keystore: reduce SC operations in the seed check

verify_seed() after storing a seed currently re-stretches the
password, which needs 5 securechip events.

We can skip that because we still have access to the stretched
password.

If the check succeeds, but there was an error involving the password
or secure chip (e.g. the stretching result was corrupted the first
time), the consequence is that the user won't be able to unlock, but
the seed itself on the MCU, on the backup and retained (encrypted) in
seed is unaffected. That trade-off is good, because a failure
there (securechip returns no error but actually had a corruption)
should be exceedingly rare.

This effort is part of mitigating Optiga's throttling mechanism that
kicks in after 133 events - users can run into this by repeatedly
resetting/restoring).

Author: benma

src/keystore.c

@@ -208,20 +208,28 @@ static keystore_error_t _get_and_decrypt_seed(
 }
 
 static bool _verify_seed(
-    const char* password,
+    const uint8_t* encryption_key,
     const uint8_t* expected_seed,
     size_t expected_seed_len)
 {
-    uint8_t decrypted_seed[KEYSTORE_MAX_SEED_LENGTH] = {0};
-    size_t seed_len;
-    UTIL_CLEANUP_32(decrypted_seed);
-    if (_get_and_decrypt_seed(password, decrypted_seed, &seed_len, NULL) != KEYSTORE_OK) {
+    uint8_t encrypted_seed_and_hmac[96];
+    UTIL_CLEANUP_32(encrypted_seed_and_hmac);
+    uint8_t encrypted_len;
+    if (!memory_get_encrypted_seed_and_hmac(encrypted_seed_and_hmac, &encrypted_len)) {
+        return false;
+    }
+
+    size_t decrypted_len = encrypted_len - 48;
+    uint8_t decrypted[decrypted_len];
+    bool password_correct = cipher_aes_hmac_decrypt(
+        encrypted_seed_and_hmac, encrypted_len, decrypted, &decrypted_len, encryption_key);
+    if (!password_correct) {
         return false;
     }
-    if (expected_seed_len != seed_len) {
+    if (expected_seed_len != decrypted_len) {
         return false;
     }
-    if (!MEMEQ(expected_seed, decrypted_seed, seed_len)) {
+    if (!MEMEQ(expected_seed, decrypted, expected_seed_len)) {
         return false;
     }
     return true;
@@ -356,7 +364,7 @@ keystore_error_t keystore_encrypt_and_store_seed(
     if (!memory_set_encrypted_seed_and_hmac(encrypted_seed, encrypted_seed_len_u8)) {
         return KEYSTORE_ERR_MEMORY;
     }
-    if (!_verify_seed(password, seed, seed_length)) {
+    if (!_verify_seed(secret, seed, seed_length)) {
         if (!memory_reset_hww()) {
             return KEYSTORE_ERR_MEMORY;
         }

src/rust/bitbox02-rust/src/hww/api/restore.rs

@@ -200,7 +200,7 @@ mod tests {
             )),
             Ok(Response::Success(pb::Success {}))
         );
-        assert_eq!(bitbox02::securechip::fake_event_counter(), 13);
+        assert_eq!(bitbox02::securechip::fake_event_counter(), 8);
         drop(mock_hal); // to remove mutable borrow of counter
         assert_eq!(counter, 2);
         assert!(!keystore::is_locked());

src/rust/bitbox02-rust/src/hww/api/set_password.rs

@@ -80,7 +80,7 @@ mod tests {
             )),
             Ok(Response::Success(pb::Success {}))
         );
-        assert_eq!(bitbox02::securechip::fake_event_counter(), 14);
+        assert_eq!(bitbox02::securechip::fake_event_counter(), 9);
         drop(mock_hal); // to remove mutable borrow of counter
         assert_eq!(counter, 2);
         assert!(!keystore::is_locked());

src/rust/bitbox02-rust/src/keystore.rs

@@ -577,7 +577,7 @@ mod tests {
 
             bitbox02::securechip::fake_event_counter_reset();
             assert!(keystore::encrypt_and_store_seed(seed, "foo").is_ok());
-            assert_eq!(bitbox02::securechip::fake_event_counter(), 12);
+            assert_eq!(bitbox02::securechip::fake_event_counter(), 7);
 
             assert!(keystore::is_locked());