Modify cert layer to support wrapped/cached certs

Author: bigbrett

docs/draft/wrapped-certs.md

@@ -0,0 +1,290 @@
+# Wrapped Cached Certificates
+
+## Overview
+
+This documents the "wrapped certificate" use case, showing how to leverage the
+certificate manager to use trusted root certificates that live in the server's
+**keystore cache** (RAM) after being unwrapped via keywrap funcitonality,
+rather than exclusively in **NVM** (flash). A root certificate is wrapped
+(AES-GCM encrypted) by the server, handed back to the client as an opaque blob,
+and later unwrapped into the server's key cache on demand. Once cached, it can
+be used in all certificate verification paths — standard, DMA, and ACERT —
+exactly like an NVM-resident root certificate.
+
+This is useful when a client needs to use a trusted root for verification but
+does not want to (or cannot) commit it to NVM. The wrapped blob can be stored
+cheaply on the client side, while the server only holds the unwrapped plaintext
+in its volatile cache for as long as it is needed.
+
+## High-Level Usage
+
+The lifecycle has three stages: **wrap**, **unwrap-and-cache**, and **use**.
+
+### 1. Provision a wrapping key (KEK)
+
+Before wrapping anything the server needs an AES-256 key to use as the
+key-encryption key. Cache it on the server with the `WH_NVM_FLAGS_USAGE_WRAP`
+flag:
+
+```c
+whKeyId kekId = 10;
+uint8_t kek[32] = { /* 256-bit AES key */ };
+
+wh_Client_KeyCache(client,
+                   WH_NVM_FLAGS_USAGE_WRAP, NULL, 0,
+                   kek, sizeof(kek), &kekId);
+```
+
+The KEK is now sitting in the server's `localCache` (or `globalCache` if marked
+global), indexed by `kekId`.
+
+### 2. Wrap the certificate
+
+Call `wh_Client_CertWrap` with the raw certificate DER and the KEK's ID. The
+server encrypts the certificate using AES-GCM and returns the wrapped blob:
+
+```c
+uint8_t  wrappedCert[2048];
+uint16_t wrappedCertSz = sizeof(wrappedCert);
+
+/* Build metadata: id embeds TYPE=WRAPPED and the client's USER id;
+ * caller controls flags, access, and optionally label */
+whNvmMetadata certMeta = {0};
+certMeta.id     = WH_CLIENT_KEYID_MAKE_WRAPPED_META(
+    client->comm->client_id, 5);
+certMeta.flags  = WH_NVM_FLAGS_USAGE_ANY;
+certMeta.access = WH_NVM_ACCESS_ANY;
+
+wh_Client_CertWrap(client, WC_CIPHER_AES_GCM, kekId,
+                    rootCaCert, rootCaCertLen,
+                    &certMeta,
+                    wrappedCert, &wrappedCertSz);
+```
+
+After this call:
+
+| Data | Location |
+|---|---|
+| KEK | Server key cache (`localCache[kekId]`) |
+| Wrapped cert blob (ciphertext + GCM tag + IV + metadata) | Client memory (`wrappedCert` buffer) |
+| Raw certificate | Nowhere on the server — only the client supplied it transiently |
+
+The client can now persist `wrappedCert` to its own storage (file, flash,
+external memory, etc.).
+
+### 3. Unwrap and cache the certificate on the server
+
+When the client needs the root for verification, it pushes the wrapped blob back
+to the server:
+
+```c
+whKeyId cachedCertId = WH_KEYID_ERASED;
+
+wh_Client_CertUnwrapAndCache(client, WC_CIPHER_AES_GCM, kekId,
+                              wrappedCert, wrappedCertSz,
+                              &cachedCertId);
+```
+
+The server decrypts the blob using the KEK, verifies the GCM authentication
+tag, and places the plaintext certificate into its key cache. The returned
+`cachedCertId` is the server-internal key ID (with `TYPE=WH_KEYTYPE_WRAPPED`
+already encoded).
+
+After this call:
+
+| Data | Location |
+|---|---|
+| KEK | Server key cache |
+| Plaintext certificate | Server key cache (`localCache[cachedCertId]`) |
+| Wrapped cert blob | Still in client memory (unchanged) |
+
+### 4. Use the cached cert for verification
+
+Pass the cached cert's ID — decorated with the wrapped flag — as the trusted
+root to any verify API:
+
+```c
+int32_t verifyResult;
+
+wh_Client_CertVerify(client,
+                      intermediateCert, intermediateCertLen,
+                      WH_CLIENT_KEYID_MAKE_WRAPPED(cachedCertId),
+                      &verifyResult);
+```
+
+`WH_CLIENT_KEYID_MAKE_WRAPPED(cachedCertId)` sets bit 9
+(`WH_KEYID_CLIENT_WRAPPED_FLAG = 0x0200`) on the ID the client sends to the
+server. This is the signal that tells the server "this root cert is in the
+cache, not in NVM."
