BNB Attestation Service (BAS) is an attestation standard and infrastructure layer in the BNB ecosystem. It is designed to let users and applications create attestations, which BAS describes as statements with proof that can be used to verify information. BAS supports both on-chain and off-chain attestations, giving developers a way to bring verifiable data into blockchain-based applications.

At a conceptual level, BAS is a framework for structuring, issuing and verifying claims about data. It uses schemas to define the format of an attestation, supports resolvers for validation logic and allows attestations to reference other attestations, so more complex data structures can be built from simpler ones. This makes BAS a general-purpose verification layer rather than a single-purpose application.

BAS also distinguishes between public verifiability and controlled access. On-chain attestations are recorded on-chain and are publicly accessible, while off-chain attestations can be stored in Greenfield so users can manage privacy and access control around the underlying data.

BAS is used to create verifiable records about identity, activity, qualifications, ownership and other forms of data that need to be checked by third parties. The official documentation presents attestations as a standard for verification, vouching, proving, certifying, authenticating and recording events or claims.

Its architecture is built around schemas and attestations. A user or application first registers a schema in the schema registry, then creates attestations against that schema. Where a schema includes a resolver, the BAS contract can call that resolver to validate the attestation before it is recorded. BAS supports revocation as well, so attestations can later be marked invalid by the attestor.

BAS is also intended for use cases where privacy and user-controlled data access matter. The documentation says off-chain attestations can be stored in Greenfield so users can retain ownership of the data while access permissions are handled through blockchain-based mechanisms. BAS describes this as part of a model for user data ownership and contract-controlled access.

The published use cases include licensing and voting. In licensing, BAS is presented as a way to verify applicant identity, qualifications and related information in a more structured and auditable format. In voting, BAS is presented as a way to verify voter eligibility and support systems such as one person one vote or quadratic voting through attestations rather than token balance alone.

The BAS documentation also includes BNB Passport, which uses attestations and proofs to represent identity, participation and reputation across Web2 and Web3 contexts. In that workflow, proofs are generated and verified before being written as attestations that other contracts and applications can use.