Security Services in Information Systems

Basic Concepts

Key exchange: Diffie-Hellman protocol

1. Picks a GF(p) at random

2. Computes TA = ga mod p

3. Sends TA

4. Receives TB

5. Computes KA = TBa mod p

1. Picks b GF(p) at random

2. Computes TB = gb mod p

3. Receives TA

4. Sends TB

5. Computes KB = TAb mod p

Where K = KA = KB, Because:

TBa = (gb)a = gba = gab = (ga)b = TA

b mod p

Machine A Machine B

Man-in-the-Middle Attack

• After this exchange, the middle-person attacker simply decrypts any messages sent out by A or B, and then reads any possibly modifies them before re-encrypting with the appropriate key and transmitting them to the correct party.• Middle-person attack is possible due to the fact that DHC does not authenticate the participants. Possible solutions are digital signatures and other protocol variants.

• Consider the following scenario:

Anita Middleperson Betito

ga = 8389 gx = 5876 gb = 9267

8389 5876

5876 9267

Shared key KAX: Shared key KBX

5876a = 8389x 9267x = 5876b

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Key Distribution Problem[proposed solutions]

Key Distribution/Management and Authentication

• two closely related subjects

• why?

Key Distribution

• symmetric schemes require both parties to share a common secret key– issue is how to securely distribute this key without

revealing that key to an adversary

• many attacks are based on poor key management and distribution – rather than breaking the codes

• This is, actually, the most difficult problem in developing secure systems

Key Distributionvarious key distribution alternatives for parties A and B :

1. A can select key and physically deliver to B– does not scale for a large and distributed group– how many keys do we need for N users?

2. third party can select & deliver key to A & B– similar comment as 1– sometimes finding a “trusted” third party is another problem

3. if A & B have communicated previously can use previous key to encrypt a new key

– good option but initially several keys to be distributed

4. if A & B have secure communications with a third party C, C can relay key between A & B

– only N master keys are enough

Session Key / Master Key

• The idea is having a master encryption key (master key) to generate random and temporary session keys

• can be implemented in several ways– Basic D-H is such an example

• public/private keys are master keys, exchanged key is a session key

– Kerberos is another example– SSL uses three layers

• D-H for master key, master key for the session key

Session Key / Master Key

• Session key lifetime is a trade-off– if small,

• more secure since attacker can obtain less ciphertext to analyze

• But this creates more delay

– If large• less secure, but less delay

Key Distribution Facts

• Conservation of trust principle– a secure communication cannot be based on nothing;

either there should be an initial direct contact or an indirect one

• Either physical delivery or a trusted third party– physical delivery is the only option to avoid a third

party• most basic system is PIN entry• the case in Bluetooth

– otherwise regardless of symmetric or asymmetric encryption, you need a trusted third party

• even D-H does not work without a third party, why?

A Key Distribution Example

• Symmetric crypto based• Each user shares a symmetric master key

with the KDC (Key Distribution Center)– e.g. Ka, Kb, Kc, …

– possibly physically distributed

• Basic idea:– whenever a user A wants to communicate with B,

it requests a session key (Ks) from KDC

• Protocol is in the next slide

Nonce Definition

• Nonce: The present or particular occasion.

• Nonce word: A word occurring, invented, or used just for a particular occasion.

A Key Distribution Example

An Alternative

• In the previous figure, can KDC send message 3 directly to B?– If not, why?– If so, what are pros and cons?

Hierarchies of KDCs

• we may have several KDCs connected to each other in a tree topology– each leaf KDC serves to a local community

• intra-domain communication passes thru only the local KDC, however inter-domain communication requires several KDC-KDC interaction

• master key delivery is only in local domains

Decentralized Key Control

• We may avoid using KDC in a small group by having a master key for each pair

Key Management in PKC

• In other words– distribution of public keys– use of PKC to distribute secret keys

• public/private key as a master key

Distribution of the Public Keys

• the most important barrier against the deployment of PKC in applications

• Basic question?– how can I make sure about the legitimacy of a

public key? – how can I make sure that Bob’s public key really

belongs to Bob, not to Charlie?

