ClaimChain

Privacy-preserving decentralized public key distribution based on cross-referencing hash chains

Paper

ClaimChain: Improving the Security and Privacy of In-band Key Distribution for Messaging
Bogdan Kulynych, Wouter Lueks, Marios Isaakidis, George Danezis, and Carmela Troncoso.
2018 Workshop on Privacy in the Electronic Society (WPES'18)

Code

Claimchain prototype implementation in Python
Reproducible simulations on the Enron email dataset
ClaimChain plugin for Muacrypt
Verified implementations of underlying data structures

Talks

Modern key distribution with ClaimChain at 34C3

Each user or device maintains repositories of claims regarding their own key material, and their beliefs about public keys and, generally, state of other users of the system. High integrity of the repositories is maintained by virtue of storing claims on authenticated data structures, namely hash chains and Merkle trees, and their authenticity and non-repudiation by the use of digital signatures.

Cross-referencing of hash chains is a way to efficiently and verifiably vouch about states of other users. This allows to detect chain compromises, manifested as forks of hash chains, and to implement various social policies for deriving decisions about the latest state of users in the system.

The claims about keys of other people can reveal the social graph. To solve this, we use cryptographic verifiable random functions to derive private identifiers that are re-randomized on each chain update. This allows to openly and verifiably publish claims that can only be read by the authorized users, ensuring privacy of the social graph. Moreover, the specific construction of Merkle trees, along with the usage of verifiable random functions, ensures users can not equivocate about the state of other people.

Claimchain is flexible with respect to deployment options, supporting fully decentralized deployments, as well as centralized, federated, and hybrid modes of operation.

Citing ClaimChain

@article{claimchain,
  title={ {ClaimChain}: Improving the Security and Privacy
         of In-band Key Distribution for Messaging},
  author={Kulynych, Bogdan and Lueks, Wouter and Isaakidis,
          Marios and Danezis, George and Troncoso, Carmela},
  journal={arXiv preprint arXiv:1707.06279},
  year={2017}
}