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Fixes for STSAFE-A120 ECDHE #9703

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Merged
JacobBarthelmeh merged 1 commit into from
Jan 23, 2026
+45 −17
Merged

Fixes for STSAFE-A120 ECDHE #9703

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14 changes: 12 additions & 2 deletions wolfcrypt/src/port/st/README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -160,12 +160,22 @@ wolfSTSAFE_CryptoCb_Ctx stsafeCtx;
stsafeCtx.devId = WOLF_STSAFE_DEVID;
wc_CryptoCb_RegisterDevice(WOLF_STSAFE_DEVID, wolfSSL_STSAFE_CryptoDevCb, &stsafeCtx);

/* Use with ECC operations */
/* For ECDSA signing operations (uses persistent slot 1) */
ecc_key key;
wc_ecc_init_ex(&key, NULL, WOLF_STSAFE_DEVID);
/* ECC operations will now use STSAFE hardware */
wc_ecc_make_key_ex(&rng, 32, &key, ECC_SECP256R1);
/* Sign operations will use STSAFE hardware */

/* For ECDH operations (uses ephemeral slot 0xFF) */
ecc_key ecdh_key;
wc_ecc_init_ex(&ecdh_key, NULL, WOLF_STSAFE_DEVID);
ecdh_key.devCtx = (void*)(uintptr_t)STSAFE_KEY_SLOT_EPHEMERAL; /* Configure for ECDH */
wc_ecc_make_key_ex(&rng, 32, &ecdh_key, ECC_SECP256R1);
/* ECDH shared secret computation will use STSAFE hardware */
```

**Note for STSAFE-A120**: ECDH operations require keys generated in the ephemeral slot (0xFF) which has key establishment enabled by default. Set `key.devCtx = (void*)(uintptr_t)STSAFE_KEY_SLOT_EPHEMERAL;` to configure keys for ECDH before generation. Persistent slots (0-4) require explicit configuration via `put_attribute` command to enable key establishment.

### Implementation Details

The STSAFE support is self-contained in `wolfcrypt/src/port/st/stsafe.c` with SDK-specific implementations selected at compile time:
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48 changes: 33 additions & 15 deletions wolfcrypt/src/port/st/stsafe.c
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Original file line number Diff line number Diff line change
Expand Up @@ -273,8 +273,12 @@ int stsafe_interface_init(void)

/**
* \brief Generate ECC key pair on STSAFE-A120
* \details Uses dedicated key slot (slot 1) for persistent keys.
* \details Uses dedicated key slot for persistent keys (typically slot 0 or 1).
* For ephemeral ECDHE keys, use stsafe_create_ecdhe_key() instead.
*
* Note: For ECDH operations on persistent slots, the key must be generated
* with appropriate usage settings. Per ST FAE: slot 0xFF with usage_limit=1
* is recommended for ephemeral ECDH (key establishment mode).
*/
static int stsafe_create_key(stsafe_slot_t slot, stsafe_curve_id_t curve_id,
uint8_t* pPubKeyRaw)
Expand All @@ -288,7 +292,11 @@ static int stsafe_create_key(stsafe_slot_t slot, stsafe_curve_id_t curve_id,

/* Generate key pair - public key is X||Y concatenated
* Note: stse_generate_ecc_key_pair expects stse_ecc_key_type_t,
* but stsafe_curve_id_t values match stse_ecc_key_type_t enum values */
* but stsafe_curve_id_t values match stse_ecc_key_type_t enum values.
*
* For persistent keys: usage_limit=255 allows multiple operations (signing)
* For ephemeral keys (slot 0xFF): usage_limit=1 for key establishment mode
*/
ret = stse_generate_ecc_key_pair(&g_stse_handler, slot,
