Create design document for MSI v2 In-Memory Key Approach

Add design document for MSI v2 In-Memory Key Approach, detailing goals, .NET reference implementation, Python implementation design, comparison with KeyGuard, and API design.

Author: gladjohn

docs/msi-v2-in-memory-key-approach.md

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+# MSI v2 In-Memory Key Approach — Design Document
+
+## Goal
+
+Implement an MSI v2 path using an **in-memory software RSA key** (no KeyGuard).
+The private key is exportable, so standard Python HTTP libraries (`requests`) work
+for both token acquisition and resource calls. **No MSAL helper needed.**
+
+This matches .NET's `InMemoryManagedIdentityKeyProvider` — the lowest tier in the
+key hierarchy.
+
+---
+
+## .NET Reference Implementation
+
+From [`InMemoryManagedIdentityKeyProvider.cs`](https://github.com/AzureAD/microsoft-authentication-library-for-dotnet/blob/main/src/client/Microsoft.Identity.Client/ManagedIdentity/KeyProviders/InMemoryManagedIdentityKeyProvider.cs):
+
+```csharp
+// Portable (non-Windows): pure in-memory RSA
+private static RSA CreatePortableRsa()
+{
+    var rsa = RSA.Create();
+    rsa.KeySize = 2048;
+    return rsa;
+}
+
+// Windows: persisted CNG key with AllowExport
+private static RSA CreateWindowsPersistedRsa()
+{
+    var creation = new CngKeyCreationParameters
+    {
+        ExportPolicy = CngExportPolicies.AllowExport,  // ← EXPORTABLE
+        Provider = CngProvider.MicrosoftSoftwareKeyStorageProvider
+    };
+    string keyName = "MSAL-MTLS-" + Guid.NewGuid().ToString("N");
+    var key = CngKey.Create(CngAlgorithm.Rsa, keyName, creation);
+    return new RSACng(key);
+}
+```
+
+Key points:
+- **`AllowExport`** — the key CAN be extracted as bytes
+- No VBS/KeyGuard flags — purely software key
+- No attestation — MAA not called
+- Named + persisted so SChannel can use it (Windows only)
+
+---
+
+## Python Implementation Design
+
+### Key Generation
+
+```python
+from cryptography.hazmat.primitives.asymmetric import rsa
+from cryptography.hazmat.primitives import serialization
+
+# Generate exportable RSA-2048 key
+private_key = rsa.generate_private_key(public_exponent=65537, key_size=2048)
+
+# Export as PEM — this is possible because key is in-memory (not KeyGuard)
+key_pem = private_key.private_bytes(
+    encoding=serialization.Encoding.PEM,
+    format=serialization.PrivateFormat.TraditionalOpenSSL,
+    encryption_algorithm=serialization.NoEncryption(),
+).decode("utf-8")
+```
+
+### CSR Building
+
+```python
+from cryptography import x509
+from cryptography.x509.oid import NameOID
+from cryptography.hazmat.primitives.hashes import SHA256
+from cryptography.hazmat.primitives.asymmetric.padding import PSS, MGF1
+
+# Build CSR using cryptography library (no manual DER needed)
+csr = (
+    x509.CertificateSigningRequestBuilder()
+    .subject_name(x509.Name([
+        x509.NameAttribute(NameOID.COMMON_NAME, client_id),
+        x509.NameAttribute(NameOID.DOMAIN_COMPONENT, tenant_id),
+    ]))
+    .add_attribute(cu_id_oid, cu_id_value)
+    .sign(private_key, SHA256(), padding=PSS(mgf=MGF1(SHA256()), salt_length=32))
+)
+csr_b64 = base64.b64encode(csr.public_bytes(serialization.Encoding.DER)).decode()
+```
+
+### IMDS Calls
+
+Same as KeyGuard path but **no attestation token**:
+
+```python
+# Step 1: getplatformmetadata (identical)
+meta = http_client.get(imds_base + "/metadata/identity/getplatformmetadata",
+                       params={"cred-api-version": "2.0"}, headers={"Metadata": "true"})
+
+# Step 2: issuecredential — empty attestation_token
+cred = http_client.post(imds_base + "/metadata/identity/issuecredential",
+                        params={"cred-api-version": "2.0"},
+                        headers={"Metadata": "true", "Content-Type": "application/json"},
+                        json={"csr": csr_b64, "attestation_token": ""})  # ← empty
+```
+
