[Paper Review] A new Privacy Preserving and Scalable Revocation Method for Self Sovereign Identity -- The Perfect Revocation Method does not exist yet

Andreas Freitag|arXiv (Cornell University)|Nov 23, 2022

Cryptography and Data Security4 citations

TL;DR

This paper proposes Linked Validity Verifiable Credentials (LVVC), a privacy-preserving and scalable revocation method for Self-Sovereign Identity (SSI) systems. LVVC enables verifiers to validate credentials without contacting the issuer, maintains full privacy through cryptographic techniques, and achieves high scalability with constant-sized credentials, outperforming existing methods in privacy and practical deployment readiness.

ABSTRACT

Digital Identities are playing an essential role in our digital lives. Today, used Digital Identities are based on central architectures. Central Digital Identity providers control and know our data and, thereby, our Identity. Self Sovereign Identities (SSI) are based on a decentralized data storage and data exchange architecture, where the user is in sole control of his data and identity. Most of the issued credentials need the possibility of revocation. For a Central Digital Identity, revocation is easy. In decentral architectures, revocation is more challenging. Revocation can be done with different methods e.g. lists, compressed lists and cryptographic accumulators. A revocation method must be privacy preserving and must scale. This paper gives an overview about the available revocation methods, include a survey to define requirements, assess different revocation groups against the requirements, highlights shortcomings of the methods and introduce a new revocation method called Linked Validity Verifiable Credentials.

Motivation & Objective

• Address the critical challenge of privacy-preserving and scalable revocation in Self-Sovereign Identity (SSI) systems.
• Identify and formalize privacy, scalability, and maturity requirements for SSI revocation from community input.
• Assess existing revocation methods—including list-based, compressed lists, cryptographic accumulators, and credential updates—against these requirements.
• Propose and define a new revocation mechanism, Linked Validity Verifiable Credentials (LVVC), to overcome limitations of current approaches.
• Demonstrate that no existing method fully satisfies the ideal combination of minimal interaction, strong privacy, and high scalability, making LVVC the most viable solution for production SSI systems.

Proposed method

• Design a new revocation mechanism called Linked Validity Verifiable Credentials (LVVC) where each credential includes a cryptographic link to a global, immutable revocation state.
• Use BBS+ signatures to bind the credential to a verifiable, cryptographically secure proof of non-revocation without revealing the credential’s status.
• Store the revocation state in a decentralized, append-only ledger or distributed database, ensuring auditability and immutability.
• Enable verifiers to validate a credential’s revocation status using a zero-knowledge proof that confirms non-revocation without revealing any additional information.
• Ensure the size of the credential remains constant (under 1100 bytes for JSON-based BBS+ signatures), enabling high scalability even at scale of 1 million credentials.
• Decouple the revocation status check from the issuer, eliminating the need for real-time issuer interaction during verification, thus enhancing privacy and performance.

Experimental results

Research questions

• RQ1 What are the essential privacy, scalability, and maturity requirements for a revocation method in Self-Sovereign Identity systems, as derived from community practice?
• RQ2 How do existing revocation methods—list-based, compressed lists, cryptographic accumulators, and credential updates—perform against these requirements?
• RQ3 What are the key limitations of current cryptographic accumulator-based revocation in terms of scalability and real-world deployment?
• RQ4 Can a new revocation mechanism be designed that achieves full privacy, minimal interaction, and high scalability without relying on issuer-side state or real-time queries?
• RQ5 To what extent does the proposed LVVC method outperform existing approaches in privacy, scalability, and maturity for SSI deployment?

Key findings

• Cryptographic accumulators, while offering full privacy, suffer from poor scalability: an RSA-based accumulator with 32,768 credentials required an 8.4 MB witness file and 7 seconds of proof generation time on an iPhone 12.
• List-based and compressed list methods offer high maturity and scalability but fail to provide strong privacy, as they enable correlation between verifiers and holders through repeated queries.
• The LVVC method maintains constant-sized credentials (under 1100 bytes) and enables verification without issuer interaction, achieving high scalability even at scale of 1 million credentials.
• LVVC provides full privacy by eliminating the need for verifiers to contact the issuer or reveal credential usage patterns, unlike list-based methods.
• The method is rated as having high maturity due to its reliance on established standards like JSON and JSON Web Token, and its compatibility with BBS+ signatures.
• No existing revocation method combines minimal interaction, full privacy, and high scalability; LVVC is identified as the most suitable for real-world SSI deployment due to its balanced performance across all key dimensions.