dpop-adoption

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# 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`):**
```javascript
// 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

```javascript

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