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An "A said that B said that C said..." approach |
An "A said that B said that C said..." approach |
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=============================================== |
=============================================== |
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We start the exposition of our system with a simplistic |
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approach based on a Trusted Third Party (TTP), an |
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actor trusted by all participants in the system. |
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To sign a message, the signer contacts the TTP, |
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authenticates themselves, and submits the message |
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to be signed. |
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To verify a message, the verifier contacts the TTP, |
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submitting the message and alleged signer; the TTP |
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tells the verifier whether the signer has submitted |
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this message or not. |
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Clearly, this system isn't very attractive in |
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practice; not only can the TTP declare arbitrary |
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messages signed by arbitrary signers, it can also |
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"change its mind" and claim a message not signed |
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that it claimed to be signed at an earlier time. |
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To make it harder for the TTP to "change its mind," |
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we improve the above system as follows: |
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- The TTP issues certificates on (signer,message) |
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pairs, stating that a given signer submitted |
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a given message. (Normally, a hash of the message |
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is included in the certificate instead of |
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the actual message itself.) |
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- Each certificate includes a cryptographic hash |
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of the previous issued certificate. |
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- At any time, the TTP publishes the hash of the |
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most recently issued certificate. |
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A certificate is valid if it is connected |
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to the most recent certificate through an unbroken |
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chain of hash links. |
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- Key intuition: each signer picks an *envoy*, |
- Key intuition: each signer picks an *envoy*, |
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a service provider. The signer has signed something |
a service provider. The signer has signed something |
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if their envoy says they have signed it. |
if their envoy says they have signed it. |