+
+The same pattern works for:
+- `wh_Client_CertVerifyDma` (DMA path)
+- `wh_Client_CertReadTrusted` / `wh_Client_CertReadTrustedDma` (read-back)
+- `wh_Client_CertVerifyAcert` / `wh_Client_CertVerifyAcertDma` (attribute certs)
+
+### 5. Cleanup
+
+Evict the cached cert and KEK when done:
+
+```c
+wh_Client_KeyEvict(client, WH_CLIENT_KEYID_MAKE_WRAPPED(cachedCertId));
+wh_Client_KeyEvict(client, kekId);
+```
+
+## Low-Level Implementation Details
+
+### Client-side functions
+
+Nine thin wrappers in `src/wh_client_cert.c` (guarded by
+`WOLFHSM_CFG_KEYWRAP`), mirroring the Key wrap/unwrap API:
+
+- **`wh_Client_CertWrap`** / **`wh_Client_CertWrapRequest`** /
+  **`wh_Client_CertWrapResponse`** — Wrap a certificate. Accepts a
+  caller-provided `whNvmMetadata*` (with `id`, `flags`, `access`, and
+  optionally `label` set by the caller), sets `meta->len = certSz`, then
+  delegates to the corresponding `wh_Client_KeyWrap*` function. The metadata's
+  `id` field must have `TYPE=WH_KEYTYPE_WRAPPED` encoded via
+  `WH_CLIENT_KEYID_MAKE_WRAPPED_META`.
+
+- **`wh_Client_CertUnwrapAndExport`** / **`wh_Client_CertUnwrapAndExportRequest`** /
+  **`wh_Client_CertUnwrapAndExportResponse`** — Unwrap a wrapped certificate
+  and export both the plaintext certificate and its metadata back to the client.
+  Delegates to the corresponding `wh_Client_KeyUnwrapAndExport*` function.
+
+- **`wh_Client_CertUnwrapAndCache`** / **`wh_Client_CertUnwrapAndCacheRequest`** /
+  **`wh_Client_CertUnwrapAndCacheResponse`** — Unwrap and cache on the server.
+  Delegates to the corresponding `wh_Client_KeyUnwrapAndCache*` function.
+  Returns the server-assigned cache slot ID in `*out_certId`.
+
+All functions accept an `enum wc_CipherType cipherType` parameter (e.g.
+`WC_CIPHER_AES_GCM`) to specify the wrapping cipher. The blocking variants
+call their respective Request/Response functions in a do-while-NOTREADY loop.
+
+These are pure convenience; a caller could use `wh_Client_KeyWrap*` /
+`wh_Client_KeyUnwrapAndExport*` / `wh_Client_KeyUnwrapAndCache*` directly if
+it needed custom metadata.
+
+### Server-side routing (the key change)
+
+#### `wh_Server_CertReadTrusted` (`src/wh_server_cert.c`)
+
+Previously accepted only `whNvmId` and always read from NVM. Now accepts
+`whKeyId` and branches on the TYPE field:
+
+```
+if WH_KEYID_TYPE(id) == WH_KEYTYPE_WRAPPED
+    → wh_Server_KeystoreReadKey(server, id, &meta, cert, &sz)   // cache path
+else
+    → wh_Nvm_GetMetadata / wh_Nvm_Read                          // NVM path (unchanged)
+```
+
+`wh_Server_KeystoreReadKey` looks up the key in the server's `localCache` (or
+`globalCache` if global keys are enabled and the USER field is 0). It copies
+both the metadata and the raw data into the caller's buffers.
+
+#### `wh_Server_CertVerify` / `wh_Server_CertVerifyAcert`
+
+Signature changed from `whNvmId trustedRootNvmId` to `whKeyId trustedRootId`.
+Internally they just call `wh_Server_CertReadTrusted`, which now handles the
+routing.
+
+#### Request handlers in `wh_Server_HandleCertRequest`
+
+Every handler that accepts a trusted root ID (`READTRUSTED`, `VERIFY`,
+`READTRUSTED_DMA`, `VERIFY_DMA`, `VERIFY_ACERT`, `VERIFY_ACERT_DMA`) was
+updated with the same pattern:
+
+1. **Translate the client ID**: If the incoming `req.id` (or
+   `req.trustedRootNvmId`) has `WH_KEYID_CLIENT_WRAPPED_FLAG` set, call
+   `wh_KeyId_TranslateFromClient(WH_KEYTYPE_NVM, server->comm->client_id, req.id)`
+   to produce a full server-internal key ID with `TYPE=WH_KEYTYPE_WRAPPED`,
+   `USER=client_id`, and the bare key `ID` in the low byte.
+
+2. **Branch on key type** for the read/verify:
+   - **Cache path** (`WH_KEYID_TYPE(certId) == WH_KEYTYPE_WRAPPED`): Calls
+     `wh_Server_KeystoreReadKey` to fetch the cert from the cache. Checks
+     `WH_NVM_FLAGS_NONEXPORTABLE` on the metadata for read-back requests.
