Module 4 Network & Application Security: Kerberos – X509 Authentication service – IP security Architecture – Secure socket layer – Electronic mail security – Pretty Good privacy – S/MIME – secure Electronic Transactions – Firewalls - Security mechanisms in JAVA platform – Applet security – Security policy and Security Manager.

Authentication Applications will consider authentication functions developed to support application-level authentication & digital signatures will consider Kerberos – a private-key authentication service then X.509 directory authentication service 2

Kerberos trusted key server system from MIT provides centralised private-key third-party authentication in a distributed network – allows users access to services distributed through out the network – without needing to trust all workstations – rather all trust a central authentication server two versions in use: 4 & 5 3

Kerberos Requirements first published report identified its requirements as: – security – reliability – transparency – scalability 4

Kerberos 4 Overview a basic third-party authentication scheme have an Authentication Server (AS) – users initially negotiate with AS to identify themselves – AS provides a non-corruptible authentication credential (ticket granting ticket TGT) have a Ticket Granting server (TGS) – users subsequently request access to other services from TGS on basis of users TGT 5

A Simple Authentication Dialogue (1) C -> AS : ID C || P C || ID V – C = client – AS = authentication server – ID C = identifier of user on C – P C = password of user on C – ID V = identifier of server V – C asks user for the password – AS checks that user supplied the right password 6

Message 2 (2) AS -> C : Ticket Ticket = E K(V) [ID C || AD C || ID V ] – K(V) = secret encryption key shared by AS and V – AD C = network address of C – Ticket cannot be altered by C or an adversary 7

Message 3 (3) C -> V: ID C || Ticket – Server V decrypts the ticket and checks various fields – AD C in the ticket binds the ticket to the network address of C – However this authentication scheme has problems 8

Problems Each time a user needs to access a different service he/she needs to enter their password – Read several times – Print, mail, or file server – Assume that each ticket can be used only once (otherwise open to replay attacks) Password sent in the clear 9

Authentication Dialogue II Once per user logon session (1) C -> AS: ID C || ID TGS (2) AS -> C: E ( K(C), [Ticket TGS ]) Ticket TGS is equal to – E( K(TGS) [ID C || AD C || ID TGS || TS 1 || Lifetime 1 ]) TGS = Ticket-granting server ID TGS = Identifier of the TGS Ticket TGS = Ticket-granting ticket or TGT K (C) = key derived from user’s password TS 1 = timestamp Lifetime 1 = lifetime for the TGT 10

Messages (3) and (4) Once per type of service (3) C -> TGS: ID C || ID V || Ticket TGS (4) TGS -> C : Ticket V Ticket V is equal to – E K(V) [ ID C || AD C || ID V || TS 2 || Lifetime 2 ] K(V): key shared between V and TGS Is called the service-granting ticket (SGT) 11

Message 5 Once per service session (5) C -> V: ID C || Ticket V C says to V “I am ID C and have a ticket from the TGS”. Let me in! Seems secure, but.. – There are problems 12

Problems Lifetime of the TGT – Short : user is repeatedly asked for their password – Long : open to replay attack – Oscar captures TGT and waits for the user to logoff – Sends message (3) with network address ID C (network address is easy to forge) Same problem with SGT 13

What should we do? A network service (TGS or server) should be able to verify that – person using the ticket is the same as the person that the ticket was issued to – Remedy : use an authenticator Server should also authenticate to user – Otherwise can setup a “fake” server – A “fake” tuition payment server and capture the student’s credit card – Remedy : use a challenge-response protocol 14

Kerberos 4 Overview

Kerberos Realms a Kerberos environment consists of: – a Kerberos server – a number of clients, all registered with server – application servers, sharing keys with server this is termed a realm – typically a single administrative domain if have multiple realms, their Kerberos servers must share keys and trust 16

Kerberos Realms: Request for service in Another Realm

Kerberos Version 5 developed in mid 1990’s provides improvements over v4 – addresses environmental shortcomings encryption algorithm, network protocol, byte order, ticket lifetime, authentication forwarding, inter-realm authentication – and technical deficiencies double encryption, non-standard mode of use, session keys, password attacks specified as Internet standard RFC

Kerberos v5 Dialogue

authentication service exchange. Message (1) is a client request for a ticket- granting ticket. Message (2) returns a ticket-granting ticket, identifying information for the client, and a block encrypted using the encryption key based on the user's password. This block includes the session key to be used between the client and the TGS.

