fix(audit): frame hash inputs with TLV

Justfile

@@ -365,6 +365,7 @@ test-unit:
     ./scripts/test-ensure-local-relay-key.sh
     if command -v cargo-nextest &>/dev/null; then
         cargo nextest run -p buzz-core -p buzz-auth --lib
+        cargo nextest run -p buzz-audit --lib
         # buzz-auth NIP-FI verifier doctests. The sealed-authority
         # `compile_fail` doctests prove the default-feature public API alone
         # cannot forge the issuer→JWKS authority; nextest does not run

crates/buzz-audit/src/entry.rs

@@ -16,10 +16,12 @@ pub struct AuditEntry {
     pub community_id: Uuid,
     /// Sequence number, monotonic within `community_id` (starts at 1).
     pub seq: i64,
+    /// Hash encoding: 1 for historical concatenation, 2 for TLV.
+    pub hash_version: i16,
     /// SHA-256 of this entry's fields including `community_id` and `prev_hash`.
     pub hash: Vec<u8>,
     /// SHA-256 of the previous entry in *this community's* chain, or `None` for
-    /// the community's first entry (hashed as [`crate::hash::GENESIS_HASH`]).
+    /// the community's first entry.
     pub prev_hash: Option<Vec<u8>>,
     /// Action that was performed.
     pub action: AuditAction,

crates/buzz-audit/src/error.rs

@@ -35,6 +35,13 @@ pub enum AuditError {
     #[error("unknown audit action in database")]
     UnknownAction,
 
+    /// The stored hash encoding version is not supported.
+    #[error("unsupported audit hash version {version}")]
+    UnsupportedHashVersion {
+        /// Unrecognised encoding version.
+        version: i16,
+    },
+
     /// A JSON serialization error occurred (e.g. while canonicalising `detail`).
     #[error("serialization error: {0}")]
     Serialization(#[from] serde_json::Error),
@@ -69,6 +76,7 @@ mod tests {
             AuditError::ChainViolation { seq: 7 },
             AuditError::HashMismatch { seq: 42 },
             AuditError::UnknownAction,
+            AuditError::UnsupportedHashVersion { version: 99 },
         ];
 
         for err in &domain_errors {

crates/buzz-audit/src/hash.rs

@@ -5,9 +5,12 @@ use crate::entry::AuditEntry;
 use crate::error::AuditError;
 
 /// The 32-byte sentinel hashed in place of `prev_hash` for a community's first
-/// entry. Stored as `prev_hash = NULL`; hashed as all-zero bytes.
+/// entry in the legacy encoding. TLV omits the absent field instead.
 pub const GENESIS_HASH: [u8; 32] = [0u8; 32];
 
+/// Encoding used for new entries. Version 1 is retained for historical rows.
+pub const CURRENT_HASH_VERSION: i16 = 2;
+
 /// Reduce a timestamp to the precision the audit store round-trips.
 ///
 /// `audit_log.created_at` is `TIMESTAMPTZ`, which Postgres keeps at microsecond
@@ -23,23 +26,54 @@ pub fn to_storage_precision(created_at: DateTime<Utc>) -> DateTime<Utc> {
     created_at.trunc_subsecs(6)
 }
 
