diff --git a/identity/identityDescription.test.ts b/identity/identityDescription.test.ts
index 3b2c544..8823c32 100644
--- a/identity/identityDescription.test.ts
+++ b/identity/identityDescription.test.ts
@@ -1,11 +1,27 @@
 import {
   type BlahIdentityDescription,
   blahIdentityDescriptionSchema,
+  getIdentityDescriptionFileURL,
+  identityDescriptionFilePath,
 } from "./identityDescription.ts";
 import { assertTypeMatchesZodSchema } from "../test/utils.ts";
+import { expect } from "@std/expect";
 
 Deno.test("type BlahIdentityDescription is accurate", () => {
   assertTypeMatchesZodSchema<BlahIdentityDescription>(
     blahIdentityDescriptionSchema,
   );
 });
+
+Deno.test("getIdentityDescriptionFileURL", () => {
+  expect(getIdentityDescriptionFileURL("https://lao.sb")).toBe(
+    "https://lao.sb" + identityDescriptionFilePath,
+  );
+
+  expect(getIdentityDescriptionFileURL("https://test.lao.sb")).toBe(
+    "https://test.lao.sb" + identityDescriptionFilePath,
+  );
+
+  expect(() => getIdentityDescriptionFileURL("https://trailing-slash.lao.sb/"))
+    .toThrow();
+});
diff --git a/identity/identityDescription.ts b/identity/identityDescription.ts
index 5d4cc5b..40840bd 100644
--- a/identity/identityDescription.ts
+++ b/identity/identityDescription.ts
@@ -1,17 +1,42 @@
 import { z } from "zod";
 import { blahSignedPayloadSchemaOf } from "../crypto/signedPayload.ts";
 import { type BlahActKeyRecord, blahActKeyRecordSchema } from "./actKey.ts";
-import { type BlahProfile, blahProfileSchema } from "./profile.ts";
+import {
+  type BlahProfile,
+  blahProfileSchema,
+  validateIDURLFormat,
+} from "./profile.ts";
 import type { BlahSignedPayload } from "../crypto/mod.ts";
 
+/** Schema for Blah identity description. */
 export const blahIdentityDescriptionSchema = z.object({
   id_key: z.string(),
   act_keys: z.array(blahSignedPayloadSchemaOf(blahActKeyRecordSchema)).min(1),
   profile: blahSignedPayloadSchemaOf(blahProfileSchema),
 });
 
+/** Type for Blah identity description. */
 export type BlahIdentityDescription = {
   id_key: string;
   act_keys: Array<BlahSignedPayload<BlahActKeyRecord>>;
   profile: BlahSignedPayload<BlahProfile>;
 };
+
+/** Path to the identity description file under a given ID URL. */
+export const identityDescriptionFilePath = "/.well-known/blah/identity.json";
+
+/**
+ * Get the full URL to the identity description file for a given ID URL.
+ *
+ * @param idURL - The ID URL to get the identity description file URL for.
+ * @returns The full URL to the identity description file.
+ * @throws Error if the ID URL format is invalid.
+ */
+export function getIdentityDescriptionFileURL(
+  idURL: string,
+): string {
+  if (!validateIDURLFormat(idURL)) throw new Error("Invalid ID URL format");
+  const url = new URL(idURL);
+  url.pathname = identityDescriptionFilePath;
+  return url.toString();
+}
diff --git a/identity/mod.ts b/identity/mod.ts
index 337c8fb..39270cd 100644
--- a/identity/mod.ts
+++ b/identity/mod.ts
@@ -5,17 +5,25 @@ export * from "./identity.ts";
 import {
   type BlahProfile,
   blahProfileSchema as internalBlahProfileSchema,
+  validateIDURLFormat,
 } from "./profile.ts";
 const blahProfileSchema: z.ZodType<BlahProfile> = internalBlahProfileSchema;
-export { type BlahProfile, blahProfileSchema };
+export { type BlahProfile, blahProfileSchema, validateIDURLFormat };
 