+   - **NVM path** (original, `WH_KEYID_TYPE != WH_KEYTYPE_WRAPPED`): Unchanged
+     behavior — reads from flash via `wh_Nvm_GetMetadata` / `wh_Nvm_Read`.
+
+### Key ID encoding walkthrough
+
+Consider a client with `client_id = 1` wrapping a cert with bare ID `5`:
+
+| Stage | Value | Encoding |
+|---|---|---|
+| `WH_CLIENT_KEYID_MAKE_WRAPPED_META(1, 5)` | `0x4105` | TYPE=4 (WRAPPED), USER=1, ID=5 — stored *inside* the wrapped blob metadata |
+| Server returns `cachedCertId` after unwrap | `0x4105` | Same — the server preserved the metadata ID |
+| Client sends `WH_CLIENT_KEYID_MAKE_WRAPPED(0x4105)` | `0x4305` | Bit 9 (0x0200) set as client flag |
+| Server calls `wh_KeyId_TranslateFromClient(...)` | `0x4105` | Flag stripped, TYPE=WRAPPED confirmed, USER=1, ID=5 |
+| `WH_KEYID_TYPE(0x4105)` | `4` | Equals `WH_KEYTYPE_WRAPPED` (4) → routes to cache |
+
+### Data stored at each point
+
+| Point in flow | Server key cache | Server NVM | Client memory |
+|---|---|---|---|
+| After `KeyCache` (KEK) | KEK at `kekId` | — | — |
+| After `CertWrap` | KEK at `kekId` | — | Wrapped blob (ciphertext + tag + IV + metadata) |
+| After `CertUnwrapAndCache` | KEK at `kekId`, plaintext cert at `cachedCertId` | — | Wrapped blob (unchanged) |
+| During `CertVerify` | KEK, plaintext cert (read into stack buffer `root_cert[WOLFHSM_CFG_MAX_CERT_SIZE]` by `CertReadTrusted`) | — | — |
+| After `KeyEvict` (cert) | KEK at `kekId` | — | Wrapped blob |
+| After `KeyEvict` (KEK) | — | — | Wrapped blob |
+
+## Interaction with Locking and Thread Safety
+
+### The NVM lock (`WH_SERVER_NVM_LOCK` / `WH_SERVER_NVM_UNLOCK`)
+
+When `WOLFHSM_CFG_THREADSAFE` is defined, `WH_SERVER_NVM_LOCK(server)` calls
+`wh_Server_NvmLock(server)`, which acquires a mutex protecting NVM state. When
+not threadsafe, the macros expand to `(WH_ERROR_OK)` (no-ops).
+
+The existing (pre-branch) code unconditionally called `WH_SERVER_NVM_LOCK`
+around every cert read/verify handler, because the cert always came from NVM.
+
+### What changes for cached certs
+
+Cached certs do not touch NVM at all — they are read from the in-memory key
+cache via `wh_Server_KeystoreReadKey`. However, the NVM lock is still
+unconditionally acquired around both cache and NVM paths. This is conservative
+but correct: the key cache (`localCache` / `globalCache`) does not have its own
+lock, so the NVM lock serves as the coarse serialization mechanism for all
+server-side storage operations (both NVM and cache) when
+`WOLFHSM_CFG_THREADSAFE` is enabled.
+
+The pattern used in every updated handler is:
+
+```c
+rc = WH_SERVER_NVM_LOCK(server);
+if (rc == WH_ERROR_OK) {
+    if (req.id & WH_KEYID_CLIENT_WRAPPED_FLAG) {
+        /* Cache path: translate and read from keystore cache */
+        whKeyId certId = wh_KeyId_TranslateFromClient(
+            WH_KEYTYPE_WRAPPED, server->comm->client_id, req.id);
+        rc = wh_Server_KeystoreReadKey(server, certId, &meta, cert_data, &cert_len);
+        /* ... exportability check for read-back requests ... */
+    } else {
+        /* NVM path (unchanged) */
+        rc = wh_Nvm_GetMetadata(server->nvm, req.id, &meta);
+        /* ... NVM reads ... */
+    }
+    (void)WH_SERVER_NVM_UNLOCK(server);
+}
+```
+
+Key points:
+
+- **Both paths hold the NVM lock**: The lock is always acquired before
+  branching. While the cache read itself doesn't strictly need NVM protection,
+  holding the lock ensures serialization with any concurrent operations that
+  access the `localCache` array on other threads.
+
+- **NVM path**: Unchanged — same behavior as before this branch.
+
+### Backward compatibility
+
+- All existing NVM-based certificate operations continue to work identically.
+  The routing branch only activates when the key type is `WH_KEYTYPE_WRAPPED`.
+- The `wh_Server_CertReadTrusted` and `wh_Server_CertVerify` function
+  signatures changed from `whNvmId` to `whKeyId`. Since `whNvmId` and `whKeyId`
+  are both `uint16_t`, this is ABI-compatible. Any existing callers passing a
+  plain NVM ID (with TYPE=0) will hit the NVM path as before.