• Why this is so important?– Name spoofing attacks become possible

• remember the man-in-the-middle attacks in anonymous Diffie-Hellman

Distribution of the Public Keys

• Some methods– Public Announcement– Publicly available databases/directories– Centralized distribution– Certificates

Public Announcement

• Broadcast your public key to the public– via newsgroups, mailing lists, from

personal website, etc.– major weakness is anyone can easily

pretend as yourself• so attacks are possible

Publicly available directories/databases

• There exists a directory/database for {name, public key} pairs

• write controlled– a trusted administrator

• if administered thoroughly, good– but a proper administration is difficult

• need secure mechanisms for registration, update, delete.

Centralized Distribution - Public-Key Authority

• Similar to directory/database approach, but access to the directory is automated via a secure protocol – users interact with directory to obtain any desired

public key securely– requires real-time access to directory when keys

are needed– users should know public key for the directory

• the directory/database contains {name, public key} pairs – write permit is restricted

Centralized Distribution - Public-Key Authority PROTOCOL

Centralized Distribution - Public-Key Authority

• Disadvantages– authority is an active entity and may create

a performance bottleneck– database should be kept secure to prevent

unauthorized modification

Public-Key Certificates

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

• a certificate binds identity to public key – usually with other info such as period of validity, rights

of use, issuer’s info, etc

• all contents signed by a trusted Certification Authority (CA)

• can be verified by anyone who knows the CA public-key

• CA must make sure about the identity of the cert owner

Public-Key Certificates

Public-Key Certificates

• Certificates are widely used in practice– previous slides only show the idea

• need lots of polishing for practical use– is a single CA sufficient?– what happens if the CA’s public key is not known?

• how to distribute CA public keys?

– what happens if a certificate is revoked?

• We will discuss the use of certificates later

What can you do with securely distributed public keys?

• Digital signatures– talked about them

• confidentiality– theoretically possible but slow– instead session keys can be distributed

• those session keys are used for symmetric encryption

Distribution of Secret Keys using PKC

• Several methods exist

• Diffie-Hellman is one way

• we will see some alternatives

Simple Secret Key Distribution

• proposed by Merkle in 1979– A generates a new temporary public key pair– A sends B its public key and identity– B generates a session key and sends it to A

encrypted using the supplied public key– A decrypts the session key and both use it

Simple Secret Key Distribution

• problem is that an opponent can intercept and impersonate both halves of protocol– man-in-the-middle attack– result: A, B think that they exchanged Ks securely but C

also knows Ks and use it to eavesdrop the communication passively

CKUc

EKUc[Ks]EKUa[Ks]

Public-Key Distribution of Secret Keys

• assumption: public-keys are securely exchanged a priori• First three steps are for authentication purposes• Last step provides both the secrecy and authenticity of

the session key

In practice

• Most systems offer a three-level approach– use of PKC to exchange master-key– use of master-key to exchange session

keys

• most important advantage is at performance

A closer look to authentication

• making sure of peer entity’s identity– mutual or one-way– non-repudiation is not an aim here

• generally implemented as a protocol– basic idea: making sure that other party knows a

common secret– also used for session key distribution

• two types– message authentication

• mostly one-way

– peer entity authentication• connection oriented approach

Two key issues

• Protection of any secret information

• Timeliness– to prevent replay attacks

• a valid message is copied and later resent

– Why replays are bad?• at minimum, disrupt proper operation by

presenting messages that appear genuine but actually are not

• may also cause impersonation and key compromise

Countermeasures - 1

• Sequence numbers– not a practical method– parties should keep track of the sequence

numbers– and should take care of message losses,

duplications in a secure manner• complicates stuff

Countermeasures - 2

• Timestamps– message contains a timestamp– accepts only fresh messages based on this

timestamp– sometimes used but some practical

problems• clocks must be synchronized in a secure

manner (some attacks are merely based on failure on clock synchronization)

