diff --git a/clients/passkeys-browser/src/base64url.ts b/clients/passkeys-browser/src/base64url.ts
index 8a24352..b063518 100644
--- a/clients/passkeys-browser/src/base64url.ts
+++ b/clients/passkeys-browser/src/base64url.ts
@@ -22,7 +22,7 @@ export function encode(input: ArrayBuffer | Uint8Array | ArrayBufferView): Base6
   return btoa(s).replaceAll("+", "-").replaceAll("/", "_").replaceAll("=", "");
 }
 
-export function decode(input: Base64Url): Uint8Array {
+export function decode(input: Base64Url): Uint8Array<ArrayBuffer> {
   const pad = "=".repeat((4 - (input.length % 4)) % 4);
   const normalized = input.replaceAll("-", "+").replaceAll("_", "/") + pad;
   const binary = atob(normalized);
@@ -43,6 +43,11 @@ export function decodeToArrayBuffer(input: Base64Url): ArrayBuffer {
 
 function toUint8Array(input: ArrayBuffer | Uint8Array | ArrayBufferView): Uint8Array {
   if (input instanceof Uint8Array) return input;
-  if (input instanceof ArrayBuffer) return new Uint8Array(input);
+  // Use toString-based check to handle cross-realm ArrayBuffers (e.g. from TextEncoder in jsdom)
+  // jsdom's TextEncoder creates an ArrayBuffer that is different from NodeJs ArrayBuffer so "instanceof"
+  // will not work. The string tag for the type will work.
+  if (input instanceof ArrayBuffer || Object.prototype.toString.call(input) === "[object ArrayBuffer]") {
+    return new Uint8Array(input as ArrayBuffer);
+  }
   return new Uint8Array(input.buffer, input.byteOffset, input.byteLength);
 }
diff --git a/clients/passkeys-browser/test/helpers/registration.test.ts b/clients/passkeys-browser/test/helpers/registration.test.ts
index 7c50064..ec86bf8 100644
--- a/clients/passkeys-browser/test/helpers/registration.test.ts
+++ b/clients/passkeys-browser/test/helpers/registration.test.ts
@@ -1,19 +1,23 @@
 import { describe, expect, it } from "vitest";
-import {RegistrationService} from "./registration";
+import { Encoder } from "cbor-x";
+import * as b64u from "../../src/base64url";
+import { decodeCreationOptions, encodeRegistrationResponse } from "../../src/ceremonies";
+import { FakeAuthenticator } from "./fakeAuthenticator";
+import { RegistrationService } from "./registration";
 
 describe("RegistrationService", () => {
-  describe("start", () => {
+  describe("start a registration", () => {
     it("returns 403 when a username is not in the allowlist", async () => {
-      const service = new RegistrationService("localhost", "alice")
-      expect(
-        () => service.start({ username: "eve", displayName: null, label: null, challenge: null }),
-      ).toThrow("HTTP 403");
+      const service = new RegistrationService("localhost", "alice");
+      await expect(
+        service.start({ username: "eve", displayName: null, label: null, challenge: null }),
+      ).rejects.toThrow("HTTP 403");
     });
 
     it("returns a StartRegistrationResponse when the request is valid", async () => {
       const service = new RegistrationService("localhost", "alice")
 
-      const response = service.start({ username: "alice", displayName: null, label: null, challenge: null })
+      const response = await service.start({ username: "alice", displayName: null, label: null, challenge: null })
 
