discussed later as public key infrastructure (PKI)


Man-in-the-Middle Attack (in key exchange using public keys)

Alice Cathysend Bob’s public key

Eve Cathysend Bob’s public key

Eve CathyeB

AliceeE Eve

Alice Bob{ ks } eE

Eve Bob{ ks } eB

Eve intercepts request

Eve intercepts message


Key Generation

• Goal: generate difficult-to-guess keys• Problem statement: given a set of K potential keys,

choose one randomly– Equivalent to selecting a random number between 0

and K–1 inclusive

• Why is this hard: generating random numbers– Actually, numbers are usually pseudo-random, that is,

generated by an algorithm


What is “Random”?

• Sequence of cryptographically ransom numbers: a sequence of numbers n1, n2, … such that for any integer k > 0, an observer cannot predict nk even if all of n1, …, nk–1 are known– Best: physical source of randomness

• Random pulses

• Electromagnetic phenomena

• Characteristics of computing environment such as disk latency

• Ambient background noise


What is “Pseudorandom”?

• Sequence of cryptographically pseudorandom numbers: sequence of numbers intended to simulate a sequence of cryptographically random numbers but generated by an algorithm– Very difficult to do this well

• Linear congruential generators [nk = (ank–1 + b) mod n]: broken

• Polynomial congruential generators [nk = (ajnk–1j + … + a1nk–1

a0) mod n]: broken too

• Here, “broken” means next number in sequence can be determined


Best Pseudorandom Numbers

• Strong mixing function: function of 2 or more inputs with each bit of output depending on some nonlinear function of all input bits– Examples: DES, MD5, SHA-1– Use on UNIX-based systems:

(date; ps gaux) | md5

where “ps gaux” lists all information about all processes on system


Exercise #1• The term “session” in “session keys” may be one of the following three

cases:a) A session is a fixed period of time, say 60 sec or 20 min, and so on.

b) A session is the time between two events. For example, a user session is established from the time the user logs in, until the user logs off.

c) A session is a message.

In all three cases, a session key is created and used throughout that session.

This exercise has two components:1. Give an example of session keys for each of the three cases. Provide

sufficient details for people to understand your examples.

2. What are the relative pros and cons of the three types of session keys? Justify your answer.


Exercise #2• Classical Key Exchange includes a set of protocols that use only

classical cryptography to generate session keys. The Needham-Schroeder protocol was proposed as a remedy against replay attacks that may be launched against the simple protocol.

This exercise has two components:1.Would the Needham-Schroeder protocol successfully mitigate the replay attacks that can be effectively launched against the simple protocol? Justify your answer.

2.Perform a vulnerability analysis on the Needham-Schroeder protocol. Can any of the five steps in that protocol be effectively replayed? Justify your answer.


Exercise #3• Public Key Cryptographic Key Exchange is briefly discussed in 9.2.3.

A simple protocol was proposed. In that protocol, when entity A wants to share a session key with entity B, A first encrypts the session key with her own private key and then enciphers the resulting ciphertext with the B’s public key.

A B: { { ks } dA } eB

This exercise has two components:1.Do you agree with the author that such a simple protocol based on public key cryptography would effectively enable key exchange between A and B. Justify your answer. Hint: What security services are provided in this protocol? Is it subject to common attacks like fabrication attacks, man-in-the-middle attacks, or replay attacks?

2.Present a complete solution relaying on public key cryptography. Explain why your protocol would be an effective solution.

Computer Security1 Bishop: Chapter 9 Key Management.

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