Abstract
We study the notion of anonymous credentials with Publicly Auditable Privacy Revocation (PAPR). PAPR credentials simultaneously provide conditional user privacy and auditable privacy revocation. The first property implies that users keep their identity private when authenticating unless and until an appointed authority requests to revoke this privacy, retroactively. The second property enforces that auditors can verify whether or not this authority has revoked privacy from an issued credential (i.e. learned the identity of the user who owns that credential), holding the authority accountable. In other words, the second property enriches conditionally anonymous credential systems with transparency by design, effectively discouraging such systems from being used for mass surveillance. In this work, we introduce the notion of a PAPR anonymous credential scheme, formalize it as an ideal functionality, and present constructions that are provably secure under standard assumptions in the Universal Composability framework. The core tool in our PAPR construction is a mechanism for randomly selecting an anonymous committee which users secret share their identity information towards, while hiding the identities of the committee members from the authority. As a consequence, in order to initiate the revocation process for a given credential, the authority is forced to post a request on a public bulletin board used as a broadcast channel to contact the anonymous committee that holds the keys needed to decrypt the identity connected to the credential. This mechanism makes the user de-anonymization publicly auditable.
J. Brorsson—This work was supported by the Wallenberg AI, Autonomous Systems and Software Program (WASP) funded by the Knut and Alice Wallenberg Foundation.
B. David—This work was supported by the Concordium Foundation and by the Independent Research Fund Denmark (IRFD) grants number 9040-00399B (\(\hbox {TrA}^{2}\hbox {C}\)), 9131-00075B (PUMA) and 0165-00079B.
L. Gentile—This work was supported by the Concordium Foundation.
E. Pagnin and P. S. Wagner—This work was supported by was supported by the Swedish Foundation for Strategic Research, grant RIT17-0035.
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Notes
- 1.
While many anonymous credential schemes strive to provide unlinkability among different showings, we restrict ourselves to the simpler case where different showings of the same credential can be linked in order to focus on our new PAPR techniques.
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A Heuristics for Efficient Subtitutions of Functionalities
A Heuristics for Efficient Subtitutions of Functionalities
To instantiate \(\mathcal {\prod }_{PC}\) efficiently without Universal Composability, the ideal functionalities \(\mathcal {F}_{BB}\), \(\mathcal {F}_{PKI}\), \(\mathcal {F}_{ZK}\) and \(\mathcal {F}_{NIZK}\) may be substituted respectively by a blockchain such as Ethereum (note that \(\mathcal {F}_{BB}\) may also be implemented starting from consensus protocols such as those in [3, 4, 26, 28, 29, 42, 45, 56]), a PKI with key transparency such as CONIKS [48], Schnorr proofs over the Tor network and Groth-Sahai proofs [40]. We stress that the security of these substitutions would be heuristic. If formally proven secure, the resulting scheme would at best be proven sequentially composable, due to the nature of Groth-Sahai proofs.
In such a system where \(\mathcal {F}_{NIZK}\) is substituted for Groth-Sahai proofs, the conditions and in \(\textsc {zk}_{esc}\) (Fig. 4) can be realized as the verification equations of a pairing-based \({\textsf {PVSS}}\) scheme, e.g. [21].
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Brorsson, J., David, B., Gentile, L., Pagnin, E., Wagner, P.S. (2023). PAPR: Publicly Auditable Privacy Revocation for Anonymous Credentials. In: Rosulek, M. (eds) Topics in Cryptology – CT-RSA 2023. CT-RSA 2023. Lecture Notes in Computer Science, vol 13871. Springer, Cham. https://doi.org/10.1007/978-3-031-30872-7_7
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