       expect(response.challengeId).toBeTypeOf("string");
       expect(response.publicKey.user.id).toBeTypeOf("string");
@@ -26,4 +30,92 @@ describe("RegistrationService", () => {
       expect(response.publicKey.attestation).toEqual("direct");
     });
   });
+
+  describe("finish a registration", () => {
+    it("returns 400 for an unknown challengeId", async () => {
+      const service = new RegistrationService("localhost", "alice");
+      await expect(
+        service.finish({
+          challengeId: "does-not-exist",
+          username: "alice",
+          label: "key",
+          response: {
+            id: "x",
+            rawId: "x",
+            type: "public-key",
+            response: { clientDataJSON: "x", attestationObject: "x", transports: [] },
+          },
+        }),
+      ).rejects.toThrow("HTTP 400");
+    });
+
+    it("returns 400 for a username that does not match the started session", async () => {
+      const service = new RegistrationService("localhost", "alice", "bob");
+      const { challengeId, publicKey } = await service.start({ username: "alice", displayName: null, label: null, challenge: null });
+
+      const auth = new FakeAuthenticator();
+      const credential = await auth.create({ publicKey: decodeCreationOptions(publicKey) });
+      const encoded = encodeRegistrationResponse(credential);
+
+      await expect(
+        service.finish({ challengeId, username: "bob", label: null, response: encoded }),
+      ).rejects.toThrow("HTTP 400");
+    });
+
+    it("verifies the attestation and returns credential metadata", async () => {
+      const service = new RegistrationService("localhost", "alice");
+      const { challengeId, publicKey } = await service.start({ username: "alice", displayName: "Alice", label: null, challenge: null });
+
+      const auth = new FakeAuthenticator();
+      const credential = await auth.create({ publicKey: decodeCreationOptions(publicKey) });
+      const encoded = encodeRegistrationResponse(credential);
+
+      const { credential: cred } = await service.finish({ challengeId, username: "alice", label: "my-key", response: encoded });
+
+      expect(cred.credentialId).toBeTypeOf("string");
+      expect(cred.userHandle).toBeTypeOf("string");
+      expect(cred.label).toBe("my-key");
+      expect(cred.transports).toEqual(["internal"]);
+      expect(cred.backupEligible).toBe(false);
+      expect(cred.backupState).toBe(false);
+    });
+
+    it("returns 400 when the attestation signature is tampered", async () => {
+      const service = new RegistrationService("localhost", "alice");
+      const { challengeId, publicKey } = await service.start({ username: "alice", displayName: null, label: null, challenge: null });
+
+      const auth = new FakeAuthenticator();
+      const credential = await auth.create({ publicKey: decodeCreationOptions(publicKey) });
+      const encoded = encodeRegistrationResponse(credential);
+
+      // decode the attestation statement; modify it by flipping bits
+      // then re-encode and submit to finish
+      const cbor = new Encoder({ useRecords: false, mapsAsObjects: false });
+      const attObjBytes = b64u.decode(encoded.response.attestationObject);
+      const attObj = cbor.decode(attObjBytes) as Map<string, unknown>;
+      const attStmt = attObj.get("attStmt") as Map<string, unknown>;
+      const sig = attStmt.get("sig") as Uint8Array;
+      sig[0]! ^= 0xff;
+      const tampered = { ...encoded, response: { ...encoded.response, attestationObject: b64u.encode(cbor.encode(attObj) as Uint8Array) } };
+
+      await expect(
+        service.finish({ challengeId, username: "alice", label: null, response: tampered }),
+      ).rejects.toThrow("HTTP 400");
+    });
+
+    it("returns 400 when the challenge in clientDataJSON belongs to a different session", async () => {
+      const service = new RegistrationService("localhost", "alice");
+      const s1 = await service.start({ username: "alice", displayName: null, label: null, challenge: null });
+      const s2 = await service.start({ username: "alice", displayName: null, label: null, challenge: null });
+
+      const auth = new FakeAuthenticator();
+      const credential = await auth.create({ publicKey: decodeCreationOptions(s2.publicKey) });
+      const encoded = encodeRegistrationResponse(credential);
+
+      // finish using s1 challengeId but with s2's credential -- this should fail
+      await expect(
+        service.finish({ challengeId: s1.challengeId, username: "alice", label: null, response: encoded }),
+      ).rejects.toThrow("HTTP 400");
+    });
+  });
 });
diff --git a/clients/passkeys-browser/test/helpers/registration.ts b/clients/passkeys-browser/test/helpers/registration.ts
index fb1c77a..afa71b9 100644
--- a/clients/passkeys-browser/test/helpers/registration.ts
+++ b/clients/passkeys-browser/test/helpers/registration.ts
@@ -1,7 +1,13 @@
 
