1. INFSCI 1075: Network Security – Spring 2013 Amir Masoumzadeh
Introduction to Protocols: Entity Authentication, Key Establishment, Integrity/Message Authentication, Confidentiality
INFSCI 1075: Network Security – Spring 2013
Amir Masoumzadeh

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

3. How Most Security Protocols Work?
Step 1: Alice verifies her “identity” to Bob and vice-versa
Alice
Bob
Step 2: Alice and Bob establish a shared “secret” (or a set of secrets)
Step 3: Data communications with confidentiality and authentication

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. Storing Passwords Obvious approach “Encrypted” password files
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

11. 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

12. 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

13. 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

14. 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

15. 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

16. Challenge-Response Protocol Based on Shared Secret Keys
Alice
Bob
Shared secret key kAB
Generate a challenge x x
Compute y = fkAB(x) y
Compute y* = fkAB(x)
Compare y* and y

17. Example Examples of functions Secret Key kAB= 7; p = 17 Alice Bob
DES encryption
Computing xkAB mod p
Used in smart cards and pass-code generators
Secret Key kAB= 7; p = 17
Alice
Bob
Challenge = 3
Response = 11
Compute
37 mod 17 = 11
Check response

18. 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 standard

19. 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?)

20. 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

21. 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

22. Key Establishment & Management
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.

23. Key Establishment Key Distribution or Transport Key Agreement
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)
Key
Establishment
Distribution
Agreement

24. 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

25. 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

26. Decentralized Key Distribution
(1) Req||N1
Alice
Bob
(2) ekAB(ks||Req||IDBob||f(N1)||N2)
Challenge
(3) eks(f(N2))
Response
kAB is the Master Key
ks is the Session Key

27. 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

28. 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

29. 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

30. Key Exchange Using Public Keys
(1) Request
Alice
Bob
(2) IDB|| KUB
(3) eKUB(IDA||ks)

31. Man-In-The-Middle Attack
(1) Request
Alice
Bob
(1) Request
(2) Request
(4) IDB||KUO
(3) IDB||KUB
(5) eKUO(ks||IDA)
y = eks(x)
(6) eKUB(ks||IDA)
Oscar

32. 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

33. 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

34. Using a Local Public Key Authority
PK Auth
(4) Req||Time2
(1) Req||Time1
(5) eKRAuth(KUA||Req||Time2)
(2) eKRAuth(KUB||Req||Time1)
Alice
Bob
(3) eKUb(IDA||N1)
(6) eKUa(N1||N2)
(7) eKUb(N2)

35. 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

36. 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

37. 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

38. General Structure The User needs The certificate authority needs
Identity: SSN, DOB, Name, 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)
ID(User)
KU(User)
sigKRauth
(IDU||KUU)

39. Public Key Distribution Using Certificates
(4)IDB||KUB
(1) IDA||KUA
(5) C(B) = ?
(2) C(A) = sigKRAuth(T1||IDA||KUA)
Alice
Bob
(3) C(A)
(6) C(B)
Alice can decrypt C(B)
Bob can decrypt C(A)
Oscar cannot generate a certificate
containing IDA||KUo or IDB||KUo

40. 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

41. Certificate Example

42. 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

43. 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

44. 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

45. Message 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

46. 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

47. Secret Key Based Encryption for Message Authentication
Insecure channel
Alice
Bob
Encrypt
Decrypt x y x k k
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

48. 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

49. 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

50. 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

51. General Idea for Message Authentication
Alice x e
Y = ek(x || f(x)) x
f(x)
Bob d k
f(x)
x || f(x) f x
Compare f k

52. 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”

53. 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

54. Example 1
Alice x x x
x || Ck(x)
Ck(x) k
Ck(x) C C
Bob k
Compare

55. Example 2 y= ek1(x || Ck(x)) Alice x e d x x Bob Compare k C C k

56. Example 3 Alice x x x Bob Compare x || ek(h(x)) h k d h e k ek(h(x))
Similar to a MAC
Hash function is cryptographically protected

57. Example 4 Alice x x x Bob Compare x || (h(s||x)) s h h s
Alice and Bob share a secret s
Similar to HMAC
Hash function must be one way to prevent s being discovered

58. Example 5 y = ek(x || h(x)) Alice x e d x x Bob Compare h h x || h(x)

59. Example 6 y = ek(x || h(x||s)) Alice x e d x x Bob Compare s h h s
h(s||x) h h
h(s||x) s
Bob
x || (h(x||s))
Compare