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Add crypto callbacks for LMS and XMSS
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‎doc/LMS_XMSS_CryptoCb.md‎

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# LMS / XMSS Crypto Callback support
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This document describes the wolfSSL-side groundwork that lets LMS / HSS
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(RFC 8554) and XMSS / XMSS^MT (RFC 8391, both profiled in NIST SP 800-208)
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participate in the `WOLF_CRYPTO_CB` framework. With this layer in place, the
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wolfSSL PKCS#11 provider and the wolfHSM client can host stateful
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hash-based keys on a device without the wolfSSL public API changing.
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No HSM-side or PKCS#11-provider code lives in this layer. It only adds the
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dispatcher surface, the per-key device binding, and the helpers a backend
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needs to answer the request.
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## Why route stateful hash-based keys through a device
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LMS and XMSS are one-time-signature trees: the private key holds a counter
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that must be incremented on every signature, and signing the same index twice
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breaks the security proof. Moving that counter to a hardware module is the
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clean way to make the scheme operationally safe — the HSM is the natural
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owner of the index, and an attacker who steals a host snapshot cannot replay
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old indices.
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## PKCS#11 mapping
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PKCS#11 v3.1 standardised HSS and v3.2 added XMSS / XMSS^MT. The CryptoCb
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surface mirrors what those mechanisms expose:
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| wolfSSL API | PKCS#11 analog | CryptoCb dispatcher |
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|-----------------------|------------------------------------------------|----------------------------------------------|
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| `wc_LmsKey_MakeKey` | `CKM_HSS_KEY_PAIR_GEN` | `wc_CryptoCb_PqcStatefulSigKeyGen` |
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| `wc_LmsKey_Sign` | `CKM_HSS` (sign) | `wc_CryptoCb_PqcStatefulSigSign` |
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| `wc_LmsKey_Verify` | `CKM_HSS` (verify) | `wc_CryptoCb_PqcStatefulSigVerify` |
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| `wc_LmsKey_SigsLeft` | `CKA_HSS_KEYS_REMAINING` attribute | `wc_CryptoCb_PqcStatefulSigSigsLeft` |
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| `wc_XmssKey_MakeKey` | `CKM_XMSS_KEY_PAIR_GEN` / `CKM_XMSSMT_KEY_PAIR_GEN` | `wc_CryptoCb_PqcStatefulSigKeyGen` |
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| `wc_XmssKey_Sign` | `CKM_XMSS` / `CKM_XMSSMT` (sign) | `wc_CryptoCb_PqcStatefulSigSign` |
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| `wc_XmssKey_Verify` | `CKM_XMSS` / `CKM_XMSSMT` (verify) | `wc_CryptoCb_PqcStatefulSigVerify` |
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| `wc_XmssKey_SigsLeft` | XMSS remaining-sigs attribute | `wc_CryptoCb_PqcStatefulSigSigsLeft` |
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The four dispatchers are shared between LMS and XMSS, following the
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`wc_CryptoCb_PqcSign*` family used for Dilithium and Falcon. A new
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discriminator enum `wc_PqcStatefulSignatureType` (`WC_PQC_STATEFUL_SIG_TYPE_LMS`,
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`WC_PQC_STATEFUL_SIG_TYPE_XMSS`) tells the callback which of `LmsKey*` or
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`XmssKey*` the `void* key` field is. XMSS vs XMSS^MT is decided inside the
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callback via the existing `XmssKey::is_xmssmt` field.
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`Reload`, `GetKid`, and `ExportPub` are not routed through CryptoCb, but each
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is aware of HSM-backed keys: `Reload` short-circuits because state lives in
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the device, `GetKid` logs a warning since `priv_raw` may be uninitialised,
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and `ExportPub` preserves the source key's `devId` so the verify-only copy
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keeps dispatching through the same device. The external-backend variants
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(`ext_lms.c` / `ext_xmss.c`, selected by `--with-liblms` / `--with-libxmss`)
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are intentionally outside the scope of this layer and execute purely in
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software.
