Abstract

did:btcr2 is a censorship-resistant Decentralized Identifier (DID) method using the Bitcoin blockchain as a Verifiable Data Registry to announce changes to the DID document. It supports zero-cost off-chain DID creation; aggregated updates for scalable on-chain update costs; long-term identifiers that can support frequent updates; private communication of the DID document; private DID resolution; and non-repudiation.

Introduction and Motivation

Public digital identity was introduced to the Internet through Pretty Good Privacy’s (PGP) foundational legal work in the 1990s. With Decentralized Identifiers (DIDs), digital identity can be preserved through a rotation of key material, without relying on a centralized party. This specification defines did:btcr2, a DID method designed and motivated by the following considerations:

Anti-censorship

Our primary motivation is to leverage the anti-censorship capability of Bitcoin to enable individuals to control decentralized identifiers without interference by centralized players. That means creating a W3C conformant DID method capable of creating identifiers usable for a range of verifiable interactions, including issuing Verifiable Credentials and proving that the current presenter is the subject of a Verifiable Credential. This DID method gives organizations and states the ability to anchor private attestations to identifiers under individuals’ direct control, giving humans an unprecedented ability to participate in digital interactions on their own terms, using the identifiers they choose, when they choose.

Privacy

The first DID method anchoring on the Bitcoin blockchain, did:btcr, focused on censorship resistance. However, self-sovereign identity is not complete without options for privacy as well, and the initial promise of privacy in the DID ecosystem was dubious, with heavy reliance on public DID documents. There has been a presumption that anyone who has the DID itself should be able to resolve it to get the current DID document. This presumption places more information in the public eye than is strictly necessary for securing interactions related to the DID.

To tackle reliance on public DID documents head-on, this DID method introduces private DID documents, in which resolving the DID requires additional information delivered either alongside or in a separate communication from the DID controller. We call this transfer Sidecar delivery. Our approach ensures that successful resolution ALWAYS results in the same DID document for all parties, so that every party in the ecosystem sees the same cryptographically anchored result.

Late Publishing

Many DID methods lack verifiable provenance: it is simply impossible to reconstruct a cryptographically verifiable history of updates. Instead, they use a Verifiable Data Registry that can rewrite history. Bitcoin’s blockchain is the premiere global network for immutably anchoring information to a specific time. This DID method ensures that every BTCR2 DID is anchored on a single, canonical, immutable history for the lifetime of the DID. It is simply not possible for anyone to modify the history of the DID. We call this feature Non-Repudiation.

Offline Creation

Finally, we wanted to support the creation of DIDs without requiring on-chain interactions. BTCR creation depended on getting a transaction on-chain. BTCR2 supports creating DIDs with sophisticated DID documents without registering them on-chain. In fact, all BTCR2 DIDs start with offline creation. Only updates to the DID document require on-chain interactions. Our approach supports both key-based deterministic DIDs and document-based deterministic DIDs. In key-based DIDs, the DID itself contains everything you need to bootstrap an initial DID Document with usable verification methods suitable for authentication, attestation, and capability invocation and delegation. For document-based DIDs, additional Sidecar Data - a Genesis Document - must be used to create the Initial DID document. Offline creation allows unlimited DID creation and use without requiring any on-chain or online interactions, making it suitable for a wide range of high-volume, low cost use cases.