# F-CT-MONTANA-1 — Cache EVP_PKEY for SK lifetime in mt-crypto-native

**Severity.** HIGH (audit finding `F-CT-MONTANA-1`).
**Status.** Architectural change. In scope for v1.0.1 closure of `S-C1 / MONT-001`.
**Closure path.** Independent CT-pass over `mt-crypto-native` per
[`MAINNET-READINESS-v1.0.0.md §2.3`](../MAINNET-READINESS-v1.0.0.md).

## Motivation

The audit found that Montana's current FFI pattern bypasses OpenSSL's internal
`CONSTTIME_SECRET` / `CONSTTIME_DECLASSIFY` annotations:

1. `keypair_from_seed_mlkem(seed)` calls `EVP_PKEY_keygen` → internally
   OpenSSL marks the secret-key polynomial as `CONSTTIME_SECRET`.
2. Montana then extracts `OSSL_PKEY_PARAM_PRIV_KEY` to **raw bytes**.
3. On every sign / decap call, Montana rebuilds the `EVP_PKEY*` from the raw
   bytes via `EVP_PKEY_fromdata(KEYPAIR)`. This re-import path:
   - hits `ossl_ml_kem_parse_private_key` (8 CT-relevant contexts in our valgrind run);
   - hits `ml_dsa_key.c:485` (`memcmp(out->priv_encoding, sk, sk_len)` — real CT leak
     because `sk` here is operator's secret material);
   - hits `ml_dsa_key.c:277` (when `EVP_PKEY_eq` is invoked during fromdata).

By **caching the `EVP_PKEY*` from the moment of keygen** through the entire
process lifetime (or identity-file unlock lifetime), Montana avoids the
re-import path entirely. Every sign / decap call after the first one operates
on the already-initialised `EVP_PKEY` opaque handle — inside which OpenSSL's
internal CONSTTIME annotations apply.

## API change

`mt-crypto-native` adds three new opaque-handle types:

```c
/* mt_crypto.h additions */

typedef struct mt_mldsa_signing_key_st mt_mldsa_signing_key;
typedef struct mt_mldsa_verify_key_st  mt_mldsa_verify_key;
typedef struct mt_mlkem_decap_key_st   mt_mlkem_decap_key;

/* Constructors take seed once, allocate & retain EVP_PKEY internally. */
mt_mldsa_signing_key *mt_mldsa_signing_key_from_seed(const uint8_t seed[32]);
void                  mt_mldsa_signing_key_free(mt_mldsa_signing_key *k);

mt_mldsa_verify_key  *mt_mldsa_verify_key_from_public(const uint8_t pk[1952]);
void                  mt_mldsa_verify_key_free(mt_mldsa_verify_key *k);

mt_mlkem_decap_key   *mt_mlkem_decap_key_from_seed(const uint8_t seed[64]);
void                  mt_mlkem_decap_key_free(mt_mlkem_decap_key *k);

/* Hot-path operations now take opaque handles, NOT raw bytes. */
int mt_mldsa_sign_with(const mt_mldsa_signing_key *k,
                       const uint8_t *msg, size_t msg_len,
                       uint8_t sig_out[3309]);

int mt_mldsa_verify_with(const mt_mldsa_verify_key *k,
                         const uint8_t *msg, size_t msg_len,
                         const uint8_t sig[3309]);

int mt_mlkem_decap_with(const mt_mlkem_decap_key *k,
                        const uint8_t ct[1088], uint8_t ss_out[32]);
```

The old `mt_sign_mldsa(sk_bytes, msg, msg_len, sig_out)` API is **retained**
as a thin compatibility shim — it constructs a transient `mt_mldsa_signing_key`,
signs, frees. This keeps existing call sites working until they migrate.

