Course Introduction

Introduction to Protocols: Entity Authentication, Key Establishment, Integrity/Message Authentication, Confidentiality INFSCI 1075: Network Security Spring 2013 Amir Masoumzadeh Overview 2 Authentication Weak entity authentication Strong entity authentication Authenticated Key Establishment Key establishment and management Public Key Infrastructure Message Confidentiality/Privacy Message Authentication How Most Security Protocols Work?

Step 1: Alice verifies her identity to Bob and vice-versa Alice 3 Step 2: Alice and Bob establish a shared secret (or a set of secrets) Step 3: Data communications with confidentiality and authentication Bob Authentication Schemes Some typical scenarios requiring electronic proof of identity Withdrawing money from an automatic teller machine

Charging purchases to a credit card over telephones 4 Needs the credit-card number and expiration date Remote login over a computer network Needs a PIN number Needs a login name and password Authenticated key establishment protocols 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 5 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 It is real-time Basis of access control & user accountability Also called identity verification Has two steps Only a claim that This is me

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 6 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

7 Oscar has successfully participated in an authentication scheme with Alice and/or Bob Oscar can simultaneously start multiple instances of the entity authentication protocol Means of User Authentication What you know What you have Something the party to be authenticated possesses: Smart card with a time variant password What you are or you do

Something known by the party to be authenticated: PIN, Password, etc. Something inherent to the party to be authenticated: Biometrics like fingerprints, voice, retinal patterns, etc. Comments: 8 Authentication can be mutual or one way It should be computationally efficient 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 9 The user supplies the password to the system (reveals the password!) The system accepts this as a corroboration of the users identity 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 10 Attacks against Fixed Passwords Replay of fixed passwords Exhaustive password search People write down passwords If transmitted in plaintext (like telnet), Oscar can capture it on the link 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

11 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 Preventive Measures Password rules Require users to have special characters, capital letters, etc. in their password Entropy = uncertainty in password

Make the password verification process slow Try to ensure that all passwords are equally likely Makes attacks more difficult Verifying a few passwords is easy Comparing millions of passwords may be very time consuming Use pass phrases 12 Increases the entropy without reducing human ability to remember Passphrases are stored as hash values and NOT truncated 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 13 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 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 14

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 15 Challenge-Response Protocol Based on Shared Secret Keys Alice

Shared secret key kAB Bob Generate a challenge x x Compute y = fkAB(x) y Compute y* = fkAB(x) Compare y* and y 16 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 Compute 37 mod 17 = 11 17 Challenge = 3 Bob Response = 11 Check response 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 18 Session Hijacking Alice identifies herself to Bob using a C-R protocol After the C-R protocol, Oscar may interject himself by spoofing Alices 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?) 19 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 20 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 21 Dial-up systems Kerberos 802.11i (WLANs) Cellular telephony 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 22 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. Key Establishment Key Distribution or Transport Key Agreement

One party chooses the secret key The secret key is securely transported to the other parties 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) 23 Key Establishment Key Distribution Key Agreement

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 24 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 Why Session Keys? Limiting the availability of ciphertext 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 The more the ciphertext, the more feasible the attack

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 25 Reduces the need to maintain state across sessions Decentralized Key Distribution (1) Req||N1 Bob Alice (2) ekAB(ks||Req||IDBob||f(N1)||N2) Challeng e (3) eks(f(N2)) Respons e 26 kAB is the Master Key ks is the Session Key

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 27 Trusted Third Party Use a Trusted Third Party (TTP) often called a Key Distribution Center (KDC)

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 This is a server based key distribution 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: 28 The TTP must be trusted to keep the master keys secret The TTP may be a bottleneck for providing the session keys

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 29 Key Exchange Using Public Keys (1) Request Alice

(2) IDB|| KUB (3) eKUB(IDA||ks) 30 Bob Man-In-The-Middle Attack (1) Request Bob Alice (1) Request (2) Request (3) IDB||KUB (4) IDB||KUO (5) eKUO(ks||IDA) (6) eKUB(ks||IDA) Oscar y = eks(x)

