The Self-Sovereign Identity (SSI) paradigm is instrumental for decentralised identity management, allowing an entity to create, manage, and present their digital credentials without relying on centralised authorities. Credential selective disclosure is one of the most attractive privacy-preserving features of SSI, allowing users to reveal only the minimum necessary information from their credentials. However, current selective disclosure mechanisms primarily focus on protecting the privacy of credential Holders, while offering limited protection to the Verifiers of credentials. Indeed, the specific credential information requested by a Verifier can inadvertently reveal to credential Holders sensitive information, including internal decision-making criteria, business rules, or strategic plans. In this work, we address this threat by proposing, to the best of our knowledge, the first approach that enforces mutual privacy in credential exchanges. To this end, we introduce COD-ssi (Claim Oblivious Disclosure for SSI), a novel framework that leverages Oblivious Pseudorandom Functions to allow Verifiers to selectively access a subset of claims without revealing which specific claims were accessed to the credential Holder. The security of our solution is formally verified and its feasibility is assessed through the experimental evaluation of our open-source prototype implementation. These results show that provable mutual privacy in the context of SSI can be achieved with just moderate computational and communication overhead.

翻译：自主主权身份（SSI）范式对于去中心化身份管理至关重要，它允许实体无需依赖中心化权威即可创建、管理和出示其数字凭证。凭证选择性披露是SSI最具吸引力的隐私保护特性之一，允许用户仅从凭证中披露最必要的信息。然而，当前的选择性披露机制主要侧重于保护凭证持有人的隐私，而对凭证验证者提供的保护有限。实际上，验证者所请求的特定凭证信息可能会无意中向凭证持有人泄露敏感信息，包括内部决策标准、业务规则或战略计划。在本工作中，我们通过提出（据我们所知）首个在凭证交换中强制实施相互隐私的方法来应对这一威胁。为此，我们引入了COD-ssi（SSI的声明不经意披露），这是一个新颖的框架，利用不经意伪随机函数允许验证者选择性地访问一组声明，而无需向凭证持有人透露具体访问了哪些声明。我们解决方案的安全性经过了形式化验证，并通过对我们开源原型实现的实验评估评估了其可行性。结果表明，在SSI背景下，只需适度的计算和通信开销即可实现可证明的相互隐私。