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## Per-key device binding
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Each key carries the device-binding fields that other key types
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(`RsaKey`, `ecc_key`, `dilithium_key`) already expose:
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```c
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struct LmsKey {
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/* ... existing fields ... */
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#ifdef WOLF_CRYPTO_CB
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int devId; /* device identifier */
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void* devCtx; /* opaque per-device state, owned by the callback */
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#endif
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#ifdef WOLF_PRIVATE_KEY_ID
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byte id[LMS_MAX_ID_LEN]; /* device-side key identifier */
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int idLen;
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char label[LMS_MAX_LABEL_LEN]; /* device-side key label */
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int labelLen;
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#endif
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};
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```
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`XmssKey` carries the equivalent set under the same macro guards, with
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`XMSS_MAX_ID_LEN` / `XMSS_MAX_LABEL_LEN`. The `*_MAX_ID_LEN` and
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`*_MAX_LABEL_LEN` constants default to 32 and can be overridden by
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predefining the macros.
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`devCtx`, `id`, and `label` are storage only — wolfSSL never reads or writes
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them internally. Backends populate `devCtx` from the callback (typically the
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first time they touch the key) and consume `id` / `label` to resolve the
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on-device handle.
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## Public API additions
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```c
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/* Bind a key to a device-side identifier or label. */
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#ifdef WOLF_PRIVATE_KEY_ID
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WOLFSSL_API int wc_LmsKey_InitId (LmsKey * key, const unsigned char * id,
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int len, void * heap, int devId);
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WOLFSSL_API int wc_LmsKey_InitLabel(LmsKey * key, const char * label,
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void * heap, int devId);
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WOLFSSL_API int wc_XmssKey_InitId (XmssKey* key, const unsigned char* id,
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int len, void* heap, int devId);
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WOLFSSL_API int wc_XmssKey_InitLabel(XmssKey* key, const char* label,
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void* heap, int devId);
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#endif
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/* Compute the digest of a message with the hash function dictated by
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* the parameter set. Useful for backends that follow the PKCS#11 v3.2
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* CKM_HSS / CKM_XMSS / CKM_XMSSMT convention of operating on a
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* pre-computed digest (see "Sign / verify input format" below). */
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WOLFSSL_API int wc_LmsKey_HashMsg (const LmsKey * key, const byte * msg,
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word32 msgSz, byte * hash,
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word32 * hashSz);
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WOLFSSL_API int wc_XmssKey_HashMsg(const XmssKey* key, const byte* msg,
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word32 msgSz, byte* hash,
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word32* hashSz);
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```
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The `Init*` helpers follow the `wc_InitRsaKey_Id` / `wc_InitRsaKey_Label`
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shape: they validate length bounds, delegate the rest of init to
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`wc_LmsKey_Init` / `wc_XmssKey_Init`, then copy id / label onto the key.
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The `HashMsg` helpers honour the parameter set:
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| Algorithm | Hash families covered |
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|-----------|-----------------------------------------------------------------|
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| LMS / HSS | SHA-256 (32 bytes), SHA-256/192 (24 bytes), SHAKE256 (32 / 24) |
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| XMSS / MT | SHA-256, SHA-512, SHAKE128, SHAKE256 (per `params->hash`) |
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`*hashSz` is in / out: callers pass the buffer size and receive the digest
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length on success.
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## Sign / verify input format
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The CryptoCb dispatcher forwards the raw message to the callback. PKCS#11
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v3.2 section 6.66.8 ("XMSS and XMSSMT without hashing") and the analogous
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text for HSS specify that those mechanisms take a pre-computed digest
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rather than the message. Backends that need that behaviour — typically
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PKCS#11 providers — call `wc_LmsKey_HashMsg` or `wc_XmssKey_HashMsg` from
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inside the callback to produce the algorithm-dictated digest. Backends
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that take the full message (typically wolfHSM) consume `msg` / `msgSz`
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directly. Picking one or the other is a callback decision; the dispatcher
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is agnostic.
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## Build configuration
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| `./configure` flag(s) | Effect |
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|--------------------------------------------------------|-------------------------------------------------------|
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| `--enable-lms --enable-xmss --enable-cryptocb` | Primary target. Full dispatcher and round-trip tests. |
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| `--enable-lms --enable-xmss` | New dispatcher code is fully `#ifdef`-elided. |
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| `--enable-cryptocb` | LMS / XMSS-less build; nothing CryptoCb-side breaks. |
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| `CPPFLAGS=-DWOLF_PRIVATE_KEY_ID …` | Adds `id` / `label` fields and the `Init*` helpers. |
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## Verification
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`./wolfcrypt/test/testwolfcrypt` exercises the full dispatcher round trip:
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inside `cryptocb_test`, `lms_test` and `xmss_test` run with the harness's
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registered `myCryptoDevCb`, which clears the key's `devId`, invokes the
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software API recursively, then restores `devId`. Sign and verify both go
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through the dispatcher, so the produced signatures self-verify within the
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harness. With no device registered, `lms_test` and `xmss_test` remain on the
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software path and produce bit-identical KAT output.
