Self-Study | Mastering Self-Sovereign Identity — Core of SSI Architecture (2/2), DID

Concept

1. Definition of DID

A string that identifies a resource — a globally unique identifier
https://www.w3.org/TR/did-core

2. Comparison with the WWW's URI 

URI: every web identifier - a string in a specific format that makes a string globally unique on the WWW
URL: network location - a URI that can be used on the Web to find the location of a representation of that resource
URN: persistent name

DID is a URI that can be either a URL or a URN, and that can be resolved to obtain a standardized set of information (metadata) about the resource identified by the DID. It depends on cryptography (cryptographic verification and decentralization).

3. DID's 4 core properties.

1) Permanent (permanent, persistent) — the identifier doesn't need to be changed.
2) Resolvable identifier — can be looked up to find metadata.
3) Cryptographically verifiable identifier — control can be proven using cryptography.
     - used to generate the DID
     - since a DID is associated with exactly one public key/private key pair, controller of the private key = controller of the DID, which is provable
4) Decentralized identifier — no centralized registration authority is required.
     - a centralized registry decides whether a particular identifier is unique and only registers it if it is unique

4. DID format

1) The DID Syntax ABNF Rules

did                = "did : method-name : method-specific-id" 
method-name        = 1*method-char
method-char        = %x61-7A / DIGIT
method-specific-id = *( *idchar ":" ) 1*idchar
idchar             = ALPHA / DIGIT / "." / "-" / "_" / pct-encoded
pct-encoded        = "%" HEXDIG HEXDIG

(1) Schema, (2) Method — based on blockchain or other distributed ledgers; on creation or update, a transaction is written to that ledger; choose a technology suitable for the specific use case from various implementation technologies, (3) sub-string identifier of the method (always guaranteed to be unique within the did method namespace, and recommended to be globally unique as well)

*Adopters' evaluation criteria for how well requirements are met [ rubric  https://w3c.github.io/did-rubric ]

2) The DID URL Syntax ABNF Rules

did-url = did path-abempty [ "?" query ] [ "#" fragment ]

3) Comparison of WWW and SSI url rules 

Comparison 01: in services running on the WWW, the port address is usually omitted. In the case of DID, the method-specific-id, which sits in a position similar to the port, is not omitted.  
Comparison 02: when multiple parameters are used in www, they are joined with & , but in DID they are joined with ?  
Comparison 03: W3C maintains recommendations for parameter naming used in parameters. While considering extensibility, to maximize interoperability they operate a recommended specification. Through this, when the same DID parameter is used even when it operates with a different semantic system, they prevent mutual conflict. 

How it works

1. DID resolver resolver

- Looks up a standardized data structure called a DID document
- Just as a DNS resolver (a protocol for resolving a domain name url to an IP address) receives a host name (www.example.com) from a web browser or other application and tracks down the IP address for that host name, a DID resolver refers to a service or server that provides a DID document for a DID
*HTTP — a protocol for retrieving a resource representation url from a web server
- Rather than thinking of it as a concrete protocol, you should regard it as an abstract function or algorithm that takes a DID (along with additional parameters) as input and returns a DID document (along with additional metadata) as a result.

2. Sample DID document

{
	"@context": "[<https://www.w3.org/ns/did/vl>](<https://www.w3.org/ns/did/vl>)",
	"id": "did:example:123456789abcdefghi",
	"authentication":[{
		"id": "did:example:123456789abcdefghi#keys-1", 
		"type": "Ed25519VerificationKey2018",
		"controller": "did:example:123456789abcdefghi", 
		"publicKeyBase58" : "H3C2AVvLMv6gmMNam3uVAjZpfkcJCwDwnZn6z3wXmqPV"
	}],
	"service": [{
		"id": "did:example:123456789abcdefghi#vcs", 
		"type": "VerifiableCredentialService",
		"serviceEndpoint": "[<https://example.com/vc/>](<https://example.com/vc/>)"
	}]
}

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