email or call for office visit. ECE-6612 http://www.csc.gatech.edu/copeland/jac/6612/ Prof. John A. Copeland john.copeland@ece.gatech.edu 404 894-5177 fax 404 894-0035 Office: Klaus Bldg 3362 email or call for office visit. Chapter 4a - Kerberos

Kerberos, v4 and v5 Provides a complete protocol for authentication and secure communications for hosts connected by a data communications network • Provides secure "tickets" to hosts that can be used to initiate a secure message exchange • Standard message formats for encrypted and signed messages, or signed plaintext messages • Formats for encoding expiration time, names, ... • Allows "read-only" slave KDC's (distributed KDCs) Wikipedia: “KDC” or http://www.zeroshell.org/kerberos/Kerberos-operation/ 2

Keberos uses Mediated Authentication ) Jack Bob Alice (with a Key Distribution Center, KDC ) Jack Bob Alice Kbob Kalice Mary Tom KDC Paul Dick Peter Jip Trudi Harry KDC has unique Secret Keys with all legitimate hosts. 3

After the 1st exchange with the KDC, Alice has a Bob has (human) Alice PC Key {Ka,{TGT;Kk}; logs on hashes Distribut. Shared Kak} to Alice's Ctr., KDC Secret Key Alice, password Alice wants (PC) to get a Bob,{TGT;Kk}, with KDC, generates Kab Ka, has Kak, Kbob DES Key, {time;Ka} Kbob Kalice>Kak (1) {Bob,Kab,Ticket -Bob; Ka} {time; Kab}, {Kab,Alice; Kbob} ="Ticket" {time + 1, Kab} After the 1st exchange with the KDC, Alice has a session key, Ka, and a "Ticket-Granting Ticket" that she can use to request "Tickets" from KDC (1) PC erases Alice's password and Kak from RAM (keeps session key Ka in RAM). No keys ever stored on disk (what about virtual memory?). (2)Time(stamp) is used as nonce (seconds after 1/1/1970) 4

The Keys of Kerberos 1. Password, Kalice - Only Alice knows it. Alice’s PC can hash what Alice types in Kak = Hash(Kalice) . KDC - also knows the hash, Kak. Uses Kak as key for encrypting TGT to Alice (not used after that). TGT contains the daily session key Ka for use with the Ticket Granting Server, TGS. Ka: Session Key (KDC gave to Alice) Now Alice’s PC, as well as TGS know it. Kab: Key for Alice & Bob - temporarily (daily) assigned by TGS Given to Alice by TGS - encrypted with Ka and also encrypted with Kb (Kkdc-Bob) inside a contact ticket. (Kb is Bob's daily session key from KDC) Alice gives Bob the Ticket from KDC which has it (Kab) encrypted with Kb (gotten by Bob from the TGS). 4. Kk: Key known only by KDC and TGS. Used to sign Ticket-Granting-Ticket for verification. Alice can only replay {TGT;Kk} back to TGS to get a contact ticket. 5

KDC 6

Slave KDC Slave KDC Master KDC Slave KDC Slave KDC Slave Realm KDC Version 5 Slave Host KDC Host Slave Host Host KDC Host Master Host KDC {db;Kmaster} Slave Host KDC Host Host Slave Host KDC Slave Host Realm KDC • Replicated KDCs (slaves) are read only. • Entire Host-KDC database is downloaded periodically 7

Alice wants to talk to Dorothy KDC (Hatter) KDC Lion (Lion) 1 2 Alice Dorothy 3 Realm Lion can also be a Realm "principal" in Wonderland Oz Wonderland (with the Queen's OK) Alice wants to talk to Dorothy 8

Cipher Block Chaining ( P PCBC) Plaintext Cipher Block Chaining ( P PCBC) m1 m2 m3 IV (+) (+) (+) E E E Key c1 c2 c3 The 1st 64-bit message segment is XOR'ed with an initial vector (IV). Each following message segment is XOR'ed with the preceding ciphertext and plaintext segments-for privacy & integrity . 9

Kerberos Message Integrity Check (Message Digest) MIC is Hash(<Ksession,message>) The Hash algorithm was never published (but source code can be obtained) It is based on a checksum algorithm designed by Juneman to use mod 2^31-1 (prime), but changed to use 2^63-1 (not prime). Cryptographers worry that it might be breakable, or reversible (to get Ksession). 10

Network Layer (IP) Addresses in Tickets Only 4 bytes available, so limited to Internet Protocol (Novel, IBM, Appletalk, IPv6... longer) Makes "spoofing" harder, IP address must be stolen from network as well as Ticket from Alice. Prevents delegation, giving the ticket to another host to represent you (which is allowed by Kerberos V5) 11

Why Study Kerberos v4 (Why doesn't everyone switch to v5) Kerberos V4 is working well in many systems Switching to V5 requires stopping the network and upgrading every host at once before restart Kerberos V5 is inefficient in some ways compared to V4 • Specified in ASN.1 (abstraction good and bad) • Example: 11 bytes required for 4-byte IP address. 12

Kerberos v5 Cryptographic Algorithms Kerberos v4 used Plaintext Cipher Block Chaining and modified Juneman hash Kerberos v5 can use a variety of encryptions (DES in practice) and hashes (MD4, MD5). Primary MIC (message integrity check) uses • { confounder + MD5(confounder & message)}K' • K' = Kalice-bob (+) F0F0F0F0F0F0F0F0 A more modern MIC that is not used is • MD5(Kalice-bob & message) 13

Password security Originally UNIX stored a hash of each User’s password in a globally readable account. This can be attacked by hashing all common words for a reverse lookup table. • Do not send in clear except over short secure channels (avoid using Telnet, FTP, http (for passwords), …) • Choose had to guess passwords, enforce. • Force changing passwords periodically Avoid keeping password in memory longer than necessary to generate the user's key. • • Send hash of (key+nonce) to KDC for authentication • Add salt before hashing passwords for pw database • Add realm name to password before hashing for pw db 14

Message Security and Integrity Only exchange messages with authenticated hosts Develop a session key and separate MIC key using initial password exchange Encrypt Diffie-Hellman exchanges to prevent Bucket Brigade (man-in-middle) attacks. Use MICs, especially with self-synchronizing encryptions that survive permuting message blocks (e.g., ECB) . Get "random" numbers from true sources Protect Master KDC Key and hashed-key database 15

Concepts Used in Kerberos Central Key Server (KDC) - n rather than n*(n-1)/2 sets of keys. Could enforce “Connection Policy.” Distributed KDCs (Slave KDCs) to prevent “Denial of Service” (DoS) Attack. Use of password hashes, for verifying password without storing password. “Dictionary Attack” - use of “salt” to improve security. Message hashes for “Message Integrity Check” (MIC). Authentication exchange - “nonce” to prevent “Replay Attack”. Standard block encryption algorithm (DES) with unique “cipher feedback.” Session keys to reduce exposure of primary keys. Version 4 to 5 upgrade difficult. Newer systems (SSL, PGP, SSH) negotiate to find the best common algorithms available to both. 16