 import * as b64u from "../../src/base64url";
-import {StartRegistrationRequest, StartRegistrationResponse} from "../../src";
+import {
+  FinishRegistrationRequest,
+  FinishRegistrationResponse,
+  StartRegistrationRequest,
+  StartRegistrationResponse
+} from "../../src";
 import {HttpError} from "./httpServer";
+import {Encoder} from "cbor-x";
 
 interface PendingEntry {
   displayName: string;
@@ -13,17 +19,15 @@ interface PendingEntry {
 }
 
 /*
-RegistrationService is a representation of a server side service that can start and finish
+RegistrationService is a server side service that can start and finish
 client registrations.
 Instantiate the service with a relying party id [rpId] and a list of allowed users.
 A registration is started with a call to start which will produce a challenge that
-the client must respond to when calling finish.
+the client must respond to with a call to finish.
  */
 export class RegistrationService {
   // pending is the map of registrations which have been started; but not yet finished.
   // Maps challenge id to a PendingEntry structure.
-  // A registration.ts cannot be finished unless it has been successfully started.
-  // A successfully started registration.ts must appear on the pending map.
   private readonly pending : Map<string, PendingEntry>;
   private readonly rpId : string;
   private readonly allowedUsernames : string[];
@@ -34,7 +38,7 @@ export class RegistrationService {
     this.allowedUsernames = allowedUsernames ?? [];
   }
 
-  start(req : StartRegistrationRequest) : StartRegistrationResponse {
+  async start(req : StartRegistrationRequest) : Promise<StartRegistrationResponse> {
     if (!this.allowedUsernames.includes(req.username)) {
       throw new HttpError(403, { error: `username '${req.username}' not allowed` });
     }
@@ -48,11 +52,141 @@ export class RegistrationService {
       userId,
       challenge,
       challengeId,
-      rpId: this.rpId };
+      rpId: this.rpId
+    };
 