## Rust-side change (`crates/mt-crypto/src/lib.rs`)

```diff
+use std::sync::Arc;
+
 pub struct SecretKey(Box<[u8; SECRET_KEY_SIZE]>);
+
+/// Live ML-DSA-65 signing context. Wraps a long-lived `EVP_PKEY*` that
+/// was constructed by `mt_keypair_from_seed_mldsa`. Sign operations hit
+/// only the constant-time annotated paths inside OpenSSL.
+pub struct SigningContext {
+    handle: Arc<MtMldsaSigningKeyHandle>,
+}
+
+impl SigningContext {
+    pub fn from_seed(seed: &[u8; KEYPAIR_SEED_SIZE]) -> Result<Self, CryptoError> {
+        let handle = unsafe { mt_mldsa_signing_key_from_seed(seed.as_ptr()) };
+        if handle.is_null() { return Err(CryptoError::KeygenFailed); }
+        Ok(SigningContext { handle: Arc::new(MtMldsaSigningKeyHandle(handle)) })
+    }
+
+    pub fn sign(&self, msg: &[u8]) -> Result<Signature, CryptoError> {
+        let mut sig = [0u8; SIGNATURE_SIZE];
+        let r = unsafe {
+            mt_mldsa_sign_with(self.handle.0, msg.as_ptr(), msg.len(), sig.as_mut_ptr())
+        };
+        if r != MT_OK { return Err(CryptoError::from_code(r)); }
+        Ok(Signature(sig))
+    }
+}
+
+struct MtMldsaSigningKeyHandle(*mut mt_mldsa_signing_key);
+
+// SAFETY: the opaque handle is reference-counted via Arc; the underlying
+// EVP_PKEY is allocated by OpenSSL and is not Send-unsafe.
+unsafe impl Send for MtMldsaSigningKeyHandle {}
+unsafe impl Sync for MtMldsaSigningKeyHandle {}
+
+impl Drop for MtMldsaSigningKeyHandle {
+    fn drop(&mut self) {
+        unsafe { mt_mldsa_signing_key_free(self.0); }
+    }
+}
+
+extern "C" {
+    fn mt_mldsa_signing_key_from_seed(seed: *const u8) -> *mut mt_mldsa_signing_key;
+    fn mt_mldsa_signing_key_free(k: *mut mt_mldsa_signing_key);
+    fn mt_mldsa_sign_with(k: *const mt_mldsa_signing_key,
+                          msg: *const u8, msg_len: usize,
+                          sig_out: *mut u8) -> c_int;
+}
+#[repr(C)] pub struct mt_mldsa_signing_key { _private: [u8; 0] }
```