• tolerance to network delays

Countermeasures - 3

• Challenge/response– Initiator sends a nonce (one-time challenge

phrase or number) and expects that nonce in the response

– easier to implement– but may require extra message transfer

Authentication using Symmetric Encryption

• We start with well-known Needham-Schroeder protocol– actually have seen it as a key distribution protocol

• There exists a Key Distribution Center (KDC)– each party shares own master key with KDC– KDC generates session keys used for connections

between parties– master keys used to distribute these keys to them

Needham-Schroeder Protocol• original three-party key distribution protocol

– for session between A and B mediated by a trusted KDC– KDC should be trusted since it knows the session key

• protocol overview1. A→KDC: IDA || IDB || N1

2. KDC→A: EKa[Ks || IDB || N1 || EKb[Ks||IDA] ]

3. A→B: EKb[Ks||IDA]

4. B→A: EKs[N2]

5. A→B: EKs[f(N2)]

• 4 and 5 are to prevent a kind of a replay attack– against replay of message 3 by an attacker

Needham-Schroeder (NS) Protocol• but still vulnerable to a replay attack

– if an old session key has been compromised, then message 3 can be resent to B by an attacker X impersonating A

– after that X intercepts message 4 and send a message 5 to B as if it is A

– now X can impersonate A for the future communications with the session key

– unlikely but a vulnerability

• modifications to address this problem– timestamps (Denning 81), B contacted at the beginning

(Needham Schroeder 87)• see http://www.lsv.ens-cachan.fr/spore/index.html for a repository of security

protocols

NS Protocol with timestamps• proposed by Dorothy Denning• A and B can understand replays by checking

the timestamp in the message– even if attacker knows Ks, he cannot generate

message 3 with a fresh timestamp since he does not know Kb

– open question: why do we need messages 4 and 5?

Needham – Schroeder Protocol Revisited

• Amended by the creators themselves in 1987 by putting B in the loop early in the protocol

1. A -> B : A

2. B -> A : EKb[A, Nb]

3. A -> KDC : A, B, Na, EKb[A, Nb]

4. KDC -> A : EKa [Na, B, Ks, EKb [Ks, Nb, A]]

5. A -> B : EKb [Ks, Nb, A]

6. B -> A : Ks[Nb]

7. A -> B : Ks[f(Nb)]

Synchronization problem• Proper use of timestamps requires synchronized

clocks• e.g. when sender’s clock is ahead of recipients

clock– attacker can intercept the message and replay later

• Neuman proposed use of nonces with timely session keys to constitute tickets in 93– nonces are used to avoid replays– timestamps are expiration dates of the session key

established– attacker only has a limited time to break the session key

• in original NS, attacker has unlimited time to break the session key

Neuman Protocol

• EKb[IDA || Ks || Tb] is a ticket that can be used later within the limit of Tb

Authentication using Public-Key Encryption

• We have given an example method that assumes public keys are known

• There exists protocols that also distributes public keys– using a central Authentication Server (AS)

or KDC– using timestamps or nonces

One-Way Authentication

• required when sender & receiver are not online at same time – e-mail is a typical application– protocol should not rely on the processing of B

• A symmetric encryption approach1. A→KDC: IDA || IDB || N1

2. KDC→A: EKa[Ks || IDB || N1 || EKb[Ks||IDA] ]

3. A→B: EKb[Ks||IDA] || EKs[M]

• Provides authentication that the sender is A • does not protect against replays (of msg. 3)

– could rely on timestamp in message, but delays in email make this problematic

Public-Key Approaches

• have seen some public-key approaches

• if confidentiality is major concern, can use:A→B: EKUb[Ks] || EKs[M]

– digital envelopes

• if authentication needed use a digital signature with a digital certificate:A→B: M || EKRa[H(M)] || EKRas[T||IDA||KUa]

– message, signature, certificate

Overview on IP Security

Internetwork Protocol (IP)

• Aim– provide interconnection across different

networks

• implemented in every network and in routers

• IP is an unreliable protocol– IP datagrams may be lost– IP datagrams may arrive out of order– TCP takes care of those problems

Internetwork Protocol (IP)

Where to provide security?

