Revocation Bitmap

A revocation bitmap is a space-efficient data structure that enables the issuer of credentials to signal which specific credentials within a batch have been revoked. It operates by assigning each credential a unique index number. The bitmap itself is a simple string of bits (0s and 1s), where each bit's position corresponds to a credential index. A 0 typically indicates the credential is valid, while a 1 indicates it has been revoked. This allows a verifier to check the status of a single credential by looking up only its specific bit position, a process known as a selective disclosure of revocation status.

The primary technical advantage of a revocation bitmap is its constant-size, on-chain footprint. Unlike a traditional revocation list that grows linearly with each revoked item, a bitmap's size is fixed at creation based on the total number of credentials issued in that batch. This makes it highly scalable and cost-effective for blockchain systems, where storing data is expensive. Common implementations, such as those defined in the W3C Verifiable Credentials Data Model, often encode the bitmap as a base64 string for compact storage and transmission.

In practice, when a user presents a credential for verification, they provide their credential index. The verifier then fetches the current revocation bitmap from its published source (e.g., a smart contract or a decentralized identifier document) and checks the bit at that index. This mechanism provides a privacy-preserving check, as the verifier does not learn about the revocation status of any other credentials in the set. It is a foundational component for decentralized identity systems, enabling efficient and scalable credential lifecycle management without relying on centralized, queryable databases.

A revocation bitmap is a compact, on-chain data structure that uses a simple binary mapping to indicate whether a specific credential, like a Verifiable Credential (VC), has been revoked. Each bit in the bitmap corresponds to a unique credential index, where a 1 (or 0, depending on the implementation's convention) signals revocation and a 0 signals the credential is still valid. This mechanism allows verifiers to check the revocation status of a credential by querying a single bit at a known position, without revealing which specific credential is being checked, thereby preserving privacy. Its core innovation is replacing large, privacy-leaking revocation lists with a tiny, fixed-size bit array.

The system's efficiency stems from its bitwise operations. When a credential issuer needs to revoke a credential, they calculate its unique index (e.g., from a credential's credentialStatus field) and publish a transaction that sets the corresponding bit in the on-chain bitmap. A verifier performs the same index calculation and reads that single bit. This operation is extremely gas-efficient on blockchains like Ethereum, as it involves minimal on-chain data storage and computation. Common standards implementing this pattern include the Iden3 Reverse Hash Service and the W3C Status List 2021 specification, which defines a bitstring status list.

A critical feature of revocation bitmaps is their support for selective disclosure and privacy. When a user presents a zero-knowledge proof (ZKP) of credential validity, they can prove that their credential's specific index bit is set to 'valid' without revealing the index itself. This prevents verifiers from linking different presentations back to the same user or credential. The bitmap itself is often stored in a compressed format, such as a base64-encoded string, and can be referenced via a URI in a credential's credentialStatus object, enabling decentralized and interoperable status checks across different ecosystems.

A revocation bitmap is a compact, on-chain data structure, often implemented as a bit array, where each bit represents the active or revoked status of a corresponding credential, token, or permission. The core mechanism is simple: a 0 (or 1) typically indicates the item is valid, while a 1 (or 0) flags it as revoked. This binary representation allows for extremely efficient storage and verification, as checking a single bit's state is a constant-time operation. In blockchain systems like those using Verifiable Credentials (VCs) or specific token standards, this provides a scalable alternative to maintaining individual revocation lists for each issued item.

Visualizing this structure reveals its efficiency. Imagine a long string of bits, such as 00101101, where each position corresponds to a unique credential ID. A verifier, knowing the specific index of the credential they are checking, queries this public bitmap. If the bit at index 3 is set to 1, the credential is revoked. This model is central to standards like W3C's Status List 2021, which encodes a gzip-compressed bitmap into a JSON Web Token (JWT) or other formats, enabling the entire status registry for thousands of credentials to be stored in a single, small on-chain transaction or a decentralized identifier (DID) document.

The practical implementation involves two key processes: bitmap management and proof generation. An issuer, acting as the bitmap controller, updates the specific bit when a credential must be revoked. The verifier's task is to cryptographically verify that the presented credential's index points to a 0 in the latest, signed version of the bitmap. This often involves checking a Merkle proof or a digital signature on the entire bitmap to ensure its integrity and freshness, preventing reliance on stale or tampered data. This creates a trust-minimized system where revocation status is as verifiable as the credential itself.

Compared to Certificate Revocation Lists (CRLs) or online status protocols, a revocation bitmap offers distinct advantages for decentralized systems. Its space efficiency is paramount, as the size grows linearly with the number of possible revocations, not the number of actual revocations. Furthermore, its privacy-preserving nature allows a user to prove their credential is valid by revealing only a single bit position, without disclosing the status of any other credentials in the registry. This makes it a foundational primitive for scalable Self-Sovereign Identity (SSI) and selective disclosure frameworks on blockchain.