fix(agglayer): enforce NoteType::Public for B2AGG bridge-out notes

CHANGELOG.md

@@ -38,6 +38,7 @@
 - Fixed `output_note::add_asset` and `output_note::set_attachment` to no longer accept invalid note indices ([#2824](https://github.com/0xMiden/protocol/pull/2824)).
 - [BREAKING] Keyed AggLayer faucet token registry by `(origin_token_address, origin_network)` instead of `origin_token_address` alone, preventing same-address cross-network mint collisions on CLAIM ([#2860](https://github.com/0xMiden/protocol/pull/2860)).
 - [BREAKING] Replaced `NoAuth` with the new `AuthNetworkAccount` auth component on the AggLayer bridge and AggLayer faucet, closing the forged-MINT attack surface where any transaction against the bridge could emit a bridge-authored MINT note ([#2797](https://github.com/0xMiden/protocol/issues/2797), [#2818](https://github.com/0xMiden/protocol/pull/2818)).
+- Enforced `NoteType::Public` for B2AGG bridge-out notes so a recipient-identical private note can no longer be folded into the Local Exit Tree and desync the aggkit mirror ([#2988](https://github.com/0xMiden/protocol/pull/2988)).
 
 ## 0.14.3 (2026-04-07)
 

crates/miden-agglayer/asm/agglayer/bridge/bridge_out.masm

@@ -22,6 +22,7 @@ use agglayer::common::eth_address::EthereumAddressFormat
 # =================================================================================================
 
 const ERR_B2AGG_DESTINATION_NETWORK_IS_MIDEN = "B2AGG note destination network ID must not be Miden's AggLayer network ID"
+const ERR_B2AGG_NOTE_MUST_BE_PUBLIC = "B2AGG note bridged to AggLayer must be of public note type"
 
 # CONSTANTS
 # =================================================================================================
@@ -97,6 +98,11 @@ const BURN_NOTE_NUM_STORAGE_ITEMS=0
 #! 4. Computes Keccak hash of the leaf data and appends it to the Local Exit Tree
 #! 5. Creates a BURN note with the bridged out asset
 #!
+#! The note being bridged out must be public: AggLayer's off-chain indexer (aggkit) mirrors the
+#! Local Exit Tree by observing notes in the public block space, so the appended leaf's pre-image
+#! must be publicly observable. The note type lives in the metadata header rather than the recipient
+#! commitment, so it is checked explicitly before the note's data is trusted.
+#!
 #! Inputs:  [ASSET_KEY, ASSET_VALUE, dest_network_id, dest_address(5), pad(2)]
 #! Outputs: [pad(16)]
 #!
@@ -107,11 +113,16 @@ const BURN_NOTE_NUM_STORAGE_ITEMS=0
 #! - dest_address(5) are 5 u32 values representing a 20-byte Ethereum address.
 #!
 #! Panics if:
+#! - the note being bridged out is not public.
 #! - destination network ID is Miden's AggLayer network ID.
 #!
 #! Invocation: call
 @locals(15)
 pub proc bridge_out
+    # The note being bridged out must be public (see above); checked before its data is trusted.
+    exec.active_note::is_public assert.err=ERR_B2AGG_NOTE_MUST_BE_PUBLIC
+    # => [ASSET_KEY, ASSET_VALUE, dest_network_id, dest_address(5), pad(2)]
+
     # Save ASSET to local memory for later BURN note creation
     locaddr.BRIDGE_OUT_BURN_ASSET_LOC
     exec.asset::store

crates/miden-agglayer/asm/note_scripts/B2AGG.masm

@@ -50,6 +50,7 @@ const ERR_B2AGG_TARGET_ACCOUNT_MISMATCH="B2AGG note attachment target account do
 #! - The note does not contain exactly 6 storage items.
 #! - The note does not contain exactly 1 asset.
 #! - The note attachment does not target the consuming account.
+#! - The note is not public (enforced by `bridge_out`).
 #! - The destination network ID equals Miden's AggLayer network ID.
 begin
     dropw

crates/miden-protocol/asm/protocol/active_note.masm

@@ -16,6 +16,14 @@ const ERR_NOTE_DATA_DOES_NOT_MATCH_COMMITMENT="note data does not match the comm
 
 const ERR_NOTE_INVALID_NUMBER_OF_STORAGE_ITEMS="the specified number of note storage items does not match the actual number"
 
+# CONSTANTS
+# =================================================================================================
+
+# Note type encoding, mirroring the Rust `NoteType` definition in `note/note_type.rs`
+# (`PUBLIC = 0b01`, `PRIVATE = 0b10`).
+const NOTE_TYPE_PUBLIC = 1
+const NOTE_TYPE_PRIVATE = 2
+
 # ACTIVE NOTE PROCEDURES
 # =================================================================================================
 #
@@ -166,6 +174,66 @@ pub proc get_metadata
     # => [NOTE_ATTACHMENT, METADATA_HEADER]
 end
 
