keystore: pass seed to unlock_bip39() to reduce secure chip events

When restoring a wallet, the seed is already known, so no need to do
another `copy_seed()` which is a secure chip security event. This
reduces the number of secure chip operations when restoring.

The hashed seed is retained so it can be compared without storing it
in plaintext.

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

@@ -37,6 +37,9 @@ static uint8_t _unstretched_retained_seed_encryption_key[32] = {0};
 // Stores the encrypted seed after unlock.
 static uint8_t _retained_seed_encrypted[KEYSTORE_MAX_SEED_LENGTH + 64] = {0};
 static size_t _retained_seed_encrypted_len = 0;
+// A hash of the unencrypted retained seed, used for comparing seeds without knowing their
+// plaintext.
+static uint8_t _retained_seed_hash[32] = {0};
 
 // Change this ONLY via keystore_unlock_bip39().
 static bool _is_unlocked_bip39 = false;
@@ -224,6 +227,17 @@ static bool _verify_seed(
     return true;
 }
 
+static keystore_error_t _hash_seed(const uint8_t* seed, size_t seed_len, uint8_t* out)
+{
+    uint8_t salted_key[32] = {0};
+    if (!salt_hash_data(NULL, 0, "keystore_retain_seed_hash", salted_key)) {
+        return KEYSTORE_ERR_SALT;
+    }
+
+    rust_hmac_sha256(salted_key, sizeof(salted_key), seed, seed_len, out);
+    return KEYSTORE_OK;
+}
+
 USE_RESULT static keystore_error_t _retain_seed(const uint8_t* seed, size_t seed_len)
 {
 #ifdef TESTING
@@ -253,7 +267,8 @@ USE_RESULT static keystore_error_t _retain_seed(const uint8_t* seed, size_t seed
         return KEYSTORE_ERR_ENCRYPT;
     }
     _retained_seed_encrypted_len = len;
-    return KEYSTORE_OK;
+
+    return _hash_seed(seed, seed_len, _retained_seed_hash);
 }
 
 USE_RESULT static bool _retain_bip39_seed(const uint8_t* bip39_seed)
@@ -298,6 +313,8 @@ static void _delete_retained_seeds(void)
         sizeof(_unstretched_retained_seed_encryption_key));
     util_zero(_retained_seed_encrypted, sizeof(_retained_seed_encrypted));
     _retained_seed_encrypted_len = 0;
+    util_zero(_retained_seed_hash, sizeof(_retained_seed_hash));
+
     util_zero(
         _unstretched_retained_bip39_seed_encryption_key,
         sizeof(_unstretched_retained_seed_encryption_key));
@@ -455,17 +472,23 @@ keystore_error_t keystore_unlock(
     return result;
 }
 
-bool keystore_unlock_bip39(const char* mnemonic_passphrase, uint8_t* root_fingerprint_out)
+bool keystore_unlock_bip39(
+    const uint8_t* seed,
+    size_t seed_length,
+    const char* mnemonic_passphrase,
+    uint8_t* root_fingerprint_out)
 {
     if (!_is_unlocked_device) {
         return false;
     }
     usb_processing_timeout_reset(LONG_TIMEOUT);
 
-    uint8_t seed[KEYSTORE_MAX_SEED_LENGTH] = {0};
-    UTIL_CLEANUP_32(seed);
-    size_t seed_length = 0;
-    if (!keystore_copy_seed(seed, &seed_length)) {
+    uint8_t seed_hashed[32] = {0};
+    UTIL_CLEANUP_32(seed_hashed);
+    if (_hash_seed(seed, seed_length, seed_hashed) != KEYSTORE_OK) {
+        return false;
+    }
+    if (!MEMEQ(seed_hashed, _retained_seed_hash, sizeof(_retained_seed_hash))) {
         return false;
     }
 

src/keystore.h

@@ -104,14 +104,19 @@ USE_RESULT keystore_error_t keystore_create_and_store_seed(
 USE_RESULT keystore_error_t
 keystore_unlock(const char* password, uint8_t* remaining_attempts_out, int* securechip_result_out);
 
