Introduction to Protocols:Entity Authentication, Key Establishment, Integrity/Message

Authentication, Confidentiality

INFSCI 1075: Network Security – Spring 2013

Amir Masoumzadeh

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Overview Authentication Weak entity authentication Strong entity authentication Authenticated Key Establishment Key establishment and management Public Key Infrastructure Message Confidentiality/Privacy Message Authentication

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How Most Security Protocols Work?

Alice Bob

Step 1: Alice verifies her “identity” to Bob and vice-versa

Step 2: Alice and Bob establish a shared “secret” (or a set of secrets)Step 3: Data communications with confidentiality and authentication

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Authentication Schemes Some typical scenarios requiring electronic

“proof of identity” Withdrawing money from an automatic teller

machine Needs a PIN number

Charging purchases to a credit card over telephones Needs the credit-card number and expiration date

Remote login over a computer network Needs a login name and password

Authenticated key establishment protocols

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Such Techniques Are NOT Secure in General Passive attacks

An eavesdropper can use the identifying information for his/her own purposes

Credit card numbers and expiration dates are in plaintext!

An ATM is somewhat more secure but someone monitoring the communication line could obtain all the information on the encoded strip as well as the PIN number

Login and passwords are transmitted in plaintext (except say with SSH)

Access to password files

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What is User Authentication? Message authentication

Involves a message and its integrity as well as where it originated (later)

User authentication There is NO meaningful message

Only a claim that “This is me” It is real-time Basis of access control & user accountability Also called “identity verification” Has two steps

identification - specify identifier verification - bind entity (person) and identifier

Formal Definition: Authentication is the procedure by which one party is assured of the identity of a second party involved in a protocol through corroborative evidence

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Objectives of Authentication In the case of honest parties Alice and Bob, Alice

should be successfully able to authenticate herself to Bob

Bob should not be able to use the identification exchange with Alice to compromise her

Oscar should not be able to impersonate Alice in an exchange with Bob, even though he is able to observe a large number of previous identification exchanges between Alice and Bob

The above hold even if Oscar has successfully participated in an authentication

scheme with Alice and/or Bob Oscar can simultaneously start multiple instances of the

entity authentication protocol

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Means of User Authentication What you know

Something known by the party to be authenticated: PIN, Password, etc.

What you have Something the party to be authenticated possesses:

Smart card with a time variant password What you are or you do

Something inherent to the party to be authenticated: Biometrics like fingerprints, voice, retinal patterns, etc.

Comments: Authentication can be mutual or one way It should be computationally efficient

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Weak Entity Authentication Features:

Uses a fixed or time-invariant password, pin, or some other quantity

The password or pin is shared between the user and the system Secret key scheme

The User ID is the claim of identity The Password is the evidence in support of the claim

Verification The user supplies the password to the system

(reveals the password!) The system accepts this as a corroboration of the

user’s identity

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Storing Passwords Obvious approach

Store passwords in plaintext Set read and write access controls Superusers can determine the password If Oscar gets superuser access, the passwords are easily available

“Encrypted” password files Password is stored as a hash value or encrypted value To verify identity, the system computes the hash of the supplied

password and compares the entry in the stored file It is called “encrypted” even though most times it is a hash value

that is stored

* A hash function is any well-defined procedure or mathematical function that converts a large, possibly variable-sized amount of data into a small datum, usually a single integer that may serve as an index to an array

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Attacks against Fixed Passwords Replay of fixed passwords

People write down passwords If transmitted in plaintext (like telnet), Oscar can capture it on

the link Exhaustive password search

Oscar keeps trying each possible password Online attacks are rare (e.g., locks up after three trials) Offline attacks are more serious

Password guessing and dictionary attacks Given access to a password file (encrypted), Oscar tests each

password to see if there is a match Easy to do since the hash function is known To improve the probability of success, Oscar tries common

words, proper names, lowercase strings etc. – dictionary attacks

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Preventive Measures Password rules

Require users to have special characters, capital letters, etc. in their password

Entropy = uncertainty in password Try to ensure that all passwords are equally likely Makes attacks more difficult

Make the password verification process slow Verifying a few passwords is easy Comparing millions of passwords may be very time

consuming Use pass phrases

Increases the entropy without reducing human ability to remember

Passphrases are stored as hash values and NOT truncated

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Preventive Measures (cont.) Salting

