creating a did:key document

gistfile1.txt

const multicodec = require('multicodec')
const multibase = require('multibase')
const toString = require('uint8arrays/to-string')
const fromString = require('uint8arrays/from-string')

const publicKeyHex = 'f9c36f8964623378bdc068d4bce07ed17c8fa486f9ac0c2613ca3c8c306d7bb61cd36717b8ac5e4fea8ad23dc8d0783c2318ee4ad7a80db6e0026ad0b072a24f'


const didKey = publicHexKeyToDIDKey(publicKeyHex);

console.log(didKey);

publicKeyToXY(publicKeyHex);

function publicHexKeyToDIDKey(publicKeyHex) {
  const publicKey = fromString(publicKeyHex,'base16');
  const publicKeywPrefix = multicodec.addPrefix('p256-pub',publicKey)
  const buf = multibase.encode('base58btc',publicKeywPrefix)
  const bufAsString = toString(buf)
  return `did:key:${bufAsString}`
}

function publicKeyToXY(publicKeyHex) {
   console.log(publicKeyHex.length);
   const xHex = publicKeyHex.slice(0,publicKeyHex.length/2);
   const yHex = publicKeyHex.slice(publicKeyHex.length/2,publicKeyHex.length);
   console.log(publicKeyHex);
   console.log(xHex);
   console.log(yHex);
   xOctet = fromString(xHex,'base16');
   yOctet = fromString(yHex,'base16');
   x = toString(multibase.encode('base64url',xOctet));
   y = toString(multibase.encode('base64url',yOctet));
   console.log(x);
   console.log(y);
}

{
  "@context": "https://w3id.org/did/v1",
  "id": "did:key:zruuPojWkzGPb8sVc42f2YxcTXKUTpAUbdrzVovaTBmGGNyK6cGFaA4Kp7SSLKecrxYz8Sc9d77Rss7rayYt1oFCaNJ",
  "publicKey": [{
    "id": "did:key:zruuPojWkzGPb8sVc42f2YxcTXKUTpAUbdrzVovaTBmGGNyK6cGFaA4Kp7SSLKecrxYz8Sc9d77Rss7rayYt1oFCaNJ#zruuPojWkzGPb8sVc42f2YxcTXKUTpAUbdrzVovaTBmGGNyK6cGFaA4Kp7SSLKecrxYz8Sc9d77Rss7rayYt1oFCaNJ",
    "type": "JsonWebKey2020",
    "controller": "did:key:zruuPojWkzGPb8sVc42f2YxcTXKUTpAUbdrzVovaTBmGGNyK6cGFaA4Kp7SSLKecrxYz8Sc9d77Rss7rayYt1oFCaNJ",
    "publicKeyJwk": {
        "kty": "EC", 
        "crv": "P-256", 
        "x": "u-cNviWRiM3i9wGjUvOB-0XyPpIb5rAwmE8o8jDBte7Y", 
        "y": "uHNNnF7isXk_qitI9yNB4PCMY7krXqA224AJq0LByok8" 
     }
  }],
  "authentication": [ "did:key:zruuPojWkzGPb8sVc42f2YxcTXKUTpAUbdrzVovaTBmGGNyK6cGFaA4Kp7SSLKecrxYz8Sc9d77Rss7rayYt1oFCaNJ#zruuPojWkzGPb8sVc42f2YxcTXKUTpAUbdrzVovaTBmGGNyK6cGFaA4Kp7SSLKecrxYz8Sc9d77Rss7rayYt1oFCaNJ" ],
  "assertionMethod": [ "did:key:zruuPojWkzGPb8sVc42f2YxcTXKUTpAUbdrzVovaTBmGGNyK6cGFaA4Kp7SSLKecrxYz8Sc9d77Rss7rayYt1oFCaNJ#zruuPojWkzGPb8sVc42f2YxcTXKUTpAUbdrzVovaTBmGGNyK6cGFaA4Kp7SSLKecrxYz8Sc9d77Rss7rayYt1oFCaNJ" ],
  "capabilityInvocation": [ "did:key:zruuPojWkzGPb8sVc42f2YxcTXKUTpAUbdrzVovaTBmGGNyK6cGFaA4Kp7SSLKecrxYz8Sc9d77Rss7rayYt1oFCaNJ#zruuPojWkzGPb8sVc42f2YxcTXKUTpAUbdrzVovaTBmGGNyK6cGFaA4Kp7SSLKecrxYz8Sc9d77Rss7rayYt1oFCaNJ" ],
  "keyAgreement": [{
    "id": "did:key:zruuPojWkzGPb8sVc42f2YxcTXKUTpAUbdrzVovaTBmGGNyK6cGFaA4Kp7SSLKecrxYz8Sc9d77Rss7rayYt1oFCaNJ#zruuPojWkzGPb8sVc42f2YxcTXKUTpAUbdrzVovaTBmGGNyK6cGFaA4Kp7SSLKecrxYz8Sc9d77Rss7rayYt1oFCaNJ", //
    "type": "JsonWebKey2020",
    "controller": "did:key:zruuPojWkzGPb8sVc42f2YxcTXKUTpAUbdrzVovaTBmGGNyK6cGFaA4Kp7SSLKecrxYz8Sc9d77Rss7rayYt1oFCaNJ",
    "publicKeyJwk": {
        "kty": "EC", 
        "crv": "P-256", 
        "x": "u-cNviWRiM3i9wGjUvOB-0XyPpIb5rAwmE8o8jDBte7Y", 
        "y": "uHNNnF7isXk_qitI9yNB4PCMY7krXqA224AJq0LByok8" 
     }
  }]
}

In the did:key document, output from function publicHexKeyToDIDKey is used.

in the did:key document coordinates "x" and "y" come from console.log(x) and console.log(y) in function publicKeyToXY.

const publicKey = 'zruuPojWkzGPb8sVc42f2YxcTXKUTpAUbdrzVovaTBmGGNyK6cGFaA4Kp7SSLKecrxYz8Sc9d77Rss7rayYt1oFCaNJ';
const buf = multibase.decode(publicKey)
const bufnoPrefix = multicodec.rmPrefix(buf)
const bbf = toString(bufnoPrefix,'base16')
console.log(bbf)

console.log(bbf) yields:

f9c36f8964623378bdc068d4bce07ed17c8fa486f9ac0c2613ca3c8c306d7bb61cd36717b8ac5e4fea8ad23dc8d0783c2318ee4ad7a80db6e0026ad0b072a24f