(stse_ecc_key_type_t)curve_id,
STSAFE_PERSISTENT_KEY_USAGE_LIMIT,
Expand All @@ -303,10 +311,11 @@ static int stsafe_create_key(stsafe_slot_t slot, stsafe_curve_id_t curve_id,

/**
* \brief Generate ECDHE ephemeral key pair on STSAFE-A120
* \details Uses stse_generate_ECDHE_key_pair() which generates truly
* ephemeral keys (not stored in slots). The private key remains
* in STSE internal memory for use with shared secret computation.
* Public key is returned in X||Y format (same as stse_generate_ecc_key_pair).
* \details Uses stse_generate_ecc_key_pair() with slot 0xFF (ephemeral slot)
* and usage_limit=1 for key establishment mode.
* Per ST FAE recommendation: slot 0xFF must be used with mode of
* operation = key establishment and usage limit = 1 for ECDH operations.
* Public key is returned in X||Y format.
*/
static int stsafe_create_ecdhe_key(stsafe_curve_id_t curve_id,
uint8_t* pPubKeyRaw)
Expand All @@ -318,11 +327,15 @@ static int stsafe_create_ecdhe_key(stsafe_curve_id_t curve_id,
return BAD_FUNC_ARG;
}

/* Generate ECDHE ephemeral key pair - public key returned as X||Y */
ret = stse_generate_ECDHE_key_pair(&g_stse_handler,
(stse_ecc_key_type_t)curve_id, pPubKeyRaw);
/* Generate ephemeral key pair in slot 0xFF with usage_limit=1
* This configures the key for key establishment mode */
ret = stse_generate_ecc_key_pair(&g_stse_handler,
STSAFE_KEY_SLOT_EPHEMERAL, /* slot 0xFF */
(stse_ecc_key_type_t)curve_id,
STSAFE_EPHEMERAL_KEY_USAGE_LIMIT, /* usage_limit = 1 */
pPubKeyRaw);
if (ret != STSE_OK) {
STSAFE_INTERFACE_PRINTF("stse_generate_ECDHE_key_pair error: %d\n", ret);
STSAFE_INTERFACE_PRINTF("stse_generate_ecc_key_pair (ephemeral) error: %d\n", ret);
rc = (int)ret;
}

Expand Down Expand Up @@ -1535,22 +1548,27 @@ int wolfSSL_STSAFE_CryptoDevCb(int devId, wc_CryptoInfo* info, void* ctx)
curve_id = stsafe_get_ecc_curve_id(ecc_curve);
key_sz = stsafe_get_key_size(curve_id);

/* For A120, generate keys in slot 1 (persistent slot) by default for ECDSA signing.
* For ECDH operations, the key slot from devCtx will be used directly.
* If ECDH is required, keys should be generated in the ephemeral slot from the start. */
/* For A120: Use ephemeral slot (0xFF) for ECDH/ECDHE operations.
* Use persistent slots (0-4) for ECDSA signing operations.
* Note: Persistent slots require key establishment to be enabled
* in their mode of operation flags via put_attribute command. */
#ifdef WOLFSSL_STSAFEA120
/* Retrieve slot from devCtx if available, otherwise use default */
slot = STSAFE_KEY_SLOT_1; /* Default fallback */
/* Check if this is for ECDH by looking at devCtx hint */
if (info->pk.eckg.key != NULL && info->pk.eckg.key->devCtx != NULL) {
slot = STSAFE_DEVCXT_TO_SLOT(info->pk.eckg.key->devCtx);
} else {
/* Default: Use slot 1 for ECDSA signing */
slot = STSAFE_KEY_SLOT_1;
}

STSAFE_INTERFACE_PRINTF("STSAFE: KeyGen slot %d, curve_id %d\n",
slot, curve_id);

/* Always use ephemeral slot (0xFF) for ECDH operations */
if (slot == STSAFE_KEY_SLOT_EPHEMERAL) {
rc = stsafe_create_ecdhe_key(curve_id, pubKeyRaw);
} else {
/* Persistent slot for signing */
rc = stsafe_create_key(slot, curve_id, pubKeyRaw);
}
if (rc != STSE_OK) {
Expand Down
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