INFSCI 1075: Network Security – Spring 2013 Amir Masoumzadeh

1. 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. Bob Alice How Most Security Protocols Work? 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

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

17. Bob Alice Challenge = 3 Response = 11 Example • Examples of functions • DES encryption • Computing xkAB mod p • Used in smart cards and pass-code generators • Secret Key kAB= 7; p = 17 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 9798-2 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 • 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.

23. Key Establishment Key Distribution Key Agreement 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)

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. Bob Alice (1) Req||N1 (2) ekAB(ks||Req||IDBob||f(N1)||N2) (3) eks(f(N2)) Decentralized Key Distribution Challenge 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. Bob Alice (1) Request (2) IDB|| KUB (3) eKUB(IDA||ks) Key Exchange Using Public Keys

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

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

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 • 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) ID(User) KU(User) sigKRauth (IDU||KUU)

39. Bob Alice (1) IDA||KUA (4)IDB||KUB CA (5) C(B) = ? (2) C(A) = sigKRAuth(T1||IDA||KUA) (3) C(A) (6) C(B) Public Key Distribution Using Certificates 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 • 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 • 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 Insecure channel Alice Bob Encrypt Decrypt y x x k k

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