# What is a Decentralized Identifier (DID)?

DIDs are identifiers that enable verifiable, decentralized digital identities. A DID refers to any subject (e.g. a person, group, organization, abstract entity, etc.) as determined by the controller of the DID. Specifically, DIDs are [URIs](https://www.rfc-editor.org/rfc/rfc3986) (unique digital addresses) that associate a DID subject with a DID document allowing trustable interactions associated with that subject. Similar to a URL, when you resolve a DID, you receive a DID document in JSON, JSON-LD, or similar format.

## A simple DID example

A DID is a simple unique text string consisting of three parts:

1. the `did` URI scheme identifier;
2. the identifier for the DID method; and
3. the DID method-specific identifier.

The example DID above resolves to a DID document.

## Multiple DIDs

A subject may have as many DIDs as they want for different use cases, such as:

- a DID for educational certificates; or
- a DID for verified identity / KYC documents; or
- a DID for online gaming accounts.

# What is a DID Document?

A DID document contains information associated with the DID, such as ways to cryptographically authenticate a DID controller.

```
{
  "@context": [
    "https://www.w3.org/ns/did/v1",
    "https://w3id.org/security/suites/ed25519-2020/v1"
  ],
  "id": "did:example:123456789abcdefghi",
  "authentication": [{
    "id": "did:example:123456789abcdefghi#keys-1",
    "type": "Ed25519VerificationKey2020",
    "controller": "did:example:123456789abcdefghi",
    "publicKeyMultibase": "zH3C2AVvLMv6gmMNam3uVAjZpfkcJCwDwnZn6z3wXmqPV"
  }]
}
```

# DID Methods

A DID method (and its spec) defines the precise operations by which DIDs and DID documents are created, resolved, updated, and deactivated. There are currently more than [150](https://www.w3.org/TR/did-spec-registries/#did-methods) DID methods out there, which can be categorized into 4 main types based on their **storage method**:

- Centralized: relies on a Web2 server
- Blockchain-based: the “original” DID method involving a blockchain
- Hybrid: uses decentralized storage to store its documents
- Static: no data registry is required; can be created and resolved by encoding/decoding data directly.

Terminal 3 currently provides a custom `did:t3n` DID.

|  | Centralized | Blockchain-based | Hybrid | Static |
| --- | --- | --- | --- | --- |
| Privacy | low | high | high | medium |
| Self-Sovereign? | no | yes | yes | yes |
| Data Registry Required? | yes | yes | yes | no |
| (web2 server) | yes |  |  |  |
| (Blockchain) |  | yes | yes |  |
| (L2 storage) |  |  | yes |  |
| Decentralized? | no | yes | yes | yes |
| Complexity | simple | complex | medium | simple |
| Transaction Fee Required? | no | yes | yes | no |
| (when updating) |  |  | yes |  |
| Mutable? | yes | no | no | no |
| Supported Operations | create<br>read<br>update<br>delete | create<br>read<br>update<br>delete | create<br>read<br>update<br>delete | create<br>read |
| Example | did:web | did:ethr<br>did:btcr<br>did:dock | did:3<br>did:ion<br>did:elem | did:key<br>did:pkh |

# DID System Architecture

### Generic DID Architecture