Kerberos v5 Dialogue ticket-granting service message (3) for includes an authenticator, a ticket, and the name of the requested service. includes requested times and options for the ticket and a nonce Message (4) has the same structure as message (2), returning a ticket plus information needed by the client, the latter encrypted with the session key now shared by the client and the TGS.

Kerberos v5 Dialogue client/server authentication exchange, several new features appear in version 5, such as a request for mutual authentication. If required, the server responds with message (6) that includes the timestamp from the authenticator. The flags field included in tickets in version 5 supports expanded functionality compared to that available in version 4

X.509 Authentication Service X.509 is the Internationally accepted standard – to construct a public key certificate, – is becoming widely used. part of CCITT X.500 directory service standards – distributed servers maintaining some info database defines framework for authentication services – directory may store public-key certificates – with public key of user – signed by certification authority also defines authentication protocols uses public-key cryptography & digital signatures – algorithms not standardised, but RSA recommended It is used by S/MIME secure , SSL/TLS secure Internet links (eg for secure web).

Public-Key Certificate Use

X.509 Certificates

issued by a Certification Authority (CA), containing: – version (1, 2, or 3) – serial number (unique within CA) identifying certificate – signature algorithm identifier – issuer X.500 name (CA) – period of validity (from - to dates) – subject X.500 name (name of owner) – subject public-key info (algorithm, parameters, key) – issuer unique identifier (v2+) – subject unique identifier (v2+) – extension fields (v3) – signature (of hash of all fields in certificate) notation CA > denotes certificate for A signed by CA 26

Obtaining a Certificate any user with access to CA can get any certificate from it only the CA can modify a certificate because cannot be forged, certificates can be placed in a public directory 27

CA Hierarchy if both users share a common CA then they are assumed to know its public key otherwise CA's must form a hierarchy use certificates linking members of hierarchy to validate other CA's – each CA has certificates for clients (forward) and parent (backward) each client trusts parents certificates enable verification of any certificate from one CA by users of all other CAs in hierarchy 28

CA Hierarchy Use 29 Track chains of certificates: A acquires B certificate using chain: X >W >V >Y >Z > B acquires A certificate using chain: Z >Y >V >W >X >

Certificate Revocation certificates have a period of validity may need to revoke before expiry 1.user's private key is compromised 2.user is no longer certified by this CA 3.CA's certificate is compromised CA’s maintain list of revoked but not expired certificates issued by CA – the Certificate Revocation List (CRL) users should check the directory each time a certificate is received, with CA’s CRL – To avoid delay user would maintain a local cache of certificates and list of revoked certificates. 30

Authentication Procedures X.509 includes three alternative authentication procedures: – One-Way Authentication for unidirectional messages (like ) – Two-Way Authentication for interactive sessions when timestamps are used – Three-Way Authentication for interactive sessions with no need for timestamps all use public-key signatures 31

One-Way Authentication A single transfer of information from A->B used to establish- – the identity of A and that message is from A – message was intended for B – integrity & originality of message message must include timestamp, nonce, B's identity and is signed by A 32

Nonce a nonce is a parameter that varies with time. – A nonce can be a time stamp, – a visit counter on a Web page, – or a special marker intended to limit or prevent the unauthorized replay or reproduction of a file.

Two-Way Authentication Two messages (A->B, B->A) which also establishes in addition: – the identity of B and that reply is from B – that reply is intended for A – integrity & originality of reply reply includes original nonce from A, also timestamp and nonce from B

Three-Way Authentication 3 messages (A->B, B->A, A->B) which enables above authentication without synchronized clocks has reply from A back to B containing a signed copy of nonce from B means that timestamps need not be checked or relied upon

tA –time stamp rA – nonce

X.509 Version 3 has been recognised that additional information is needed in a certificate – /URL, policy details, usage constraints rather than explicitly naming new fields defined a general extension method extensions consist of: – extension identifier – criticality indicator – extension value 37

Certificate Extensions key and policy information – convey info about subject & issuer keys, plus indicators of certificate policy certificate subject and issuer attributes – support alternative names, in alternative formats for certificate subject and/or issuer certificate path constraints – allow constraints on use of certificates by other CA’s 38