DPoP Adoption & Identity Security Architecture

Demonstrating Proof-of-Possession (DPoP, RFC 9449) secures OAuth 2.0 refresh tokens against interception and replay attacks by cryptographically binding them to a private key held exclusively by the client. In Google's OAuth 2.0 platform, DPoP binds the refresh token at the token endpoint, while access tokens issued for Google APIs are standard Bearer tokens (token_type: "Bearer").

1. Core Cryptographic & Architectural Invariants

When implementing DPoP helpers or upgrading HTTP clients, you MUST adhere to the following strict security invariants:

A. Universal WebCrypto & Runtime Compatibility

• In modern ES6 JavaScript ("type": "module" for Node 18+ and browsers), ALWAYS access globalThis.crypto directly after verifying the environment context.
• NEVER import legacy CommonJS modules via require('node:crypto') or reference browser-scoped window.crypto, as these cause module initialization crashes across hybrid runtimes.

B. Hardware-Backed Non-Extractable Key Persistence

• Generate an Elliptic Curve key pair on the SECP256R1 (P-256) curve: { name: 'ECDSA', namedCurve: 'P-256' }.
• CRITICAL SECURITY GUARDRAIL: The private key MUST be configured as non-extractable (extractable: false). This guarantees the private key can never leave the hardware cryptographic boundary (Secure Enclave, Android KeyStore, or JS sandbox memory), thwarting XSS and dependency token theft attacks.
• The public key MUST remain exportable (extractable: true) to allow emitting JSON Web Keys (JWKs).

C. Public JWK Formatting Standards

• When exporting public keys to attach to DPoP Proof JWT headers, construct a clean JWK dictionary containing strictly:
  • "kty": "EC"
  • "crv": "P-256"
  • "x": Base64URL-encoded x-coordinate without trailing equal sign padding (=).
  • "y": Base64URL-encoded y-coordinate without trailing equal sign padding (=).
• NEVER expose private key parameters ("d") or superfluous metadata.

D. IEEE P1363 vs. ASN.1 DER Signature Disambiguation

• DPoP Proof JWTs require raw concatenated coordinate signatures ($R \parallel S$, exactly 64 bytes for P-256) per IEEE P1363 and RFC 7518.
• WebCrypto Native Rule: In standard WebCrypto (crypto.subtle.sign), ECDSA signatures are ALREADY emitted natively in raw IEEE P1363 format (concatenated 32-byte r and s buffers, 64 bytes total). DO NOT attempt DER-to-Raw conversion on crypto.subtle.sign outputs, as parsing a 64-byte raw buffer as ASN.1 DER causes an immediate runtime exception (Invalid DER sequence). Directly base64url-encode the raw ArrayBuffer.
• Legacy API Fallback: If and only if implementing in legacy Java/Android (java.security.Signature) or Node CommonJS (crypto.createSign), convert ASN.1 DER output to raw 64-byte IEEE P1363 format before base64url encoding.

E. SPA & Backend-for-Frontend (BFF) Architecture

• Secretless SPAs Limitation: Pure client-side single-page applications (SPAs) without a backend cannot use DPoP directly with Google APIs due to client_secret requirements on server endpoints and browser CORS limitations on the DPoP-Nonce response header.
• BFF Pattern: To secure SPAs with DPoP, route authorization and token refresh requests through a Backend-for-Frontend (BFF) server-side client. The BFF sets access_type=offline, binds refresh tokens server-side using DPoP, and maintains secure session cookies with the frontend.

2. Implementation Rules & Mandatory Public API

When creating new modules, your module MUST explicitly export all functions below to integrate cleanly with CI/CD verification harnesses and automated probers. When inspecting or refactoring existing codebases, ensure equivalent cryptographic and RFC 9449 logic is present. Obey strict claim derivation logic in all cases:

A. DPoP Proof JWT Claim Derivation Rules (createDPoPProof)

When generating the DPoP Proof JWT in createDPoPProof:

1. JOSE Header (typ, alg, jwk):

// Header
{
  "typ": "dpop+jwt",
  "alg": "ES256",
  "jwk": await exportPublicJWK(publicKey)
}

2. Payload Claims:

• "htm": Uppercase HTTP Method ("POST" for token requests).
• "htu": Target URI stripped of query parameters and hash fragments using sanitizeHTU(htu). For token requests, this is https://oauth2.googleapis.com/token.
• "iat": Current integer epoch timestamp in seconds (Math.floor(Date.now() / 1000)).
• "jti" (Critical Invariant):
  1. If an explicit jti argument is provided to createDPoPProof, use that exact string over all others.
  2. Otherwise, if an authCode argument is provided (during initial code exchange), set jti = await calculateAuthCodeJti(authCode) where calculateAuthCodeJti computes base64url(sha256(authCode)) to ensure the DPoP proof is cryptographically bound to the authorization code.
  3. Only if neither jti nor authCode is provided, generate a fresh cryptographic random string via generateRandomString() (such as crypto.getRandomValues(new Uint8Array(24)) base64url encoded).
• "ath" (Optional): If an accessToken argument is provided for RFC 9449 resource requests, compute base64url(sha256(accessToken)) via calculateATH(accessToken) and inject it (RFC 9449 Section 6.1).
• "nonce" (Optional): If a nonce argument is provided, inject it directly into the payload.