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## Design notes
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- **Shared dispatcher, separate type tag.** The eight LMS / XMSS operations
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collapse to four shared dispatchers (`KeyGen`, `Sign`, `Verify`,
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`SigsLeft`) keyed on `wc_PqcStatefulSignatureType`. The pattern matches
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the `PqcSign` family used for Dilithium / Falcon and reduces the surface
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area a backend has to implement.
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- **Verify carries `int* res`.** Following the Ed25519 / ECC / PqcVerify
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convention, the verify dispatcher reports validity through a separate
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`*res` flag, so a backend can distinguish a transport error from a
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forged signature. The wrapping wolfSSL function still translates
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`res != 1` to `SIG_VERIFY_E` for callers that do not see `res`.
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- **`SigsLeft` carries `word32* sigsLeft`.** PKCS#11 defines
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`CKA_HSS_KEYS_REMAINING` as a `CK_ULONG`-sized attribute; the callback
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uses `word32*` so an HSS key at its 2^32 limit can be expressed
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unambiguously. The wolfSSL public API still returns `int` and clamps at
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`0x7FFFFFFF`.
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- **HSM-backed keys skip the software write / read callbacks.**
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`wc_LmsKey_MakeKey` / `_Sign` and the XMSS equivalents dispatch through
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CryptoCb *before* validating `write_private_key` / `read_private_key` /
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`context`. A device-backed key does not need dummy software callbacks.
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On `CRYPTOCB_UNAVAILABLE` fall-through the software validations are
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re-applied as normal.

‎wolfcrypt/src/cryptocb.c‎

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}
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#endif /* HAVE_ED25519 */
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#if defined(WOLFSSL_HAVE_LMS) || defined(WOLFSSL_HAVE_XMSS)
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int wc_CryptoCb_PqcStatefulSigGetDevId(int type, void* key)
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{
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int devId = INVALID_DEVID;
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if (key == NULL)
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return devId;
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#if defined(WOLFSSL_HAVE_LMS)
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if (type == WC_PQC_STATEFUL_SIG_TYPE_LMS) {
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devId = ((LmsKey*)key)->devId;
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}
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#endif
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#if defined(WOLFSSL_HAVE_XMSS)
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if (type == WC_PQC_STATEFUL_SIG_TYPE_XMSS) {
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devId = ((XmssKey*)key)->devId;
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}
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#endif
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return devId;
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}
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int wc_CryptoCb_PqcStatefulSigKeyGen(int type, void* key, WC_RNG* rng)
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{
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int ret = WC_NO_ERR_TRACE(CRYPTOCB_UNAVAILABLE);
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int devId = INVALID_DEVID;
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CryptoCb* dev;
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if (key == NULL)
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return ret;
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devId = wc_CryptoCb_PqcStatefulSigGetDevId(type, key);
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if (devId == INVALID_DEVID)
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return ret;
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dev = wc_CryptoCb_FindDevice(devId, WC_ALGO_TYPE_PK);
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if (dev && dev->cb) {
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wc_CryptoInfo cryptoInfo;
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XMEMSET(&cryptoInfo, 0, sizeof(cryptoInfo));
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cryptoInfo.algo_type = WC_ALGO_TYPE_PK;
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cryptoInfo.pk.type = WC_PK_TYPE_PQC_STATEFUL_SIG_KEYGEN;
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cryptoInfo.pk.pqc_stateful_sig_kg.rng = rng;
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cryptoInfo.pk.pqc_stateful_sig_kg.key = key;
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cryptoInfo.pk.pqc_stateful_sig_kg.type = type;
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ret = dev->cb(dev->devId, &cryptoInfo, dev->ctx);
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}
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return wc_CryptoCb_TranslateErrorCode(ret);
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}
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int wc_CryptoCb_PqcStatefulSigSign(const byte* msg, word32 msgSz, byte* out,
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word32* outSz, int type, void* key)
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{
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int ret = WC_NO_ERR_TRACE(CRYPTOCB_UNAVAILABLE);
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int devId = INVALID_DEVID;
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CryptoCb* dev;
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if (key == NULL)
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return ret;