-    this.pending.set(challengeId, pendingEntry); // save the requests which will be validated in finish
+    // save the requests which will be validated in finish
+    this.pending.set(challengeId, pendingEntry);
     return newStartRegistrationResponse(pendingEntry)
   }
+
+  async finish(req: FinishRegistrationRequest): Promise<FinishRegistrationResponse> {
+    const entry = this.pending.get(req.challengeId);
+    if (!entry) {
+      throw new HttpError(400, { error: `unknown challengeId: ${req.challengeId}` });
+    }
+    if (entry.username !== req.username) {
+      throw new HttpError(400, { error: `username mismatch: ${req.username}` });
+    }
+    const clientData = parseClientData(req.response.response.clientDataJSON);
+    if (clientData.type !== "webauthn.create") {
+      throw new HttpError(400, { error: `wrong clientDataJSON type: ${clientData.type}` });
+    }
+    if (clientData.challenge !== entry.challenge) {
+      throw new HttpError(400, { error: "challenge mismatch" });
+    }
+
+    const attestation = decodeAttestation(req.response.response.attestationObject)
+    const publicKey = await parsePublicKey(attestation.publicKeyBytes)
+    const isValid = await validateChallenge(publicKey, attestation, req.response.response.clientDataJSON)
+    if (!isValid) {
+      throw new HttpError(400, { error: "invalid signature" });
+    }
+
+    const credId = attestation.authData.slice(55, 55 + attestation.credentialIdLength);
+    const flags = attestation.authData[32]!;
+    this.pending.delete(req.challengeId);
+    return {
+      credential: {
+        credentialId: b64u.encode(credId),
+        userHandle: entry.userId,
+        label: req.label ?? "key",
+        transports: req.response.response.transports ?? [],
+        counter: attestation.dv.getUint32(33),
+        backupEligible: (flags & 0x08) !== 0,
+        backupState: (flags & 0x10) !== 0,
+        authenticatorData: b64u.encode(attestation.authData),
+      },
+    };
+  }
+}
+
+async function validateChallenge(publicKey: CryptoKey, attestation: Attestation, clientDataJson: string) : Promise<boolean> {
+  const data = b64u.decode(clientDataJson);
+  const clientDataHash = new Uint8Array(
+    await crypto.subtle.digest("SHA-256", data),
+  );
+  const verificationData = new Uint8Array(attestation.authData.length + clientDataHash.length);
+  verificationData.set(attestation.authData);
+  verificationData.set(clientDataHash, attestation.authData.length);
+
+  return crypto.subtle.verify(
+    { name: "ECDSA", hash: "SHA-256" },
+    publicKey,
+    attestation.signature,
+    verificationData,
+  );
+}
+
+function parsePublicKey(publicKeyBytes : Uint8Array<ArrayBuffer>) : Promise<CryptoKey> {
+  try {
+    return crypto.subtle.importKey(
+      "raw",
+      publicKeyBytes,
+      { name: "ECDSA", namedCurve: "P-256" },
+      false,
+      ["verify"],
+    );
+  } catch(e) {
+    const eMessage = e as {message: string}
+    const message = eMessage && eMessage.message ? eMessage.message : "unknown error";
+    throw new HttpError(400, { error: `invalid public key in authData: ${message}` });
+  }
+}
+
+type Attestation = {
+  dv: DataView,
+  credentialIdLength: number,
+  signature: Uint8Array<ArrayBuffer>,
+  authData: Uint8Array<ArrayBufferLike>,
+  publicKeyBytes: Uint8Array<ArrayBuffer>,
+}
+
+function decodeAttestation(attestation: string) : Attestation {
+  const cbor = new Encoder({ useRecords: false, mapsAsObjects: false });
+  const attestationObjectBytes = b64u.decode(attestation);
+  const attObj = cbor.decode(attestationObjectBytes) as Map<string, unknown>;
+  if (attObj.get("fmt") !== "packed") {
+    throw new HttpError(400, { error: `unsupported attestation format: ${attObj.get("fmt")}` });
+  }
+  const authData = attObj.get("authData") as Uint8Array;
+  const attStmt = attObj.get("attStmt") as Map<string, unknown>;
+  const sig = attStmt.get("sig") as Uint8Array<ArrayBuffer>;
+
+  // Extract COSE public key from authData
+  const dv = new DataView(authData.buffer, authData.byteOffset);
+  const credIdLen = dv.getUint16(53);
+  const coseKey = cbor.decode(authData.slice(55 + credIdLen)) as Map<number, unknown>;
+  const xBytes = coseKey.get(-2) as Uint8Array;
+  const yBytes = coseKey.get(-3) as Uint8Array;
+
+  const rawPoint = new Uint8Array(65);
+  rawPoint[0] = 0x04;
+  rawPoint.set(xBytes, 1);
+  rawPoint.set(yBytes, 33);
+
+  return {
+    dv: dv,
+    credentialIdLength: credIdLen,
+    signature: sig,
+    authData: authData,
+    publicKeyBytes: rawPoint
+  }
+}
+
+function parseClientData(clientDataJson: string) : { type:string, challenge: string } {
+  // Decode and validate clientDataJSON
+  const data = b64u.decode(clientDataJson);
+  try {
+    return JSON.parse(new TextDecoder().decode(data)) as {
+      type: string;
+      challenge: string;
+    };
+  } catch(e){
+    const eMessage = e as {message: string}
+    const message = eMessage && eMessage.message ? eMessage.message : "unknown error";
+    throw new HttpError(400, { error: "invalid client data json: " + message });
+  }
 }
 
 function newStartRegistrationResponse(pendingEntry: PendingEntry) : StartRegistrationResponse {