B. Explicit Export Signatures

// 1. Key generation & JWK export
export async function generateDPoPKeyPair() // -> { publicKey, privateKey } (private key extractable=false)
export async function exportPublicJWK(publicKey) // -> { kty: 'EC', crv: 'P-256', x, y }

// 2. Proof generation & validation
export async function createDPoPProof({ privateKey, publicKey, htm, htu, nonce, accessToken, authCode, jti }) // -> signed JWT string
export async function verifyDPoPProof(dpopProofJwt) // -> { isValid: boolean, header, payload, error }
export function sanitizeHTU(htu) // -> URL stripped of query and hash: const u = new URL(htu); return `${u.origin}${u.pathname}`;

// 3. Cryptographic & encoding utilities
export function base64UrlEncode(buffer) // -> Uint8Array/ArrayBuffer to base64url string without '=' padding
export function base64UrlDecode(str) // -> base64url string to Uint8Array/Buffer
export function stringToBase64Url(str) // -> UTF-8 string to base64url
export function base64UrlToString(str) // -> base64url to UTF-8 string
export function generateRandomString(byteLength = 32) // -> cryptographic random base64url string
export async function calculateATH(accessToken) // -> base64url(sha256(accessToken)) per RFC 9449 Sec 6.1
export async function calculateAuthCodeJti(code) // -> base64url(sha256(code))
export async function generatePKCE() // -> { codeVerifier (>=43 chars), codeChallenge, codeChallengeMethod: 'S256' }

3. Token Endpoint & Resource Request Workflow

When integrating with Google's OAuth 2.0 platform:

1. Token Endpoint Requests (oauth2.googleapis.com/token):
   • Attach the DPoP Proof JWT in the DPoP HTTP header: `DPoP: ${proofJwt}` when making POST requests for code exchange (grant_type=authorization_code) and token refresh (grant_type=refresh_token).
2. Resource API Requests:
   • Google's token endpoint returns "token_type": "Bearer". Downstream requests to Google APIs (e.g. Calendar, Drive, Gmail) use standard `Authorization: Bearer ${accessToken}` headers without DPoP headers.
3. Single-Retry Nonce Challenge Loop & Workflow Isolation:
   • If Google's token endpoint returns HTTP 400 Bad Request with error: "use_dpop_nonce" and a "DPoP-Nonce" response header:
     • Workflow Isolation: Google's authorization server enforces workflow isolation between authorization code exchange and token refresh, returning an HTTP 400 use_dpop_nonce challenge to establish a fresh nonce namespace. This is standard RFC-compliant protocol behavior, not a server failure.
     • Cache the fresh nonce in client state (this.dpopNonce).
     • Immediately synthesize a new DPoP Proof JWT incorporating the updated nonce claim and a fresh jti.
     • Replay the failed token request exactly once. If the retried request fails, terminate immediately with an error to prevent infinite recursion.

4. Concise Agent Egress Protocol

When prompted to synthesize or output code deliverables under this skill, prioritize returning clean, directly importable code blocks without redundant conversational preambles or repetitive filler. For conceptual or architectural inquiries, provide standard direct answers.

5. References & Supporting Documentation

Developer Documentation (Google for Developers)

• DPoP Adoption Guide — Official Google Identity guide for implementing DPoP across authorization code exchange and token refresh.
• Using OAuth 2.0 for Web Server Applications — Offline access, refresh tokens, and server-side authorization flows.
• OAuth 2.0 Best Practices: Sender-Constrain Tokens — Recommendations for token storage, rotation, and sender-constraining.

Developer Knowledge MCP Server

• Agents equipped with Model Context Protocol (MCP) can query real-time Google Developer documentation using the Google Developer Knowledge MCP Server (npx -y @google/mcp-developer-knowledge-server) via developer_knowledge:search_documents and developer_knowledge:get_documents.

Standards & RFC Specifications

• RFC 9449: OAuth 2.0 Demonstrating Proof-of-Possession (DPoP)
• RFC 7519: JSON Web Token (JWT)
• RFC 7636: Proof Key for Code Exchange by OAuth Public Clients (PKCE)