+### Token Acquisition (mTLS)
+
+Since the key is exportable, use `requests` with cert + key PEM:
+
+```python
+import requests
+import tempfile, os
+
+# Write cert + key to temp files (requests needs file paths)
+with tempfile.NamedTemporaryFile(mode='w', suffix='.pem', delete=False) as cf:
+    cf.write(cert_pem)
+    cert_path = cf.name
+with tempfile.NamedTemporaryFile(mode='w', suffix='.pem', delete=False) as kf:
+    kf.write(key_pem)
+    key_path = kf.name
+
+try:
+    token_resp = requests.post(
+        token_endpoint,
+        cert=(cert_path, key_path),
+        data={
+            "grant_type": "client_credentials",
+            "client_id": client_id,
+            "scope": scope,
+            "token_type": "mtls_pop",
+        },
+    )
+finally:
+    os.unlink(cert_path)
+    os.unlink(key_path)
+```
+
+### Auth Result
+
+```python
+{
+    "access_token": "eyJ...",
+    "token_type": "mtls_pop",
+    "expires_in": 86399,
+    "cert_pem": "-----BEGIN CERTIFICATE-----\n...",
+    "key_pem": "-----BEGIN RSA PRIVATE KEY-----\n...",  # ← AVAILABLE
+    "cert_thumbprint_sha256": "abc123...",
+}
+```
+
+### Resource Call — No Helper Needed!
+
+The caller uses standard `requests`:
+
+```python
+result = client.acquire_token_for_client(
+    resource="https://vault.azure.net",
+    mtls_proof_of_possession=True,
+)
+
+# Write cert+key to temp files (or use in-memory with urllib3)
+# ... (same temp file pattern as above)
+
+resp = requests.get(
+    "https://tokenbinding.vault.azure.net/secrets/boundsecret/?api-version=2015-06-01",
+    cert=(cert_path, key_path),
+    headers={
+        "Authorization": f"{result['token_type']} {result['access_token']}",
+        "x-ms-tokenboundauth": "true",
+    },
+)
+```
+
+---
+
+## Comparison: KeyGuard vs In-Memory
+
+| Aspect | KeyGuard (current PR) | In-Memory (this design) |
+|--------|----------------------|------------------------|
+| Key type | Non-exportable CNG/VBS | Exportable software RSA |
+| Attestation | MAA (proves hardware) | None |
+| `key_pem` in result? | ❌ Impossible | ✅ Yes |
+| Token acquisition | WinHTTP/SChannel (ctypes) | `requests` + cert/key PEM |
+| Resource call | `mtls_http_request()` helper | Standard `requests` |
+| Helper needed? | **Yes** | **No** |
+| Platform | Windows + Credential Guard | **Any** (cross-platform) |
+| Dependencies | `msal-key-attestation`, ctypes | `cryptography` (already used) |
+| Security | ★★★★★ | ★★☆☆☆ |
+
+---
+
+## API Design
+
+```python
+# KeyGuard + attestation (high security, helper required)
+result = client.acquire_token_for_client(
+    resource=...,
+    mtls_proof_of_possession=True,
+    with_attestation_support=True,     # ← KeyGuard path
+)
+# result has cert_pem, cert_der_b64 but NO key_pem
+# Must use: mtls_http_request() for resource calls
+
+# In-memory (lower security, no helper needed)
+result = client.acquire_token_for_client(
+    resource=...,
+    mtls_proof_of_possession=True,
+    # with_attestation_support=False (default) ← In-memory path
+)
+# result has cert_pem AND key_pem
+# Standard: requests.get(url, cert=(cert, key)) just works
+```
+
+---
+
+## Implementation Effort
+
+| Component | KeyGuard (done) | In-Memory (new) |
+|-----------|----------------|-----------------|
+| Key generation | NCrypt via ctypes | `cryptography.rsa.generate_private_key()` |
+| CSR building | Manual DER builder (500+ LOC) | `cryptography.x509.CertificateSigningRequestBuilder` (~20 LOC) |
+| IMDS calls | Shared | Shared |
+| Token acquisition | WinHTTP/SChannel via ctypes | `requests.post(cert=...)` |
+| Platform | Windows only | Cross-platform |
+| Complexity | High (ctypes, Win32 APIs) | Low (pure Python) |
+
+The in-memory path is **significantly simpler** — most of the complexity in `msi_v2.py`
+(NCrypt, Crypt32, WinHTTP, manual DER) is specifically for KeyGuard. The in-memory path
+can be implemented with `cryptography` + `requests` in ~200 lines.