+#! Returns whether the active note is public.
+#!
+#! The note type is stored in the metadata header rather than the recipient commitment, so this
+#! reads the active note's metadata and compares the note type against the public encoding.
+#!
+#! Inputs:  []
+#! Outputs: [is_public]
+#!
+#! Where:
+#! - is_public is 1 if the active note is public, 0 otherwise.
+#!
+#! Panics if:
+#! - no note is currently active.
+#!
+#! Invocation: exec
+pub proc is_public
+    exec.get_metadata
+    # => [NOTE_ATTACHMENT, METADATA_HEADER]
+
+    # drop the attachment word, keeping the metadata header
+    dropw
+    # => [METADATA_HEADER]
+
+    exec.note::extract_note_type_from_metadata
+    # => [note_type]
+
+    eq.NOTE_TYPE_PUBLIC
+    # => [is_public]
+end
+
+#! Returns whether the active note is private.
+#!
+#! The note type is stored in the metadata header rather than the recipient commitment, so this
+#! reads the active note's metadata and compares the note type against the private encoding.
+#!
+#! Inputs:  []
+#! Outputs: [is_private]
+#!
+#! Where:
+#! - is_private is 1 if the active note is private, 0 otherwise.
+#!
+#! Panics if:
+#! - no note is currently active.
+#!
+#! Invocation: exec
+pub proc is_private
+    exec.get_metadata
+    # => [NOTE_ATTACHMENT, METADATA_HEADER]
+
+    # drop the attachment word, keeping the metadata header
+    dropw
+    # => [METADATA_HEADER]
+
+    exec.note::extract_note_type_from_metadata
+    # => [note_type]
+
+    eq.NOTE_TYPE_PRIVATE
+    # => [is_private]
+end
+
 #! Returns the sender of the active note.
 #!
 #! Inputs:  []

crates/miden-protocol/asm/protocol/note.masm

@@ -217,3 +217,30 @@ pub proc extract_attachment_info_from_metadata
     u32split swap
     # => [attachment_kind, attachment_scheme]
 end
+
+#! Extracts the note type from the provided metadata header.
+#!
+#! Inputs:  [METADATA_HEADER]
+#! Outputs: [note_type]
+#!
+#! Where:
+#! - METADATA_HEADER is the metadata of a note.
+#! - note_type is the note type of the note. It is stored in the low byte of the merged
+#!   sender_id_suffix_and_note_type felt and is not part of the recipient commitment.
+#!
+#! Invocation: exec
+pub proc extract_note_type_from_metadata
+    # => [sender_id_suffix_and_note_type, sender_id_prefix, tag, attachment_kind_scheme]
+
+    # keep the merged suffix/note_type felt and drop the other three header felts
+    movdn.3 drop drop drop
+    # => [sender_id_suffix_and_note_type]
+
+    # take the low 32 bits of the felt
+    u32split swap drop
+    # => [suffix_and_note_type_lo]
+
+    # mask to the low byte, which holds the note type
+    u32and.0xff
+    # => [note_type]
+end

crates/miden-testing/src/kernel_tests/tx/test_active_note.rs

@@ -21,12 +21,12 @@ use miden_protocol::testing::account_id::{
     ACCOUNT_ID_SENDER,
 };
 use miden_protocol::transaction::memory::{ASSET_SIZE, ASSET_VALUE_OFFSET};
-use miden_protocol::{EMPTY_WORD, Felt, ONE, WORD_SIZE, Word};
+use miden_protocol::{EMPTY_WORD, Felt, ONE, WORD_SIZE, Word, ZERO};
 use miden_standards::code_builder::CodeBuilder;
 use miden_standards::testing::mock_account::MockAccountExt;
 
 use crate::kernel_tests::tx::ExecutionOutputExt;
-use crate::utils::create_public_p2any_note;
+use crate::utils::{create_p2any_note, create_public_p2any_note};
 use crate::{
     Auth,
     MockChain,
@@ -128,6 +128,74 @@ async fn test_active_note_get_metadata() -> anyhow::Result<()> {
     Ok(())
 }
 
