Add DOMAIN_APPLICATION_MASK validation for builder domain (#4542)

packages/params/src/index.ts

@@ -118,7 +118,17 @@ export const DOMAIN_SYNC_COMMITTEE = Uint8Array.from([7, 0, 0, 0]);
 export const DOMAIN_SYNC_COMMITTEE_SELECTION_PROOF = Uint8Array.from([8, 0, 0, 0]);
 export const DOMAIN_CONTRIBUTION_AND_PROOF = Uint8Array.from([9, 0, 0, 0]);
 
+// Application specfic domains
+
+/**
+ * `DOMAIN_APPLICATION_MASK` reserves the rest of the bitspace in `DomainType` for application
+ * usage. This means for some `DomainType` `DOMAIN_SOME_APPLICATION`, `DOMAIN_SOME_APPLICATION
+ * & DOMAIN_APPLICATION_MASK` **MUST** be non-zero. This expression for any other `DomainType`
+ * in the consensus specs **MUST** be zero.
+ */
+export const DOMAIN_APPLICATION_MASK = Uint8Array.from([0, 0, 0, 1]);
 export const DOMAIN_APPLICATION_BUILDER = Uint8Array.from([0, 0, 0, 1]);
+
 // Participation flag indices
 
 export const TIMELY_SOURCE_FLAG_INDEX = 0;

packages/params/test/unit/applicationDomains.test.ts

@@ -0,0 +1,14 @@
+import {expect} from "chai";
+import {DOMAIN_APPLICATION_MASK, DOMAIN_APPLICATION_BUILDER} from "../../src/index.js";
+
+describe("validate application domains", async () => {
+  [{name: "builder domain", domain: DOMAIN_APPLICATION_BUILDER}].map(({name, domain}) => {
+    it(name, () => {
+      let r = 0;
+      for (let i = 0; i < DOMAIN_APPLICATION_MASK.length; i++) {
+        r += DOMAIN_APPLICATION_MASK[i] & domain[i];
+      }
+      expect(r > 0).to.equal(true, `${name} mask application should be valid`);
+    });
+  });
+});