Augment passwords by a random string of t-bits before applying the hash function

The hash value and the salt are both stored This increases the effort of a dictionary attack (by how

much?) but not an exhaustive search Used in the UNIX operating system

One-time passwords Major security threat is eavesdropping and replay Each password is used only once to prevent this problem

System and user share a sequence of t passwords that are used one after the other

Sequentially updated – during authentication, the user and system exchange the password to be used the next time

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Challenge-Response or Strong Entity Authentication Idea: Alice proves her identity to Bob by

demonstrating “knowledge” of a secret known to be associated with her rather than revealing the secret itself to Bob during the protocol

Use a Nonce* or some other time varying quantity as a challenge

Use knowledge of the “secret” and the nonce in the response

Oscar, who is monitoring the communications medium gains no useful information

* Nonce: a number or bit string used only once, in security engineering

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Nonce A nonce is a quantity that is not used for the same

purpose more than once Examples:

Sequence numbers Time stamps Random numbers Concatenation of a combination of these

Typically serves to prevent otherwise undetectable replay

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Challenge-Response Protocol Based on Shared Secret Keys

Alice BobShared secret key kAB

Generate a challenge xx

Compute y = fkAB(x)

y

Compute y* = fkAB(x)

Compare y* and y

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Example Examples of functions

DES encryption Computing xkAB mod p

Used in smart cards and pass-code generators

Secret Key kAB= 7; p = 17

Alice BobChallenge = 3

Compute 37 mod 17 = 11

Response = 11

Check response

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Assumptions of previous protocol Alice and Bob share a secret key The authentication protocol is unilateral The claim of identity is presumably

completed earlier to the C-R protocol Possibly in cleartext

Modified version of this protocol is specified in the ISO/IEC 9798-2 standard

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Session Hijacking Alice identifies herself to Bob using a C-R protocol After the C-R protocol, Oscar may interject himself

by spoofing Alice’s address This is called session hijacking

How do we prevent session hijacking? A secret key MUST be exchanged as part of the

identification/authentication The secret key can be used to prevent spurious

messages from being sent with the same address (how?)

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The “Key” Problems Two communicating parties must share a secret

key The keys should change frequently to prevent

Oscar from getting too much of information about it The more ciphertext Oscar can have, the better

the attack The more often you use the key, the better the

attack Solution: use a hierarchy of keys

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Authenticated Key Establishment (AKE) Establish a secret key with an entity whose

identity has been verified Also called Authentication and Key Agreement (AKA)

in some specific protocols Used in many applications

Dial-up systems Kerberos 802.11i (WLANs) Cellular telephony

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Key Establishment & Management Key Establishment

A “secure” process by which a shared secret key becomes available to two or more parties for use later on for encryption, authentication, etc.

Result of the protocol is the creation of a shared secret “session key”

The session key is restricted for use for only a short time after which it is obliterated

Key Management Set of processes and mechanisms which support key

establishment and maintenance of ongoing “key” relationships such as replacing older keys, updating keys, storing keys, the roles of trusted third parties, etc.

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Key Establishment Key Distribution or Transport

One party chooses the secret key The secret key is securely

transported to the other parties Key Agreement

Two or more parties jointly establish a secret key by communicating over a public channel

e.g., Diffie-Hellman Key Exchange

Sometimes we make use of a Trusted Authority (TA), a Trusted Third party (TTP) or a Key Distribution Center (KDC)

KeyEstablishment

KeyDistribution

KeyAgreement

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Key Distribution Using Secret Keys Session Keys

Used to encrypt communication between two end systems

Used only for the duration of the logical connection (or for a fixed duration)

Transported to communicating entities using a “master key”

Master Key The key used for transporting session keys It is considered to be a long-term key Shared between communicating end-systems Usually it is physically delivered or manually

installed

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Why Session Keys? Limiting the availability of ciphertext

The more the ciphertext, the more feasible the attack Limiting exposure

If the key is compromised, only the data that has used the particular session key is compromised

Avoiding long term storage of a number of keys If Alice needs to communicate with N possible users,

she will use a session key only when the need arises Independence across communication sessions

and applications Reduces the need to maintain state across sessions

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Decentralized Key Distribution

Alice Bob

(1) Req||N1

(2) ekAB(ks||Req||IDBob||f(N1)||N2)

(3) eks(f(N2))