• Application-layer?– S/MIME, PGP – email security– Kerberos – client server– SSH – secure telnet

• Transport level?– SSL / TLS– between TCP and Application

• IP level– IPSec

IPv4• The IP version that most LANs are

currently using

Data (Payload) follows the header

IPv6

• Next generation IP– driving force was the inadequateness of IPv4

address space

• IPv6 header– modular approach– base header + extension headers– base header is longer than v4, but number of

fields is smaller

IPv6 header

Is IP Secure?

• Content (Payload) is not encrypted– confidentiality is not provided– IP sniffers are available on the net

• IP addresses may be spoofed– authentication based on IP addresses can be

broken

• So IP is not secure

IPSec• general IP Security mechanisms• provides authentication and confidentiality at IP level

– also has key management features

• Applications– VPNs (Virtual Private Networks)

• Interconnected LANs over the insecure common carrier network (mostly the Internet)

• router-to-router

– Secure remote access, e.g. to ISPs• individual-to-router

• IPSec is mandatory for IPv6, optional for v4– many manufacturers support IPSec in their v4 products

IPSec Application Scenarios

AH – Anti-replay Service• Detection of duplicate packets • Sequence numbers

– associated with SAs – 32-bit value– when an SA is created, initialized to 0

• when it reaches 232-1, SA must be terminated• not to allow overflows

– sender increments the replay counter and puts into each AH (sequence number field)

• Problem: IP is unreliable, so the receiver may receive IP packets out of order– Solution is using windows

AH – Anti-replay

Service

• If a received packet falls in the window– if authenticated and unmarked, mark it– if marked, then replay!

• If a received packet is > N– if authenticated, advance the window so that this packet is at the rightmost edge

and mark it

• If a received packet is <= N-W– packet is discarded; this is an auditable event

•Fixed window size W (default is 64)

–employed by the receiver

Key Management in IPSec

• Ultimate aim– generate and manage SAs for AH and ESP– asymmetric

• receiver and initiator have different SAs

• can be manual or automated– manual key management

• sysadmin manually configures every system

– automated key management• on demand creation of keys for SA’s in large systems

Key Management in IPSec• Complex system

– not a single protocol (theoretically)– different protocols with different roles

• intersection is IPSec• but may be used for other purposes as well

• Several protocols are offered by IPSec WG of IETF– Oakley, SKEME, SKIP, Photuris– ISAKMP, IKE

• IKE seems to be the IPSec key management protocol but it is actually a combination of Oakley, SKEME and uses ISAKMP structure

• See IPSec WG effort at http://www.ietf.org/html.charters/ipsec-charter.html

Internet Key Exchange.

X.509 Authentication

X.509 Authentication Protocols

• X.509 is a ITU-T standard part of the “directory services”– mostly for certificates, but also propose

three generic authentication protocols– one-way authentication– two-way authentication– three-way authentication– use both nonces and timestamps

• nonces are unique only within the lifetime of timestamp

X.509 one-way authentication• Proves that the message generated by A and

intended for B– also proves that message is not a replay– proves the identity of the sender, but not the recipient– optionally includes a session key

tA: timestamp rA: nonce sgnData: Data signed

X.509 two-way Authentication• both parties verify identity of each other

• reply message– generated by B– not a replay (guaranteed by tB and rB)

X.509 three-way Authentication• Nonces are signed and echoed back

– each side can check the replay– timestamps are not needed

• synchronized clocks are not needed either• what is a potential risk in such a case?

X.509 Certificate Format