CSCE 815 Network Security Lecture 10 KerberosX.509 February 13, 2003

– 2 – CSCE 815 Sp 03 Version 4 Authentication Dialogue Authentication Service Exchange, To obtain Ticket-Granting Ticket (1) C AS: ID c || ID tgs ||TS 1 (2) AS C: E Kc [K c,tgs || ID tgs || TS 2 || Lifetime 2 || Ticket tgs ] Ticket-Granting Service Exchange: To obtain Service-Granting Ticket (3) C TGS: ID v ||Ticket tgs ||Authenticator c TGS C: EK c [Kc,¨v|| ID v || TS 4 || Ticket v ] Client/Server Authentication Exchange: To Obtain Service (5) C V: Ticketv || Authenticatorc (6) V C: EK c,v [TS 5 +1]

– 3 – CSCE 815 Sp 03 Kerberos 4 Overview Fig 4.1

– 4 – CSCE 815 Sp 03 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

– 5 – CSCE 815 Sp 03 Request for Service in Another Realm

– 6 – CSCE 815 Sp 03 Kerberos Version 5 developed in mid 1990s provides improvements over v4 addresses environmental shortcomings encryption algorithm – v4 DES based, v5 tags ciphertext by type of encryption IP dependence – v4 requires internet Protocol (IP), v5 general byte order - v5 message defined using Abstract Syntax Notation(ASN.1) and encoded with Basic Encoding Rules (BER) ticket lifetime – 8 bits (255)x five minutes = 21+ hours; v5 start/stop times authentication forwarding: In v5 one server can forward credentials to another e.g., a print server can forward credentials to file server so that a file can be printed interrealm authorization – In v4 n realms n(n-1)/2 relationships and technical deficiencies specified as Internet standard RFC 1510

– 7 – CSCE 815 Sp 03 Kerberos Version 5 V5 solves technical deficiencies of v4 double encryption: tickets provided to clients are encrypted twice (fig 4.1) PCBC encryption: v4 uses propagating block chaining (PCBC) - non-std and vulnerable to ciphertext block interchange attack session keys – a key used by the client to encrypt the AS to the service; however it may be reused to gain access to the service again. V5 allows subsession keys to prevent replays. password attacks – both versions are susceptible to attacks on the password. The message from the AS to the client is encrypted with a key based on the clients password. This can be captured and then attempts to decrypt and figure out the password.

– 8 – CSCE 815 Sp 03 Kerberos Version 5 Message Exchanges Table 4.3 Authentication Service Exchange: to obtain the ticket- granting ticket New additions Realm of the user Options Times: from, till, renewTime Nonce – random value to be repeated in response to insure freshness

– 9 – CSCE 815 Sp 03 Ticket-Granting Service Exchange To obtain a service-granting ticket Both versions include Authenticator a ticket Name of the requested service In addition v5 includes Requested times for the ticket Options for the ticket And a nonce

– 10 – CSCE 815 Sp 03 Client/Server Authentication Exchange To obtain service Both versions include Authenticator Ticket Name of the requested service In addition v5 includes Options for mutual authentication Subkey – clients choice of encryption key (default is K c,v ) Sequence number – used to detect replays Ticket Flags table 4.4

– 11 – CSCE 815 Sp 03 X.509 Authentication Service part of CCITT X.500 directory service standards distributed servers maintaining some user info database defines framework for authentication services directory may store public-key certificates with public key of user signed with private key by certification authority also defines authentication protocols uses public-key cryptogrraphy & digital signatures algorithms not standardised, but RSA recommended Used in a variety of contexts S/MIME chapter 5 IP secuirty chapter 6 SSL/TLS, SET chapter 7

– 12 – CSCE 815 Sp 03 X.509 Certificates issued by a Certification Authority (CA), containing (fig 4.3): version (1, 2, or 3) serial number (unique within CA) identifying certificate signature algorithm identifier and parameters 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

– 13 – CSCE 815 Sp 03 X.509 Certificates

– 14 – CSCE 815 Sp 03 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

– 15 – CSCE 815 Sp 03 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

– 16 – CSCE 815 Sp 03 CA Hierarchy Use

– 17 – CSCE 815 Sp 03 Certificate Revocation certificates have a period of validity may need to revoke before expiry, eg: 1. user's private key is compromised 2. user is no longer certified by this CA 3. CA's certificate is compromised CAs maintain list of revoked certificates the Certificate Revocation List (CRL) users should check certs with CAs CRL

– 18 – CSCE 815 Sp 03 Authentication Procedures X.509 includes three alternative authentication procedures: One-Way Authentication Two-Way Authentication Three-Way Authentication all use public-key signatures

– 19 – CSCE 815 Sp 03 One-Way Authentication 1 message ( 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

– 20 – CSCE 815 Sp 03 Two-Way Authentication 2 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

– 21 – CSCE 815 Sp 03 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 signed copy of nonce from B means that timestamps need not be checked or relied upon

– 22 – CSCE 815 Sp 03 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

– 23 – CSCE 815 Sp 03 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 CAs

– 24 – CSCE 815 Sp 03 Summary have considered: Kerberos trusted key server system X.509 authentication and certificates