• kAB is the Master Key• ks is the Session Key

Challenge

Response

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Drawbacks A master key needs to be shared between all

the nodes that need to communicate If there are N hosts we need N(N – 1)/2 keys If we add a node to the network, all the other

nodes must now create a shared master key with it

Physical distribution of pairs of master keys is hard

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Trusted Third Party Use a Trusted Third Party (TTP) often called a Key

Distribution Center (KDC) This is a server based key distribution

Each node or user shares a secret master key with the KDC

A session key is generated by the KDC each time two nodes wish to communicate If Alice or Bob generate the session key, we call this a “Key

Translation Center” or KTC Can also be used for distributing public keys and

associated certificates - PKI later Drawbacks:

The TTP must be trusted to keep the master keys secret The TTP may be a bottleneck for providing the session keys

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Key Exchange Using Public Keys Why public keys?

We do not wish to trust a third party Communications are created between entities that

do not know each other a priori Example: Diffie-Hellman Key Exchange Protocol RSA for key exchange

Problem: Man-in-the-Middle Attack

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Key Exchange Using Public Keys

Alice Bob

(1) Request

(2) IDB|| KUB

(3) eKUB(IDA||ks)

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Man-In-The-Middle Attack

Alice Bob

Oscar

(1) Request

(1) Request (2) Request

(3) IDB||KUB(4) IDB||KUO

(5) eKUO(ks||IDA) (6) eKUB(ks||IDA)

y = eks(x)

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Man-In-The-Middle Attack (cont.) Transmitting a session key by encrypting it with a

public key is secure against “passive attacks” It is NOT secure against ACTIVE attacks Oscar can read and alter x without detection Public keys are not authenticated (so far)

Applies also to the Diffie-Hellman Key Exchange Protocol

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Distribution of Public Keys Public Announcement

No authentication Easy to masquerade Example: PGP

Publicly available Directory A TTP maintains an authenticated directory of names and

associated public keys Each user registers his/her public key with the directory

authority Keys can be updated Directories can be published periodically Authenticated communication is possible via MACs to access

the directory electronically Records may be tampered Accessing the directory server could be a bottleneck

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Using a Local Public Key Authority

Alice Bob

PK Auth(1) Req||Time1

(2) eKRAuth(KUB||Req||Time1)

(3) eKUb(IDA||N1)

(6) eKUa(N1||N2)

(4) Req||Time2

(5) eKRAuth(KUA||Req||Time2)

(7) eKUb(N2)

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Using Public-Key Authority Provides stronger encryption The first four messages can be used

infrequently to check if the public keys have changed

Records might be manipulated at source (need third party trust)

Bottleneck at the Authority

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Public Key Certificates Idea:

Bind the user’s identity to his public key via his SSN, name, etc.

Have a trusted authority to “certify” the binding Keep everything autonomous

Anyone should be able to read the certificate Anyone should be able to verify the authenticity

and currency of the certificate No one should be able to create a certificate

except the trusted authority How?

Use Digital Signatures

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Requirements Anyone should be able to read a certificate

to determine the identity and public key of the user

The certificate must be tamperproof Only the Certificate Authority (CA) can

change or update a certificate The certificate should have a verifiable

lifetime

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General Structure The User needs

Identity: SSN, DOB, Name, E-mail address, URL etc.

Private Key KRU

Public Key KUU

The certificate authority needs A secret signature algorithm sigKRAuth(x) A public verification algorithm verKUAuth(y)sigKRauth

(IDU||KUU)

KU(User)

ID(User)

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Public Key Distribution Using Certificates

Alice Bob

CA

(1) IDA||KUA

(2) C(A) = sigKRAuth(T1||IDA||KUA)

(3) C(A)

(6) C(B)

(4)IDB||KUB

(5) C(B) = ?

Alice can decrypt C(B)Bob can decrypt C(A)

Oscar cannot generate a certificatecontaining IDA||KUo or IDB||KUo

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Advantages There is no serious bottleneck since

certificates rest with the users and are “public” They can be downloaded and kept offline before

an actual communication They cannot be forged and can be placed in

public directories Hierarchical certification and directories can be

used Certificates can be used for the validity of the

lifetime

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Certificate Example

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Public Key Infrastructure (PKI) Components that are necessary to securely

distribute public keys Ideally consists of

Certificates A repository for retrieving certificates A method for revoking certificates A method of evaluating a chain of certificates from

public keys that are known and trusted in advance of the target public key

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Message Confidentiality/Privacy Protection of transmitted data from

unauthorized access Interception & release of information

Clearly, the solution is encryption If the data is encrypted (say using a block cipher

in an appropriate mode of operation) the contents are quite secure

Traffic analysis Frequency of packets and dependence on time Source and destination networks Much harder to prevent

