livecodingspace-app/public/lib/gundb/sea/secret.js

    var SEA = require('./root');
    var shim = require('./shim');
    var S = require('./settings');
    // Derive shared secret from other's pub and my epub/epriv 
    SEA.secret = SEA.secret || (async (key, pair, cb, opt) => { try {
      opt = opt || {};
      if(!pair || !pair.epriv || !pair.epub){
        pair = await SEA.I(null, {what: key, how: 'secret', why: opt.why});
      }
      var pub = key.epub || key;
      var epub = pair.epub;
      var epriv = pair.epriv;
      var ecdhSubtle = shim.ossl || shim.subtle;
      var pubKeyData = keysToEcdhJwk(pub);
      var props = Object.assign({ public: await ecdhSubtle.importKey(...pubKeyData, true, []) },{name: 'ECDH', namedCurve: 'P-256'}); // Thanks to @sirpy !
      var privKeyData = keysToEcdhJwk(epub, epriv);
      var derived = await ecdhSubtle.importKey(...privKeyData, false, ['deriveBits']).then(async (privKey) => {
        // privateKey scope doesn't leak out from here!
        var derivedBits = await ecdhSubtle.deriveBits(props, privKey, 256);
        var rawBits = new Uint8Array(derivedBits);
        var derivedKey = await ecdhSubtle.importKey('raw', rawBits,{ name: 'AES-GCM', length: 256 }, true, [ 'encrypt', 'decrypt' ]);
        return ecdhSubtle.exportKey('jwk', derivedKey).then(({ k }) => k);
      })
      var r = derived;
      if(cb){ try{ cb(r) }catch(e){console.log(e)} }
      return r;
    } catch(e) {
      console.log(e);
      SEA.err = e;
      if(SEA.throw){ throw e }
      if(cb){ cb() }
      return;
    }});

    // can this be replaced with settings.jwk?
    var keysToEcdhJwk = (pub, d) => { // d === priv
      //var [ x, y ] = Buffer.from(pub, 'base64').toString('utf8').split(':') // old
      var [ x, y ] = pub.split('.') // new
      var jwk = d ? { d: d } : {}
      return [  // Use with spread returned value...
        'jwk',
        Object.assign(
          jwk,
          { x: x, y: y, kty: 'EC', crv: 'P-256', ext: true }
        ), // ??? refactor
        {name: 'ECDH', namedCurve: 'P-256'}
      ]
    }

    module.exports = SEA.secret;