-/** Unlocks the bip39 seed.
+/** Unlocks the bip39 seed. The input seed must be the keystore seed (i.e. must match the output
+ * of `keystore_copy_seed()`).
+ * @param[in] seed the input seed to BIP39.
+ * @param[in] seed_length the size of the seed
  * @param[in] mnemonic_passphrase bip39 passphrase used in the derivation. Use the
  * empty string if no passphrase is needed or provided.
  * @param[out] root_fingerprint_out must be 4 bytes long and will contain the root fingerprint of
  * the wallet.
  * @return returns false if there was a critital memory error, otherwise true.
  */
 USE_RESULT bool keystore_unlock_bip39(
+    const uint8_t* seed,
+    size_t seed_length,
     const char* mnemonic_passphrase,
     uint8_t* root_fingerprint_out);
 

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

@@ -65,7 +65,8 @@ pub async fn from_file(
     }
 
     let password = password::enter_twice(hal).await?;
-    if let Err(err) = bitbox02::keystore::encrypt_and_store_seed(data.get_seed(), &password) {
+    let seed = data.get_seed();
+    if let Err(err) = bitbox02::keystore::encrypt_and_store_seed(seed, &password) {
         hal.ui()
             .status(&format!("Could not\nrestore backup\n{:?}", err), false)
             .await;
@@ -87,7 +88,7 @@ pub async fn from_file(
     // Ignore non-critical error.
     let _ = bitbox02::memory::set_device_name(&metadata.name);
 
-    unlock::unlock_bip39(hal).await;
+    unlock::unlock_bip39(hal, seed).await;
     Ok(Response::Success(pb::Success {}))
 }
 
@@ -157,7 +158,7 @@ pub async fn from_mnemonic(
 
     bitbox02::memory::set_initialized().or(Err(Error::Memory))?;
 
-    unlock::unlock_bip39(hal).await;
+    unlock::unlock_bip39(hal, &seed).await;
     Ok(Response::Success(pb::Success {}))
 }
 
@@ -199,7 +200,7 @@ mod tests {
             )),
             Ok(Response::Success(pb::Success {}))
         );
-        assert_eq!(bitbox02::securechip::fake_event_counter(), 14);
+        assert_eq!(bitbox02::securechip::fake_event_counter(), 13);
         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

@@ -40,7 +40,7 @@ pub async fn process(
         hal.ui().status(&format!("Error\n{:?}", err), false).await;
         return Err(Error::Generic);
     }
-    unlock::unlock_bip39(hal).await;
+    unlock::unlock_bip39(hal, &keystore::copy_seed()?).await;
     Ok(Response::Success(pb::Success {}))
 }
 

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

@@ -573,7 +573,7 @@ mod tests {
             keystore::lock();
             let seed = &seed[..test.seed_len];
 
-            assert!(keystore::unlock_bip39(test.mnemonic_passphrase).is_err());
+            assert!(keystore::unlock_bip39(seed, test.mnemonic_passphrase).is_err());
 
             bitbox02::securechip::fake_event_counter_reset();
             assert!(keystore::encrypt_and_store_seed(seed, "foo").is_ok());
@@ -582,8 +582,8 @@ mod tests {
             assert!(keystore::is_locked());
 
             bitbox02::securechip::fake_event_counter_reset();
-            assert!(keystore::unlock_bip39(test.mnemonic_passphrase).is_ok());
-            assert_eq!(bitbox02::securechip::fake_event_counter(), 2);
+            assert!(keystore::unlock_bip39(seed, test.mnemonic_passphrase).is_ok());
+            assert_eq!(bitbox02::securechip::fake_event_counter(), 1);
 
             assert!(!keystore::is_locked());
             assert_eq!(

src/rust/bitbox02-rust/src/workflow/unlock.rs

@@ -108,7 +108,7 @@ pub async fn unlock_keystore(
 
 /// Performs the BIP39 keystore unlock, including unlock animation. If the optional passphrase
 /// feature is enabled, the user will be asked for the passphrase.
-pub async fn unlock_bip39(hal: &mut impl crate::hal::Hal) {
+pub async fn unlock_bip39(hal: &mut impl crate::hal::Hal, seed: &[u8]) {
     // Empty passphrase by default.
     let mut mnemonic_passphrase = zeroize::Zeroizing::new("".into());
 
