# Derived key use from parent key in DID Doc:

(Notes from DIF I&D Working Group meeting on 1 June 2020)

https://docs.google.com/document/d/1gV99R7G2bUlRerxznyd0ucwWPfc1v2F2gQVaTawIPYI/

This document has notes about a discussion during the I&D Working Group meeting. The basic idea is to add a DID parameter to a DID URL that contains a chain code, which will result in derived keys in the DID document.

#### Example DID URL:

did:example:123?derivation-scheme=bip32&derivation-value=...

Syntax of the parameter values?

#### Example DID document:

TODO

{
  "didDocument": {
    "id": "did:example:123",
    ...
    "publicKey": [{
      "id": "#parent",
      "type": "EcdsaSecp256k1VerificationKey2019",
      "controller": "did:example:123",
      "publicKeyJwk": {
        "crv": "secp256k1",
        "x": "NtngWpJUr-rlNNbs0u-Aa8e16OwSJu6UiFf0Rdo1oJ4",
        "y": "qN1jKupJlFsPFc1UkWinqljv4YE0mq_Ickwnjgasvmo",
        "kty": "EC",
        "kid": "WjKgJV7VRw3hmgU6--4v15c0Aewbcvat1BsRFTIqa5Q"
      }
    }]
  },
  "metaData": {
   "query": {
     "derivation-scheme": ...,
     "derivation-value": ...
   }
 } 
}

#### Register DID parameter in the DID Spec Registries

https://w3c.github.io/did-spec-registries/#parameters

#### Hierachical Deterministic Key Generation

From https://github.com/WebOfTrustInfo/rwot6-santabarbara/blob/master/final-documents/DecentralizedAutonomicData.pdf

As previously mentioned, reproduction has to do with the generation of new keys. One way to accomplish this is with a deterministic procedure for generating new public/private keys pairs where the private keys may be reproduced securely from some public information without having to be stored. A hierarchically deterministic (HD) key-generation algorithm does this by using a master or root private key and then generating new key pairs using a deterministic key-derivation algorithm. A derived key is expressed as a branch in a tree of parent/child keys. Each public key includes the path to its location in the tree. The private key for a given public key in the tree can be securely regenerated using the root private key and the key path, also called a chain code. Only one private key, the root, needs to be stored.

The [BIP-32](https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki) specification, for example, uses an indexed path representation for its HD *chain* code, such as, "0/1/2/0".
The BIP-32 algorithm needs a master or root key pair and a chain code for each derived key. Then only the master key pair needs to be saved and only the master private key needs to be kept securely secret. The other private keys can be reproduced on the fly given the key generation algorithm and the chain code. An extended public key would include the chain code in its representation so that the associated private key can be derived by the holder of the master private key any time the extended public key is presented. This is the procedure for hardened keys.

The query part of the DID syntax may be used to represent an HD chain code or HD key path for an HD key that is derived from a root DID. This provides an economical way to specify derived DIDs (DDIDs) that are used to identify DADS. An example follows:

did:dad:Xq5YqaL6L48pf0fu7IUhL0JRaU2_RxFP0AL43wYn148=?chain=0\1\2

This expression above discloses the root public DID as well as the key derivation path or chain via the query part. For the sake of brevity this will be call an extended DID. The actual derived DDID is create by applying the HD algorithm such as:

did:dad:Qt27fThWoNZsa88VrTkep6H-4HA8tr54sHON1vWl6FE=

Thus a database of DDIDs could be indexed by DDID expressions with each value being the extended DID. Looking up the extended DID allows the holder to recreate on the fly the associated private key for the DDID without ever having to store the private key. This might look like the following:

{
    "did:dad:Qt27fThWoNZsa88VrTkep6H-4HA8tr54sHON1vWl6FE=": "did:dad:Xq5YqaL6L48pf0fu7IUhL0JRaU2_RxFP0AL43wYn148=?chain=0\1\2",
   ... 
}

Or given that the same DID method is used throughout:

{
    "Qt27fThWoNZsa88VrTkep6H-4HA8tr54sHON1vWl6FE=": "Xq5YqaL6L48pf0fu7IUhL0JRaU2_RxFP0AL43wYn148=?chain=0\1\2",
   ... 
}

The namespacing of the DID idstring also provides information that could be used to help formulate an HD path to generate a DDID. The following example shows two different DDIDs using the same public key and the same chain code but with a different extended idstring.

did:dad:Xq5YqaL6L48pf0fu7IUhL0JRaU2_RxFP0AL43wYn148=:blue?chain=0/1
did:dad:Xq5YqaL6L48pf0fu7IUhL0JRaU2_RxFP0AL43wYn148=:red?chain=0/1