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Traffic Confidentiality Attack

Identification of communicating parties Frequency of communication Message pattern (length, quantity, etc.) Event correlation

Security measures Link encryption Traffic padding Pad data units to be of fixed size Insert null messages

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Message Authentication Authentication

Assurance that a message is coming from an entity that supposedly sent it

Protection against masquerade or fraud Integrity

Assurance that the message has not been modified Contents – insertion, deletion, transposition, etc. Sequence – insertion, deletion, reordering Timing – delay or replay

Message Authentication = Authentication + Integrity

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Message Authentication How do we know whether or not a message is

coming from the “claimed” source? How do we know that the message has not been

modified in between? There must be an “authenticator” to verify the

authenticity of the message Message encryption Hash functions Message authentication code

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Secret Key Based Encryption for Message Authentication

Alice and Bob share a key k Nobody else is aware of the key k

If a message is received by Bob that can be decrypted using the key k, the message MUST have originated at Alice

Encrypt DecryptInsecure channelAlice Bobyx x

k k

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Drawbacks of Simple Encryption for Message Authentication If the ciphertext y can be anything (e.g. a

block of 64 bits that look random), Oscar can send spurious or meaningless messages to Bob

Bob cannot automatically say whether Alice sent the meaningless messages Need some structure in the plaintext that can be

used to determine spurious messages The structure MUST be secure

Oscar can “replay” the messages sent by Alice without being detected We look at this later

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General Idea of Using a “Function” for Message Authentication Generate a function or fingerprint of the

message Store it securely if the data is in an insecure place Transmit it securely if the data is transmitted over

an insecure channel If the data gets altered

Hopefully the altered data will NOT have the same fingerprint as the original data

If the fingerprint is secure, we can detect the modification

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A Simple Method for Securing the Fingerprint Append it to the message Encrypt the message and the appended function A random sequence of bits will not have the

properties that the above ciphertext has Advantages

Using layered communications protocols automatically creates a form of authentication because of the structure

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General Idea for Message Authentication

Alicex x

x

f(x)

e

Y = ek(x || f(x))

k

Bob

d

kf(x)

x || f(x)

f

Compare

f

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How to Generate auth(x) ? Use Hash Functions

Takes as input a binary string of arbitrary length Creates as output a fingerprint of this string The fingerprint is also called “message digest”

Typically a very short string Important in the use of digital signatures

Use Message Authentication Codes (MAC) or Keyed Hash Functions The hash function is dependent on a shared secret

key between Alice and Bob No need for securely keeping the fingerprint Also called an “authentication tag”

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Message Authentication without Privacy In some applications, it is only necessary to

authenticate but not keep the information secret Broadcast messages and alarm signals Load on receiving side Plaintext messages like shareware etc. SNMPv3 and network management messages

Since the plaintext is sent without encryption, there is a need to now add a secure authenticator to the message The function auth(x) should be dependent on the message It should not be easily created given the message It should not be easily modified given the message Computational security

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Example 1

Alice

Bob

x x

Ck(x)

Compare

x || Ck(x)

x

Ck(x)

CC

k

k

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Example 2

Alice

Bob

x xe d

Ck(x)

Compare

y= ek1(x || Ck(x))

x || Ck(x)

x

Ck(x)

k1k1

CC

k

k

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Example 3

Alice

Bob

x x

ek(h(x))

Compare

x || ek(h(x))

x

eh

k

h kd

ek(h(x))

h(x)

Similar to a MACHash function is cryptographically protected

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Example 4

Alice

Bob

x x

h(s||x)

Compare

x || (h(s||x))

x

h hs

Alice and Bob share a secret sSimilar to HMACHash function must be one way to prevent s being discovered

h(s||x)

s

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Example 5

Alice

Bob

x xe d

h(x)

Compare

y = ek(x || h(x))

x || h(x)

x

h(x)

k k

hh

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Example 6

e d

y = ek(x || h(x||s))

k k

Alice

Bob

x x

h(s||x)

x || (h(x||s))

x

h hs

h(s||x)

s

Compare