 import {
   type BlahIdentityDescription,
   blahIdentityDescriptionSchema as internalBlahIdentityDescriptionSchema,
+  getIdentityDescriptionFileURL,
+  identityDescriptionFilePath,
 } from "./identityDescription.ts";
 const blahIdentityDescriptionSchema: z.ZodType<BlahIdentityDescription> =
   internalBlahIdentityDescriptionSchema;
-export { type BlahIdentityDescription, blahIdentityDescriptionSchema };
+export {
+  type BlahIdentityDescription,
+  blahIdentityDescriptionSchema,
+  getIdentityDescriptionFileURL,
+  identityDescriptionFilePath,
+};
 
 import {
   type BlahActKeyRecord,
diff --git a/identity/profile.test.ts b/identity/profile.test.ts
index ad0dda5..68b827f 100644
--- a/identity/profile.test.ts
+++ b/identity/profile.test.ts
@@ -1,6 +1,36 @@
-import { type BlahProfile, blahProfileSchema } from "./profile.ts";
+import {
+  type BlahProfile,
+  blahProfileSchema,
+  validateIDURLFormat,
+} from "./profile.ts";
 import { assertTypeMatchesZodSchema } from "../test/utils.ts";
+import { expect } from "@std/expect";
 
 Deno.test("type BlahProfile is accurate", () => {
   assertTypeMatchesZodSchema<BlahProfile>(blahProfileSchema);
 });
+
+Deno.test("ID URL format - valid", () => {
+  expect(validateIDURLFormat("https://lao.sb")).toBe(true);
+  expect(validateIDURLFormat("https://test.lao.sb")).toBe(true);
+  expect(validateIDURLFormat("https://🧧.lao.sb")).toBe(true);
+});
+
+Deno.test("ID URL format - invalid", () => {
+  // Must be valid URL
+  expect(validateIDURLFormat("lao.sb")).toBe(false);
+  // No trailing slash
+  expect(validateIDURLFormat("https://lao.sb/")).toBe(false);
+  // No search params
+  expect(validateIDURLFormat("https://lao.sb?query=1")).toBe(false);
+  // No fragment
+  expect(validateIDURLFormat("https://lao.sb#fragment")).toBe(false);
+  // No path
+  expect(validateIDURLFormat("https://lao.sb/path")).toBe(false);
+  // No username
+  expect(validateIDURLFormat("https://user@lao.sb")).toBe(false);
+  // No password
+  expect(validateIDURLFormat("https://user:123@lao.sb")).toBe(false);
+  // No non-HTTPS protocol
+  expect(validateIDURLFormat("http://lao.sb")).toBe(false);
+});
diff --git a/identity/profile.ts b/identity/profile.ts
index 07cf40d..f86fda4 100644
--- a/identity/profile.ts
+++ b/identity/profile.ts
@@ -1,13 +1,15 @@
 import z from "zod";
 
+/** Schema for Blah user profile. */
 export const blahProfileSchema = z.object({
   typ: z.literal("profile"),
   preferred_chat_server_urls: z.array(z.string().url()),
-  id_urls: z.array(z.string().url()).min(1),
+  id_urls: z.array(z.string().refine(validateIDURLFormat)).min(1),
   name: z.string(),
   bio: z.string().optional(),
 });
 
+/** Type for Blah user profile. */
 export type BlahProfile = {
   typ: "profile";
   preferred_chat_server_urls: string[];
@@ -15,3 +17,16 @@ export type BlahProfile = {
   name: string;
   bio?: string;
 };
+
+/** Validate the format of an ID URL. */
+export function validateIDURLFormat(url: string): boolean {
+  const idURL = URL.parse(url);
+  return !!idURL &&
+    idURL.protocol === "https:" &&
+    idURL.pathname === "/" &&
+    !url.endsWith("/") &&
+    !idURL.search &&
+    !idURL.hash &&
+    !idURL.username &&
+    !idURL.password;
+}