+/// `is_public` / `is_private` return the correct flag for the active note's type.
+#[rstest::rstest]
+#[case::public(NoteType::Public)]
+#[case::private(NoteType::Private)]
+#[tokio::test]
+async fn test_active_note_is_public_and_is_private(
+    #[case] note_type: NoteType,
+) -> anyhow::Result<()> {
+    let tx_context = {
+        let account =
+            Account::mock(ACCOUNT_ID_REGULAR_PUBLIC_ACCOUNT_UPDATABLE_CODE, Auth::IncrNonce);
+        let mut rng = RandomCoin::new(Default::default());
+        let input_note = create_p2any_note(
+            ACCOUNT_ID_SENDER.try_into().unwrap(),
+            note_type,
+            [FungibleAsset::mock(100)],
+            &mut rng,
+        );
+        TransactionContextBuilder::new(account)
+            .extend_input_notes(vec![input_note])
+            .build()?
+    };
+
+    // The Rust and MASM `NoteType` encodings are identical, so derive the expected flags from the
+    // note type rather than passing them as separate test cases.
+    let (expected_public, expected_private) = match note_type {
+        NoteType::Public => (ONE, ZERO),
+        NoteType::Private => (ZERO, ONE),
+    };
+
+    let code = format!(
+        r#"
+        use $kernel::prologue
+        use $kernel::note->note_internal
+        use miden::protocol::active_note
+
+        begin
+            exec.prologue::prepare_transaction
+            exec.note_internal::prepare_note
+            dropw dropw dropw dropw
+
+            # check whether the active note is public
+            exec.active_note::is_public
+            # => [is_public]
+
+            push.{expected_public}
+            assert_eq.err="active note public flag did not match expected value"
+            # => []
+
+            # check whether the active note is private
+            exec.active_note::is_private
+            # => [is_private]
+
+            push.{expected_private}
+            assert_eq.err="active note private flag did not match expected value"
+            # => []
+
+            # truncate the stack
+            swapw dropw
+        end
+        "#,
+    );
+
+    tx_context.execute_code(&code).await?;
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn test_active_note_get_sender() -> anyhow::Result<()> {
     let tx_context = {

crates/miden-testing/tests/agglayer/bridge_out.rs

@@ -2,6 +2,7 @@ extern crate alloc;
 
 use miden_agglayer::errors::{
     ERR_B2AGG_DESTINATION_NETWORK_IS_MIDEN,
+    ERR_B2AGG_NOTE_MUST_BE_PUBLIC,
     ERR_B2AGG_TARGET_ACCOUNT_MISMATCH,
     ERR_FAUCET_NOT_REGISTERED,
 };
@@ -29,12 +30,13 @@ use miden_protocol::account::{
     StorageMapKey,
 };
 use miden_protocol::asset::{Asset, FungibleAsset};
-use miden_protocol::note::{NoteAssets, NoteType};
+use miden_protocol::errors::MasmError;
+use miden_protocol::note::{Note, NoteAssets, NoteAttachment, NoteMetadata, NoteType};
 use miden_protocol::transaction::RawOutputNote;
 use miden_protocol::{Felt, Word};
 use miden_standards::account::faucets::TokenMetadata;
 use miden_standards::account::mint_policies::OwnerControlledInitConfig;
-use miden_standards::note::StandardNote;
+use miden_standards::note::{NetworkAccountTarget, NoteExecutionHint, StandardNote};
 use miden_testing::{Auth, MockChain, assert_transaction_executor_error};
 use miden_tx::utils::hex_to_bytes;
 use rand::rngs::StdRng;
@@ -541,10 +543,37 @@ async fn test_bridge_out_fails_with_unregistered_faucet() -> anyhow::Result<()>
     Ok(())
 }
 
-/// B2AGG / bridge-out must reject a note whose `destination_network` equals the Miden network ID,
-/// even when the faucet is registered and the rest of the bridge-out path would otherwise succeed.
+/// The kind of malformation applied to an otherwise-valid B2AGG note in
+/// [`test_bridge_out_rejects_invalid_b2agg_note`].
+#[derive(Debug, Clone, Copy)]
+enum InvalidB2aggNote {
+    /// `destination_network` equals Miden's AggLayer network ID.
+    DestinationIsMiden,
+    /// Recipient-identical note marked `NoteType::Private` instead of public.
+    PrivateNoteType,
+}
+
+/// B2AGG / bridge-out must reject malformed notes even when the faucet is registered and the rest
+/// of the bridge-out path would otherwise succeed.
+///
+/// - `DestinationIsMiden`: the destination network equals Miden's own network ID, which
+///   `bridge_out` rejects outright.
+/// - `PrivateNoteType`: the note type lives in `NoteMetadata`, not in the recipient commitment, so
+///   an attacker can assemble a note with an identical recipient/attachment/asset but
+///   `NoteType::Private`. If accepted, the leaf would be folded into the on-chain Local Exit Tree
+///   while AggLayer's off-chain indexer (aggkit) could never recover the pre-image, permanently
+///   desyncing the LET mirror. The bridge branch must reject it.
+#[rstest::rstest]
+#[case::destination_is_miden(
+    InvalidB2aggNote::DestinationIsMiden,
+    ERR_B2AGG_DESTINATION_NETWORK_IS_MIDEN
+)]
+#[case::private_note_type(InvalidB2aggNote::PrivateNoteType, ERR_B2AGG_NOTE_MUST_BE_PUBLIC)]
 #[tokio::test]
-async fn test_bridge_out_fails_when_destination_is_miden_network() -> anyhow::Result<()> {
+async fn test_bridge_out_rejects_invalid_b2agg_note(
+    #[case] invalid_note: InvalidB2aggNote,
+    #[case] expected_err: MasmError,
+) -> anyhow::Result<()> {
     let mut builder = MockChain::builder();
 