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devId = wc_CryptoCb_PqcStatefulSigGetDevId(type, key);
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if (devId == INVALID_DEVID)
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return ret;
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dev = wc_CryptoCb_FindDevice(devId, WC_ALGO_TYPE_PK);
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if (dev && dev->cb) {
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wc_CryptoInfo cryptoInfo;
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XMEMSET(&cryptoInfo, 0, sizeof(cryptoInfo));
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cryptoInfo.algo_type = WC_ALGO_TYPE_PK;
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cryptoInfo.pk.type = WC_PK_TYPE_PQC_STATEFUL_SIG_SIGN;
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cryptoInfo.pk.pqc_stateful_sig_sign.msg = msg;
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cryptoInfo.pk.pqc_stateful_sig_sign.msgSz = msgSz;
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cryptoInfo.pk.pqc_stateful_sig_sign.out = out;
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cryptoInfo.pk.pqc_stateful_sig_sign.outSz = outSz;
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cryptoInfo.pk.pqc_stateful_sig_sign.key = key;
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cryptoInfo.pk.pqc_stateful_sig_sign.type = type;
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ret = dev->cb(dev->devId, &cryptoInfo, dev->ctx);
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}
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return wc_CryptoCb_TranslateErrorCode(ret);
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}
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int wc_CryptoCb_PqcStatefulSigVerify(const byte* sig, word32 sigSz,
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const byte* msg, word32 msgSz, int* res, int type, void* key)
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{
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int ret = WC_NO_ERR_TRACE(CRYPTOCB_UNAVAILABLE);
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int devId = INVALID_DEVID;
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CryptoCb* dev;
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if (key == NULL)
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return ret;
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devId = wc_CryptoCb_PqcStatefulSigGetDevId(type, key);
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if (devId == INVALID_DEVID)
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return ret;
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dev = wc_CryptoCb_FindDevice(devId, WC_ALGO_TYPE_PK);
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if (dev && dev->cb) {
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wc_CryptoInfo cryptoInfo;
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XMEMSET(&cryptoInfo, 0, sizeof(cryptoInfo));
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cryptoInfo.algo_type = WC_ALGO_TYPE_PK;
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cryptoInfo.pk.type = WC_PK_TYPE_PQC_STATEFUL_SIG_VERIFY;
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cryptoInfo.pk.pqc_stateful_sig_verify.sig = sig;
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cryptoInfo.pk.pqc_stateful_sig_verify.sigSz = sigSz;
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cryptoInfo.pk.pqc_stateful_sig_verify.msg = msg;
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cryptoInfo.pk.pqc_stateful_sig_verify.msgSz = msgSz;
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cryptoInfo.pk.pqc_stateful_sig_verify.res = res;
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cryptoInfo.pk.pqc_stateful_sig_verify.key = key;
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cryptoInfo.pk.pqc_stateful_sig_verify.type = type;
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ret = dev->cb(dev->devId, &cryptoInfo, dev->ctx);
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}
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return wc_CryptoCb_TranslateErrorCode(ret);
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}
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int wc_CryptoCb_PqcStatefulSigSigsLeft(int type, void* key, word32* sigsLeft)
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{
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int ret = WC_NO_ERR_TRACE(CRYPTOCB_UNAVAILABLE);
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int devId = INVALID_DEVID;
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CryptoCb* dev;
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if (key == NULL)
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return ret;
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devId = wc_CryptoCb_PqcStatefulSigGetDevId(type, key);
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if (devId == INVALID_DEVID)
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return ret;
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dev = wc_CryptoCb_FindDevice(devId, WC_ALGO_TYPE_PK);
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if (dev && dev->cb) {
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wc_CryptoInfo cryptoInfo;
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XMEMSET(&cryptoInfo, 0, sizeof(cryptoInfo));
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cryptoInfo.algo_type = WC_ALGO_TYPE_PK;
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cryptoInfo.pk.type = WC_PK_TYPE_PQC_STATEFUL_SIG_SIGS_LEFT;
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cryptoInfo.pk.pqc_stateful_sig_sigs_left.key = key;
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cryptoInfo.pk.pqc_stateful_sig_sigs_left.sigsLeft = sigsLeft;
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cryptoInfo.pk.pqc_stateful_sig_sigs_left.type = type;
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ret = dev->cb(dev->devId, &cryptoInfo, dev->ctx);
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}
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return wc_CryptoCb_TranslateErrorCode(ret);
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}
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#endif /* WOLFSSL_HAVE_LMS || WOLFSSL_HAVE_XMSS */
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#if defined(WOLFSSL_HAVE_MLKEM)
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int wc_CryptoCb_PqcKemGetDevId(int type, void* key)
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{

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