1. Key Escrow System “like leaving your key with a neighbour in case of an emergency” 10-11-2009 SSIN – MIEIC Micael Fernando Fonseca Oliveira

2. Sumary Key Escrow System (KES) Escrow third party KES advantages KES disadvantages Clipper Chip Clipper System Example Clipper System Vulnerability Recovery system and session keys 2

3. Key Escrow System (KES) A data security measure in which a cryptographic key is entrusted to a third party and are released under certain situation. Ensure that there is a backup of the cryptographic key in case the parties with access to key lose the data. 3

4. Escrow third party Businesses who may want access to employees' private communications. Governments, who may wish to be able to view the contents of encrypted communications. 4

5. KES advantages Ensure that there is a backup of the cryptographic key in case the parties with access to key lose the data through a disaster or malicious intent. 5

6. KES disadvantages New Vulnerabilities & Risks New Complexities New Costs 6

7. Clipper Chip (1) The Clipper chip is a chipset that was developed and promoted by the U.S. Government as an encryption device to be adopted by telecommunications companies for voice transmission. 7

8. Clipper Chip (2) Designed by the NSA Includes a classified encryption algorithm - SKIPJACK Voice encryption chip (for phones) - Clipper Key-escrow system - key is split - half of key held by NIST, half of key held by Treasury Department Manufactured by Mykotronx 8

9. Clipper Chip Message F = Family key (common to all Clipper Chips) - 80 bits N = serial Number of chip - 32 bits K = Key specific to particular conversation - 80 bits U = secret key for chip - 80 bits M = the Message 9

10. Clipper System Example (1) Let’s say that Alice, using a telephone containing a Clipper chip, wants to talk to Bob, who has a similar device. Alice’s chip has unique ID ID A and secret key K A 10

11. Clipper System Example (2) What key will Alice and Bob use for communicating? Alice and Bob use Diffie-Hellman mechanism to produce a shared key K. The chip use K to encrypt and decrypt the data. 11

12. Clipper System Example (3) How does the government know the ID A in order to obtain K A ? How would the government, knowing K A, be able to decrypt the conversation? The information the government needs is in a field known as the LEAF (Law Enforcment Access Field) 12

13. Clipper System Example (4) The government: use F to decrypt outer layer of LEAF revealing ID A and K encrypted by K A obtain escrowed key halves for chip with serial number ID A put key halves together (with XOR) to reveal K A use K A to decrypt K use K to decrypt M (the message) 13

14. Clipper System Vulnerability In 1994, Matt Blaze pointed out that Clipper’s escrow system has a vulnerability. To prevent the software that transmitted the message from tampering with the LEAF, a 16-bit hash was included. A brute force attack would produce another LEAF value that would give the same hash. 14

15. Recovery system and session keys Is it possible to use key-recovery systems to recover session keys? 15

16. References http://www.cdt.org/crypto/risks98/ http://lorrie.cranor.org/pubs/crypt1.html http://en.wikipedia.org/wiki/Clipper_chip http://en.wikipedia.org/wiki/Key_escrow http://www.yourdictionary.com/hacker/key-escrow Kaufman, C., Network Security Private communication on a public world, second edition, 2002. 16