Key Escrow System“like leaving your key with a neighbour in case of an emergency”

10-11-2009SSIN – MIEICMicael Fernando Fonseca Oliveira

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

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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.

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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.

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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.

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KES disadvantages

• New Vulnerabilities & Risks

• New Complexities

• New Costs

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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.

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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

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

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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 IDA and secret key KA

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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.

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Clipper System Example (3)

• How does the government know the IDA in order to obtain KA?

• How would the government, knowing KA, be able to decrypt the conversation?

• The information the government needs is in a field known as the LEAF (Law Enforcment Access Field)

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Clipper System Example (4)

• The government:• use F to decrypt outer layer of LEAF revealing IDA and K

encrypted by KA

• obtain escrowed key halves for chip with serial number IDA

• put key halves together (with XOR) to reveal KA

• use KA to decrypt K

• use K to decrypt M (the message)

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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.

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Recovery system and session keys

• Is it possible to use key-recovery systems to recover session keys?

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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.

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