applied cryptography - protocols, algorithms, and source code in c

Applied cryptography protocols algorithms and source code in c

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Unformatted text preview: h other’s public keys, creating an interconnected community of PGP users. For example, Alice might physically give her public key to Bob. Bob knows Alice, so he signs her public key. He then gives the signed key back to her and keeps a copy for himself. When Alice wants to communicate with Carol, Alice sends Carol a copy of the key Bob signed. Carol, who already has Bob’s public key (she got it at some other time) and trusts Bob to certify other people’s keys, verifies his signature on Alice’s key and accepts it as valid. Bob has introduced Alice to Carol. PGP does not specify a policy for establishing trust; users are free to decide who they trust and who they do not. PGP provides mechanisms for associating trust with public keys and for using trust. Each user keeps a collection of signed public keys in a file called a public-key ring. Each key in the ring has a key legitimacy field that indicates the degree to which the particular user trusts the validity of the key. The higher the trust level, the more the user believes the key is legitimate. A signature trust field measures how far the user trusts the signer to certify the public keys of other users. And finally, an owner trust field indicates the degree to which the particular user trusts the key’s owner to sign other public keys; this field is set manually by the user. PGP continuously updates these fields as users supply new information. Figure 24.7 shows how this model might look for a particular user, Alice. Alice’s key is at the top, and the owner trust value is ultimate trust. Alice has signed Bob’s, Carol’s, Dave’s, Ellen’s, and Frank’s keys. She trusts Bob and Carol to sign other people’s public keys, and she partially trusts Dave and Ellen to sign other people’s public keys. And she trusts Gail to sign other people’s public keys, even though she has not signed Gail’s key herself. Two partially trusted signatures may be sufficient to certify a key. Alice believes that Kurt’s key is legitimate because both Dave and Ellen have signed it. This is not automatic in PGP; Alice c...
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This note was uploaded on 10/18/2010 for the course MATH CS 301 taught by Professor Aliulger during the Fall '10 term at Koç University.

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