31 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 32 Distribution of Public Keys Public Announcement

No authentication Easy to masquerade Example: PGP Publicly available Directory 33 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 Using a Local Public Key Authority (4) Req||Time2 (1) Req||Time1 PK Auth (5) eKRAuth(KUA||Req||Time2) Alice (2) eKRAuth(KUB||Req||Time1) (3) eKUb(IDA||N1) (6) eKUa(N1||N2) (7) eKUb(N2) 34 Bob 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 35 Public Key Certificates Idea: Keep everything autonomous

Bind the users identity to his public key via his SSN, name, etc. Have a trusted authority to certify the binding 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? 36 Use Digital Signatures 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

37 General Structure The User needs Identity: SSN, DOB, Name, E-mail address, URL etc. Private Key KRU Public Key KUU The certificate authority needs ID(User) KU(User)

A secret signature algorithm sigKRAuth(x) A public verification algorithm verKUAuth(y)sig KRauth (IDU||KUU) 38 Public Key Distribution Using Certificates (4)IDB||KUB (1) IDA||KUA CA Alice (5) C(B) = ? (2) C(A) = sigKRAuth(T1||IDA||KUA) Bob (3) C(A) (6) C(B)

Alice can decrypt C(B) Bob can decrypt C(A) 39 Oscar cannot generate a certificate containing IDA||KUo or IDB||KUo Advantages There is no serious bottleneck since certificates rest with the users and are public They cannot be forged and can be placed in public directories They can be downloaded and kept offline before an actual communication Hierarchical certification and directories can be used

Certificates can be used for the validity of the lifetime 40 Certificate Example 41 Public Key Infrastructure (PKI) Components that are necessary to securely distribute public keys Ideally consists of 42 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 Message Confidentiality/Privacy Protection of transmitted data from unauthorized access Clearly, the solution is encryption Interception & release of information If the data is encrypted (say using a block cipher in an appropriate mode of operation) the contents are quite secure Traffic analysis

43 Frequency of packets and dependence on time Source and destination networks Much harder to prevent Traffic Confidentiality Attack Identification of communicating parties Frequency of communication Message pattern (length, quantity, etc.) Event correlation Security measures

44 Link encryption Traffic padding Pad data units to be of fixed size Insert null messages Message Authentication Authentication Integrity Assurance that a message is coming from an entity that supposedly sent it Protection against masquerade or fraud 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 45 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 46 Message encryption Hash functions Message authentication code

Secret Key Based Encryption for Message Authentication Alice x Encrypt Insecure channel y k k Alice and Bob share a key k Decrypt Bob x 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 47 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 48 We look at this later 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

49 Hopefully the altered data will NOT have the same fingerprint as the original data If the fingerprint is secure, we can detect the modification 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 50 Using layered communications protocols automatically creates a form of authentication because of the structure

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

51 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

52 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 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 53 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 Example 1 Alice x x x x || Ck(x) Ck(x) C

Ck(x) k C Bob k Compare 54 Example 2 y= ek1(x || Ck(x)) x e d Alice x Ck(x) k1 C

k1 Ck(x) k x C Bob k Compare x || Ck(x) 55 Example 3 Alice x x x || ek(h(x)) h ek(h(x))

h(x) e k ek(h(x)) d h Bob k Similar to a MAC Hash function is cryptographically protected 56 x Compare Example 4 Alice x

x x || (h(s||x)) h(s||x) s h x s h(s||x) h Bob Compare Alice and Bob share a secret s Similar to HMAC Hash function must be one way to prevent s being discovered 57 Example 5 y = ek(x || h(x))

Alice x h(x) x e d k k h(x) h x h Bob Compare x || h(x) 58 Example 6

y = ek(x || h(x||s)) Alice x e h(s||x) s k h x d s k h(s||x) h x || (h(x||s))

Compare 59 x Bob

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