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Trust Analysis of PGP based on Mean Shortest Distance School of Engineering 2001814 Kyusuk Han

Contents n 1. Introduction n 2. What is PGP? n 3. Web of trust n 4. Difference to Hierarchical trust structure n 5. Trust analysis n 6. source code n 7. Sample result n 8. Conclusion n 9. Further works n 10. Reference 2

What is PGP? n What is PGP? PGP - Pretty Good Privacy n Developed by Phil Zimmerman in 1991 n From v 2. 0, well-known algorithms was begun to be used. 3

Web of Trust § Assume Alice signed Bob, Bob signed Charlie, and Charlie signed Alice. They can reach any one. If Alice cannot reach Charlie directly, Alice can reach him via Bob. It's strong set § There are many small strong set in whole PGP users. With exchanging keys between these small strong set, strong set will be combined and be bigger. After all, strong set will be one. (In ideal assumption) 4

Web of Trust 5

Difference between Hierarchical trust and web of trust § Root CA can selfcertificate, and ultimately trusted. § Alice and Bob are under the CAs. § bottom one trust upper CA. 6

Difference between Hierarchical trust and web of trust § The most difference between Hierarchical trust and web of trust is existence of Root CA. In Hierarchical trust, A users can ultimately trust himself and Root CA. § In hierarchical trust structure, CA manage user's key information. In PGP, users manage their key ring in their personal computer. 7

Trust analysis § To measure each key's mean shortest distance. Sometimes user can reach others in 1 or 2 times, but also 20 or 30 times. § Mean shortest distance, MSD to signee is more important than MSD from signer, because of the possibility like any user sign all key in the world. § Lowest MSD means the user are in the centre of web of trust. § But more than anything else, every decision is up to users themselves. Even though Phil Zimmerman, the inventor of PGP, is not trustful than user himself. 8

Example: Finding Mean Shortest Distance of A When A can reach B, and B can reach A, there is 'strong set'. 'hop' means unit distance to contact beside one. To contact C : 1 hop To contact B : 2 hop To contact D : 2 hop To contact E : 3 hop To contact F : 4 hop To contact G : 3 hop Total distance : 15 MSD : 15/6 = 2. 5 9

Source Code § pre-process. keys raw data of public keys and signed keys. It can be gathered by export_keys. sh, precess_keys. pl. (They are shell script, and perl script. I used the scrpit made by Drew Streib. ) 10