     // CREATE BRIDGE ADMIN ACCOUNT (sends CONFIG_AGG_BRIDGE notes)
@@ -607,24 +636,47 @@ async fn test_bridge_out_fails_when_destination_is_miden_network() -> anyhow::Re
     )?;
     builder.add_output_note(RawOutputNote::Full(config_note.clone()));
 
-    // CREATE B2AGG NOTE (targets the bridge)
-    // Set destination_network to exactly `AggLayerBridge::MIDEN_NETWORK_ID` so `bridge_out`
-    // fails immediately.
+    // CREATE THE INVALID B2AGG NOTE (targets the bridge)
     // --------------------------------------------------------------------------------------------
     let amount = Felt::new(100);
     let bridge_asset: Asset =
         FungibleAsset::new(faucet.id(), amount.as_canonical_u64()).unwrap().into();
     let eth_address =
         EthAddress::from_hex(&vectors.destination_addresses[0]).expect("valid destination address");
 
-    let b2agg_note = B2AggNote::create(
-        AggLayerBridge::MIDEN_NETWORK_ID,
-        eth_address,
-        NoteAssets::new(vec![bridge_asset])?,
-        bridge_account.id(),
-        faucet.id(),
-        builder.rng_mut(),
-    )?;
+    let b2agg_note = match invalid_note {
+        // Destination network equals Miden's own network ID, which `bridge_out` rejects.
+        InvalidB2aggNote::DestinationIsMiden => B2AggNote::create(
+            AggLayerBridge::MIDEN_NETWORK_ID,
+            eth_address,
+            NoteAssets::new(vec![bridge_asset])?,
+            bridge_account.id(),
+            faucet.id(),
+            builder.rng_mut(),
+        )?,
+        // Build a legitimate (public) B2AGG note with a valid destination network, then assemble an
+        // attack note reusing its exact recipient and assets but flipping the note type to Private.
+        // The note type is not part of the recipient commitment, so the two notes share a recipient
+        // and are indistinguishable to consensus apart from the type bit.
+        InvalidB2aggNote::PrivateNoteType => {
+            let public_note = B2AggNote::create(
+                origin_network,
+                eth_address,
+                NoteAssets::new(vec![bridge_asset])?,
+                bridge_account.id(),
+                faucet.id(),
+                builder.rng_mut(),
+            )?;
+
+            let attachment = NoteAttachment::from(
+                NetworkAccountTarget::new(bridge_account.id(), NoteExecutionHint::Always)
+                    .map_err(|e| anyhow::anyhow!(e.to_string()))?,
+            );
+            let metadata =
+                NoteMetadata::new(faucet.id(), NoteType::Private).with_attachment(attachment);
+            Note::new(public_note.assets().clone(), metadata, public_note.recipient().clone())
+        },
+    };
 
     builder.add_output_note(RawOutputNote::Full(b2agg_note.clone()));
 
@@ -644,7 +696,7 @@ async fn test_bridge_out_fails_when_destination_is_miden_network() -> anyhow::Re
     mock_chain.add_pending_executed_transaction(&config_executed)?;
     mock_chain.prove_next_block()?;
 
-    // TX1: EXECUTE B2AGG NOTE AGAINST BRIDGE (must fail: destination_network is Miden's ID)
+    // TX1: EXECUTE THE INVALID B2AGG NOTE AGAINST BRIDGE (must fail)
     // --------------------------------------------------------------------------------------------
     let foreign_account_inputs = mock_chain.get_foreign_account_inputs(faucet.id())?;
 
@@ -655,7 +707,7 @@ async fn test_bridge_out_fails_when_destination_is_miden_network() -> anyhow::Re
         .execute()
         .await;
 
-    assert_transaction_executor_error!(result, ERR_B2AGG_DESTINATION_NETWORK_IS_MIDEN);
+    assert_transaction_executor_error!(result, expected_err);
 
     Ok(())
 }