refactor: offchain reception on entity store

noir-projects/aztec-nr/aztec/src/messages/processing/offchain/message.nr

@@ -0,0 +1,75 @@
+use crate::{
+    entities::EntityBody,
+    messages::encoding::MESSAGE_CIPHERTEXT_LEN,
+    protocol::{address::AztecAddress, hash::{poseidon2_hash, sha256_to_field}, traits::{Deserialize, Serialize}},
+};
+
+/// Entity type id shared by every offchain message reception entity in the entity store.
+pub(crate) global OFFCHAIN_RECEPTION_ENTITY: Field = sha256_to_field("AZTEC_NR::OFFCHAIN_RECEPTION_ENTITY".as_bytes());
+
+/// A message delivered via the `offchain_receive` utility function.
+#[derive(Serialize, Deserialize)]
+pub struct OffchainMessage {
+    /// The encrypted message payload.
+    pub ciphertext: BoundedVec<Field, MESSAGE_CIPHERTEXT_LEN>,
+    /// The intended recipient of the message.
+    pub recipient: AztecAddress,
+    /// The hash of the transaction that produced this message. `Option::none` indicates a tx-less message.
+    pub tx_hash: Option<Field>,
+    /// Anchor block timestamp at message emission.
+    pub anchor_block_timestamp: u64,
+}
+
+/// Number of fields produced by serializing a `BoundedVec<Field, MESSAGE_CIPHERTEXT_LEN>`.
+///
+/// A `BoundedVec` serializes as its storage fields followed by a trailing length field, so the total is
+/// `MESSAGE_CIPHERTEXT_LEN + 1`.
+global SERIALIZED_CIPHERTEXT_LEN: u32 = MESSAGE_CIPHERTEXT_LEN + 1;
+
+impl EntityBody for OffchainMessage {
+    /// All offchain messages share the reception state-machine entity type.
+    fn entity_type_id() -> Field {
+        OFFCHAIN_RECEPTION_ENTITY
+    }
+
+    /// Computes the entity id that identifies this message's reception entity in the entity store.
+    ///
+    /// The id is a poseidon2 hash over the serialized ciphertext fields and the normalized tx hash (0 when absent).
+    /// Re-delivering the same `(ciphertext, tx_hash)` pair therefore maps to the same entity, making `receive`
+    /// idempotent. The `recipient` and `anchor_block_timestamp` fields are intentionally excluded, so identity depends
+    /// only on the message content and its originating transaction.
+    fn entity_id(self) -> Field {
+        let serialized_ciphertext: [Field; SERIALIZED_CIPHERTEXT_LEN] = self.ciphertext.serialize();
+        let mut inputs: [Field; SERIALIZED_CIPHERTEXT_LEN + 1] = [0; SERIALIZED_CIPHERTEXT_LEN + 1];
+        for i in 0..SERIALIZED_CIPHERTEXT_LEN {
+            inputs[i] = serialized_ciphertext[i];
+        }
+        inputs[SERIALIZED_CIPHERTEXT_LEN] = self.tx_hash.unwrap_or(0);
+        poseidon2_hash(inputs)
+    }
+}
+
+mod test {
+    use crate::{
+        messages::encoding::MESSAGE_CIPHERTEXT_LEN,
+        protocol::{address::AztecAddress, traits::{Deserialize, FromField, Serialize}},
+    };
+    use super::OffchainMessage;
+
+    #[test]
+    unconstrained fn pack_unpack_round_trips_offchain_message() {
+        let ciphertext: BoundedVec<Field, MESSAGE_CIPHERTEXT_LEN> = BoundedVec::new();
+        let msg = OffchainMessage {
+            ciphertext,
+            recipient: AztecAddress::from_field(42),
+            tx_hash: Option::some(99),
+            anchor_block_timestamp: 12345,
+        };
+        let packed = msg.serialize();
+        let roundtripped: OffchainMessage = Deserialize::deserialize(packed);
+        assert_eq(roundtripped.recipient, msg.recipient);
+        assert_eq(roundtripped.tx_hash, msg.tx_hash);
+        assert_eq(roundtripped.anchor_block_timestamp, msg.anchor_block_timestamp);
+        assert_eq(roundtripped.ciphertext.len(), msg.ciphertext.len());
+    }
+}