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README.md
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README.md
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# fuzzymetatag
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Experimental Rust implementation of https://eprint.iacr.org/2021/089 using Ristretto.
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Anonymous messaging systems (and other privacy-preserving applications) often require a mechanism for one party
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to learn that another party has messaged them.
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A tag is a probabilistic cryptographic structure. When constructed for a given FuzzyMetaPublicKey it will pass the FuzzyMetaDetectionKey::test 100% of the time. For other public keys it will pass the test with probability gamma related to the security parameter of the system. This system provides the following security properties:
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Many schemes rely on a bandwidth-intensive "download everything and attempt-decryption" approach. Others rely on a trusted
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3rd party to provide the service.
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* Correctness: Valid tags for a public key for a key pair always validate when tested against the secret key
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* Fuzziness: Invalid matches should produce false positives with probability p related to the security property (γ)
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* Security: An adversarial server with access to Test oracle (i.e. the detection key) is unable to distinguish false positives from true positives. (Detection Ambiguity)
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It would be awesome if we could get an untrusted server to do the work for us without compromising metadata-resistance!
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fuzzymetatag is a probabilistic cryptographic structure to do just that! Specifically it provides the following
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properties:
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* Correctness: Valid tags constructed for a specific public key will always validate when tested using the detection key
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* Fuzziness: Invalid tags will produce false positives with probability _p_ related to the security property (_γ_)
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* Security: An adversarial server with access to the detection key is unable to distinguish false positives from true positives. (Detection Ambiguity)
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## Security (hic sunt dracones)
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This crate provides an experimental implementation of the `FMD2` scheme described in ["Fuzzy Message Detection"](https://eprint.iacr.org/2021/089). Using
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Ristretto as the prime order group.
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This code has not undergone any significant review.
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Further, the properties provided by this system are highly dependent on selecting a good security parameter _γ_ for
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your system. There is no one-size-fits-all approach.
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If _γ_ is too low, then the probability of false positives will be very high.
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If _γ_ is too high, then an adversarial server will be able to link messages to recipients with low probability.
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## Usage
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Generate a key pair:
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let gamma = 3;
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let key = FuzzyMetaTagKeyPair::generate(gamma);
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`key.public_key` can be given to parties who you want to be able to communicate with you over a specific anonymous
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messaging service / privacy-preserving application.
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`key.detection_key` can be given to untrusted _adversarial_ servers.
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`gamma` is security property (_γ_) in the system. For a given gamma, a tag generated for a specific public key will
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validate against a random public key with probability 2^-gamma. The actual value of gamma needed to protect the metadata
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of a given application is specific to that application, the number of parties involved, the number of messages involved etc.
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## Generating Tags
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let tag = public_key.flag()
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This tag can be attached to a message in a metadata resistant system.
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## Verifying Tags
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An adversarial server can test a given tag against a detection key:
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if detection_key.test(tag) {
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// the message attached to this tag *might* be for the party associated with the detection key
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} else {
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// the message attached to this tag is definitely *not* for the party associated with the detection key.
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}
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@ -17,9 +17,10 @@ use std::ops::{Add, Mul, Sub};
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/// it will pass the `FuzzyMetaDetectionKey::test` 100% of the time. For other public keys
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/// it will pass the test with probability `gamma` related to the security parameter of the system.
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/// This system provides the following security properties:
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/// - Correctness: Valid tags for a public key for a key pair always validate when tested against the secret key
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/// - Fuzziness: Invalid matches should produce false positives with probability p related to the security property (γ)
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/// - Security: An adversarial server with access to Test oracle (i.e. the detection key) is unable to distinguish false positives from true positives. (Detection Ambiguity)
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/// * Correctness: Valid tags constructed for a specific public key will always validate when tested using the detection key
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/// * Fuzziness: Invalid tags will produce false positives with probability _p_ related to the security property (_γ_)
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/// * Security: An adversarial server with access to the detection key is unable to distinguish false positives from true positives. (Detection Ambiguity)
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#[derive(Debug)]
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pub struct FuzzyMetaTag {
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u: RistrettoPoint,
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