Bishop Chapter 10 Cont Key Management Storage

Bishop: Chapter 10 (Cont. ) Key Management: Storage & Revoking csci 5233 Computer Security 1

Topics • Key exchange – – – • Cryptographic key infrastructure – • Certificates Key storage – – • Session vs interchange keys Classical vs public key methods Key generation Key escrow Key revocation Digital signatures csci 5233 Computer Security 2

Storing Keys • Multi-user or networked systems: attackers may defeat access control mechanisms – Encipher the file containing keys • Attacker can monitor keystrokes to decipher files • Key will be resident in memory that attacker may be able to read – Use physical devices like “smart card” • Key never enters system • Card can be stolen, so have 2 devices combine bits to make a single key csci 5233 Computer Security 3

Key Escrow • Key escrow system allows authorized third party to recover key – Useful when keys belong to roles, such as system operator, rather than individuals – Business: recovery of backup keys – Law enforcement: recovery of keys that authorized parties require access to • Goal: provide this without weakening cryptosystem • Very controversial. Why? csci 5233 Computer Security 4

Desirable Properties 1. 2. 3. 4. 5. Escrow system should not depend on encipherment algorithm. That is, no inter-dependence. Privacy protection mechanisms must work from end to end and be part of user interface. That is, only authorized parties with the escrowed keys can access the messages. Requirements must map to key exchange protocol. That is, a person cannot bypass the key escrow system. System supporting key escrow must require all parties to authenticate themselves. That is, only authorized parties may use the escrowed keys. If message to be observable for limited time, key escrow system must ensure keys valid for that period of time only. violated by the Clipper project csci 5233 Computer Security 5

Components • User security component – Does the encripherment, decipherment – Supports the key escrow component • Key escrow component – Manages storage, use of data recovery keys • Data recovery component – Does key recovery csci 5233 Computer Security 6

Example: ESS, Clipper Chip • Escrow Encryption Standard – Set of interlocking components – Designed to balance need for law enforcement access to enciphered traffic with citizens’ right to privacy • Clipper chip prepares per-message escrow information – Each chip numbered uniquely by UID – Special facility programs chip • Key Escrow Decrypt Processor (KEDP) – Available to agencies authorized to read messages csci 5233 Computer Security 7

User Security Component • UID: Unique ID for Device, unique for each chip • Unique device key: kunique • Nonunique family key: kfamily , an 80 -bit encryption key for the entire family of Clipper chips • Cipher is Skipjack – Classical cipher: 80 bit key, 64 bit input, output blocks • Each piece of enciphered message is accompanied by a law enforcement agents’ field (LEAF): – { UID || { ksession } kunique || hash } kfamily – hash: 16 bit authenticator from session key and initialization vector csci 5233 Computer Security 8

Programming User Components • Done in a secure facility • Two escrow agencies needed – – Agents from each present Each supplies a random seed and key number Family key components combined to get kfamily Key numbers combined to make key component enciphering key kcomp – Random seeds mixed with other data to produce sequence of unique keys kunique • Each chip imprinted with UID, kunique, kfamily csci 5233 Computer Security 9

The Escrow Components • During initialization of user security component, the process creates ku 1 and ku 2 where kunique = ku 1 ku 2 – First escrow agency gets { ku 1 } kcomp – Second escrow agency gets { ku 2 } kcomp csci 5233 Computer Security 10

Obtaining Access • Alice obtains legal authorization to read message • She runs message LEAF through KEDP – LEAF is { UID || { ksession } kunique || hash } kfamily • KEDP uses (known) kfamily to validate LEAF, obtain sending device’s UID • Authorization, LEAF taken to escrow agencies csci 5233 Computer Security 11

Agencies’ Role • Each validates authorization • Each supplies { kui } kcomp and the corresponding key number • KEDP takes these and LEAF: – Key numbers produce kcomp – kcomp produces ku 1 and ku 2 – ku 1 and ku 2 produce kunique – kunique and LEAF produce ksession csci 5233 Computer Security 12

Problems • (minor) hash too short – LEAF 128 bits, so given a hash of 16 bits: • 2112 LEAFs show this as a valid hash • 1 has actual session key, UID • Takes about 42 minutes to generate a LEAF with a valid hash but meaningless session key and UID; in fact, deployed devices would prevent this attack • (major) Scheme does not meet temporal requirement • As kunique fixed for each unit, once message is read, any future messages can be read. csci 5233 Computer Security 13

Yaksha Security System • Key escrow system meeting all 5 criteria • Based on RSA, central server – Central server (Yaksha server) generates session key • Each user has 2 private keys – Alice’s modulus n. A, public key e. A – First private key d. AA known only to Alice – Second private key d. AY known only to Yaksha central server – d. AA d. AY = d. A mod n. A (meaning d. AA d. AY mod n. A = d. A mod n. A csci 5233 Computer Security 14

Alice and Bob • Alice wants to send message to Bob 1. Alice asks Yaksha server for session key 2. Yaksha server generates ksession 3. Yaksha server sends Alice the key as: CA = (ksession) d. AY e. A mod n. A 4. Alice computes (CA)d. AA mod n. A = ksession Remember: d. AA d. AY = d. A mod n. A • Questions: – – Who knows d. A? How would Alice determine d. AA without knowing d. AY ? csci 5233 Computer Security 15

Analysis • Authority can read only one message per escrowed key – Meets requirement 5 (temporal one), because “time” interpreted as “session” • Independent of message enciphering key – Meets requirement 1 – Interchange algorithm, keys fixed • Others met by supporting infrastructure csci 5233 Computer Security 16

Alternate Approaches to Escrow • Tie to time [Beth, etc. 1994] – The secret key used to generate the session key is not given as escrow key, but a related key is. – To derive the actual key from the related key, the authority must solve an instance of the discrete log problem. • Tie to probability: Translucent cryptography [Bellare/Rivest 1999] – Oblivious transfer: message received with specified probability – Idea: translucent cryptography allows fraction f of messages to be read by third party – Not key escrow, but similar in spirit csci 5233 Computer Security 17

Key Revocation • Certificates invalidated before expiration – Usually due to compromised key – May be due to change in circumstance (e. g. , someone leaving company) • Problems – Entity revoking certificate authorized to do so – Revocation information circulates to everyone fast enough • Network delays, infrastructure problems may delay information csci 5233 Computer Security 18

CRLs • Certificate revocation lists certificates that are revoked • X. 509: only certificate issuer can revoke certificate – Added to CRL • PGP: signers can revoke signatures; owners can revoke certificates, or allow others to do so – Revocation message placed in PGP packet and signed – Flag marks it as revocation message csci 5233 Computer Security 19

Next Bishop, Chapter 10 (Cont. ): q • Digital Signatures csci 5233 Computer Security 20