@@ -133,7 +133,8 @@ pub async fn unlock_bip39(hal: &mut impl crate::hal::Hal) {
         }
     }
 
-    let result = bitbox02::ui::with_lock_animation(|| keystore::unlock_bip39(&mnemonic_passphrase));
+    let result =
+        bitbox02::ui::with_lock_animation(|| keystore::unlock_bip39(seed, &mnemonic_passphrase));
     if result.is_err() {
         abort("bip39 unlock failed");
     }
@@ -160,6 +161,6 @@ pub async fn unlock(hal: &mut impl crate::hal::Hal) -> Result<(), ()> {
         .is_err()
     {}
 
-    unlock_bip39(hal).await;
+    unlock_bip39(hal, &bitbox02::keystore::copy_seed()?).await;
     Ok(())
 }

src/rust/bitbox02/src/keystore.rs

@@ -95,10 +95,12 @@ pub fn lock() {
     unsafe { ROOT_FINGERPRINT.write(None) }
 }
 
-pub fn unlock_bip39(mnemonic_passphrase: &str) -> Result<(), Error> {
+pub fn unlock_bip39(seed: &[u8], mnemonic_passphrase: &str) -> Result<(), Error> {
     let mut root_fingerprint = [0u8; 4];
     if unsafe {
         bitbox02_sys::keystore_unlock_bip39(
+            seed.as_ptr(),
+            seed.len(),
             crate::util::str_to_cstr_vec(mnemonic_passphrase)
                 .unwrap()
                 .as_ptr()
@@ -409,7 +411,7 @@ mod tests {
             .unwrap();
         assert!(encrypt_and_store_seed(&seed, "password").is_ok());
         assert!(is_locked()); // still locked, it is only unlocked after unlock_bip39.
-        assert!(unlock_bip39("foo").is_ok());
+        assert!(unlock_bip39(&seed, "foo").is_ok());
         assert!(!is_locked());
         lock();
         assert!(is_locked());
@@ -498,7 +500,10 @@ mod tests {
         assert!(root_fingerprint().is_err());
         assert!(encrypt_and_store_seed(&seed, "password").is_ok());
         assert!(root_fingerprint().is_err());
-        assert!(unlock_bip39("foo").is_ok());
+        // Incorrect seed passed
+        assert!(unlock_bip39(b"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", "foo").is_err());
+        // Correct seed passed.
+        assert!(unlock_bip39(&seed, "foo").is_ok());
         assert_eq!(root_fingerprint(), Ok(vec![0xf1, 0xbc, 0x3c, 0x46]),);
 
         let expected_bip39_seed = hex::decode("2b3c63de86f0f2b13cc6a36c1ba2314fbc1b40c77ab9cb64e96ba4d5c62fc204748ca6626a9f035e7d431bce8c9210ec0bdffc2e7db873dee56c8ac2153eee9a").unwrap();

src/rust/bitbox02/src/testing.rs

@@ -22,7 +22,7 @@ pub fn mock_unlocked_using_mnemonic(mnemonic: &str, passphrase: &str) {
     unsafe {
         bitbox02_sys::keystore_mock_unlocked(seed.as_ptr(), seed.len() as _, core::ptr::null())
     }
-    keystore::unlock_bip39(passphrase).unwrap();
+    keystore::unlock_bip39(&seed, passphrase).unwrap();
 }
 
 pub const TEST_MNEMONIC: &str = "purity concert above invest pigeon category peace tuition hazard vivid latin since legal speak nation session onion library travel spell region blast estate stay";