## C-side change (`crates/mt-crypto-native/csrc/mt_crypto.c`)

```diff
+struct mt_mldsa_signing_key_st {
+    EVP_PKEY *pkey;       /* retained from EVP_PKEY_generate */
+};
+
+MT_EXPORT mt_mldsa_signing_key *mt_mldsa_signing_key_from_seed(
+    const uint8_t seed[MT_MLDSA65_SEED_SIZE]
+) {
+    if (seed == NULL) return NULL;
+    EVP_PKEY_CTX *ctx = EVP_PKEY_CTX_new_from_name(NULL, "ML-DSA-65", NULL);
+    if (ctx == NULL) return NULL;
+    if (EVP_PKEY_keygen_init(ctx) != 1) { EVP_PKEY_CTX_free(ctx); return NULL; }
+    OSSL_PARAM params[2];
+    params[0] = OSSL_PARAM_construct_octet_string(
+        OSSL_PKEY_PARAM_ML_DSA_SEED, (void*)seed, MT_MLDSA65_SEED_SIZE);
+    params[1] = OSSL_PARAM_construct_end();
+    if (EVP_PKEY_CTX_set_params(ctx, params) != 1) {
+        EVP_PKEY_CTX_free(ctx); return NULL;
+    }
+    EVP_PKEY *pkey = NULL;
+    if (EVP_PKEY_generate(ctx, &pkey) != 1 || pkey == NULL) {
+        EVP_PKEY_CTX_free(ctx); return NULL;
+    }
+    EVP_PKEY_CTX_free(ctx);
+    mt_mldsa_signing_key *k = OPENSSL_zalloc(sizeof(*k));
+    if (k == NULL) { EVP_PKEY_free(pkey); return NULL; }
+    k->pkey = pkey;
+    return k;
+}
+
+MT_EXPORT void mt_mldsa_signing_key_free(mt_mldsa_signing_key *k) {
+    if (k == NULL) return;
+    EVP_PKEY_free(k->pkey);  /* clears internal SK material via OpenSSL_clear_free */
+    OPENSSL_clear_free(k, sizeof(*k));
+}
+
+MT_EXPORT int mt_mldsa_sign_with(
+    const mt_mldsa_signing_key *k,
+    const uint8_t *msg, size_t msg_len,
+    uint8_t sig_out[MT_MLDSA65_SIGNATURE_SIZE]
+) {
+    if (k == NULL || k->pkey == NULL || sig_out == NULL
+        || (msg == NULL && msg_len != 0)) {
+        return MT_ERR_INVALID_INPUT;
+    }
+    EVP_MD_CTX *md_ctx = EVP_MD_CTX_new();
+    if (md_ctx == NULL) return MT_ERR_OPENSSL_INIT;
+    int rc = MT_ERR_SIGN_FAILED;
+    OSSL_PARAM sig_params[2];
+    int deterministic = 1;
+    sig_params[0] = OSSL_PARAM_construct_int(
+        OSSL_SIGNATURE_PARAM_DETERMINISTIC, &deterministic);
+    sig_params[1] = OSSL_PARAM_construct_end();
+    if (EVP_DigestSignInit_ex(md_ctx, NULL, NULL, NULL, NULL, k->pkey, sig_params) != 1) {
+        goto cleanup;
+    }
+    size_t sig_len = MT_MLDSA65_SIGNATURE_SIZE;
+    if (EVP_DigestSign(md_ctx, sig_out, &sig_len, msg, msg_len) != 1) {
+        goto cleanup;
+    }
+    if (sig_len != MT_MLDSA65_SIGNATURE_SIZE) { rc = MT_ERR_SIGN_LENGTH_MISMATCH; goto cleanup; }
+    rc = MT_OK;
+cleanup:
+    EVP_MD_CTX_free(md_ctx);
+    return rc;
+}
```

## ML-KEM analogous

The same pattern applies to `mt_mlkem_decap_key`. The `EVP_PKEY*` returned by
`EVP_PKEY_generate` for ML-KEM-768 is held for the lifetime of the identity;
every decap call hits `EVP_PKEY_decapsulate(k->pkey, ...)` without going
through `EVP_PKEY_fromdata`.

This eliminates the 8 CT-violation contexts in `ossl_ml_kem_parse_private_key`
we observed in the valgrind run (§11.3.6 of the audit report).

## What happens to the existing `SecretKey` type

`SecretKey(Box<[u8; 4032]>)` is retained for:
- mnemonic-recovery flow (regenerate from seed, then convert to `SigningContext`);
- identity-file persistence (load 4032 bytes from `~/.montana/identity.bin`).

After loading, the application **immediately** constructs `SigningContext::from_seed(...)`
and drops the raw `SecretKey`. The hot path (consensus signing, libp2p Noise handshake)
holds only `SigningContext`.

## Verification plan

1. `cargo test -p mt-crypto --test security_invariants` — 13 invariants must still pass;
   add new invariant `signing_context_is_not_clone`.
2. `cargo test -p mt-crypto-native --test nist_acvp_kat` — all 3 NIST KATs must
   pass byte-exact through the new `mt_mldsa_sign_with` path.
3. Re-run the ctgrind from §11.3.6 with the new flow:
   ```bash
   valgrind --tool=memcheck --track-origins=yes /tmp/ctgrind_mlkem_cached
   ```
   Expected: **0 errors** in `ossl_ml_kem_parse_private_key` (path never hit).

## Estimated effort

~ 12 hours of focused work: 4h for the C API, 3h for the Rust wrapper, 2h for
migrating call sites in `mt-mnemonic`, `montana-node`, `mt-noise-pq` to use
`SigningContext` instead of raw `SecretKey`, 3h for testing + ctgrind re-run.