V2.1 (#21)

Working version Reviewed-on: WikiDeck/wikideck#21

shared/chain-identity.ts

@@ -0,0 +1,16 @@
+import { CONSENSUS, MAX_TARGET, NETWORK } from './testnet.ts';
+import { NETWORK_GENESIS_TIME } from './network.ts';
+
+export type ChainIdentity = { chainId: string; genesis: string };
+let identity: Promise<ChainIdentity> | undefined;
+
+/** Derive the same public manifest as the node from this immutable browser build. */
+export function expectedChainIdentity(): Promise<ChainIdentity> {
+  if (!identity) identity = (async () => {
+    const digestHex = async (value: string) => Array.from(new Uint8Array(await crypto.subtle.digest(
+      'SHA-256', new TextEncoder().encode(value))), byte => byte.toString(16).padStart(2, '0')).join('');
+    const chainId = await digestHex(JSON.stringify([NETWORK, CONSENSUS, NETWORK_GENESIS_TIME, MAX_TARGET]));
+    return { chainId, genesis: await digestHex(`${NETWORK}:genesis:${chainId}`) };
+  })();
+  return identity;
+}