-/// SHA-256 over the entry's identity, chain, and context fields.
-///
-/// Field order is fixed — changing it invalidates all existing chains. The
-/// `community_id` is hashed first so chain identity carries the tenant: an entry
-/// cannot be lifted out of one community's chain and re-verified inside another.
-///
-/// `created_at` is normalized through [`to_storage_precision`] here rather than
-/// hashed as given. Write paths truncate before storing so the row matches the
-/// in-memory entry, but normalizing again at the single point that consumes the
-/// value means no future caller can reintroduce the write/read preimage split
-/// by forgetting to. Values already at storage precision are unaffected —
-/// truncation is idempotent — so this does not change any digest.
+/// SHA-256 over the entry's fields using its stored encoding version.
 ///
-/// `detail` is serialized via [`canonical_json`] (sorted keys) so the hash is
-/// stable across machines and Rust versions. A serialization failure is a hard
-/// error, never silently hashed as empty.
+/// Version 2 uses one-byte tags and big-endian u64 byte lengths. Fields are
+/// appended in tag order; absent optional fields are omitted, while present
+/// empty values retain their tag and zero length. JSON keys are sorted and
+/// timestamps are truncated to storage precision before encoding.
 pub fn compute_hash(entry: &AuditEntry) -> Result<[u8; 32], AuditError> {
+    match entry.hash_version {
+        1 => compute_legacy_hash(entry),
+        CURRENT_HASH_VERSION => compute_tlv_hash(entry),
+        version => Err(AuditError::UnsupportedHashVersion { version }),
+    }
+}
+
+fn compute_tlv_hash(entry: &AuditEntry) -> Result<[u8; 32], AuditError> {
+    let mut encoded = b"buzz:audit:v2\0".to_vec();
+    append_tlv(&mut encoded, 1, entry.community_id.as_bytes());
+    append_tlv(&mut encoded, 2, &entry.seq.to_be_bytes());
+    append_tlv(
+        &mut encoded,
+        3,
+        to_storage_precision(entry.created_at)
+            .to_rfc3339()
+            .as_bytes(),
+    );
+    append_tlv(&mut encoded, 4, entry.action.as_str().as_bytes());
+    if let Some(pk) = &entry.actor_pubkey {
+        append_tlv(&mut encoded, 5, pk);
+    }
+    if let Some(id) = &entry.object_id {
+        append_tlv(&mut encoded, 6, id.as_bytes());
+    }
+    append_tlv(&mut encoded, 7, canonical_json(&entry.detail)?.as_bytes());
+    if let Some(hash) = &entry.prev_hash {
+        append_tlv(&mut encoded, 8, hash);
+    }
+    Ok(Sha256::digest(encoded).into())
+}
+
+fn append_tlv(encoded: &mut Vec<u8>, tag: u8, value: &[u8]) {
+    encoded.push(tag);
+    encoded.extend_from_slice(&(value.len() as u64).to_be_bytes());
+    encoded.extend_from_slice(value);
+}
+
+// Preserve historical digests. This encoding retains its original ambiguity;
+// only the version 2 writer prevents bytes from moving between fields.
+fn compute_legacy_hash(entry: &AuditEntry) -> Result<[u8; 32], AuditError> {
     let mut hasher = Sha256::new();
     // Tenant binding: community_id leads the hash.
     hasher.update(entry.community_id.as_bytes());
@@ -124,6 +158,7 @@ mod tests {
 
     fn sample_entry() -> AuditEntry {
         AuditEntry {
+            hash_version: CURRENT_HASH_VERSION,
             community_id: Uuid::from_u128(1),
             seq: 1,
             hash: Vec::new(),
@@ -156,6 +191,25 @@ mod tests {
         assert_eq!(compute_hash(&entry).unwrap().len(), 32);
     }
 
+    #[test]
+    fn encoding_versions_have_stable_digests() {
+        let mut entry = sample_entry();
+        assert_eq!(
+            hex::encode(compute_hash(&entry).unwrap()),
+            "e12c899d941af9794679602ae83ef2ab1c8f029fe380349263a046a93374d580"
+        );
+        entry.hash_version = 1;
+        assert_eq!(
+            hex::encode(compute_hash(&entry).unwrap()),
+            "5fd6c48ddbd39979bd7fcc94357d78b5fcffa0d1443a0cdda997ad7e22f4f2ce"
+        );
+        entry.hash_version = 99;
+        assert!(matches!(
+            compute_hash(&entry),
+            Err(AuditError::UnsupportedHashVersion { version: 99 })
+        ));
+    }
+
     #[test]
     fn storage_precision_drops_sub_microsecond_digits() {
         let stored = to_storage_precision(nanosecond_instant());
@@ -254,13 +308,46 @@ mod tests {
     }
 
     #[test]
-    fn presence_tag_distinguishes_none_from_empty() {
-        // Some(empty) must not collide with None — the presence tag prevents it.
+    fn omitted_fields_are_distinct_from_empty_values() {
         let mut none = sample_entry();
         none.actor_pubkey = None;
         let mut empty = sample_entry();
         empty.actor_pubkey = Some(Vec::new());
         assert_ne!(compute_hash(&none).unwrap(), compute_hash(&empty).unwrap());
+
+        none.object_id = None;
+        empty = none.clone();
+        empty.object_id = Some(String::new());
+        assert_ne!(compute_hash(&none).unwrap(), compute_hash(&empty).unwrap());
+
+        empty = none.clone();
+        empty.prev_hash = Some(Vec::new());
+        assert_ne!(compute_hash(&none).unwrap(), compute_hash(&empty).unwrap());
+        empty.prev_hash = Some(vec![0; 32]);
+        assert_ne!(compute_hash(&none).unwrap(), compute_hash(&empty).unwrap());
+    }
+
+    #[test]
+    fn object_id_and_detail_have_distinct_boundaries() {
+        let mut a = sample_entry();
+        a.object_id = Some("record1".into());
+        a.detail = serde_json::json!(23);
+        let mut b = a.clone();
+        b.object_id = Some("record12".into());
+        b.detail = serde_json::json!(3);
+        // The old encoding concatenated both pairs as "record123".
+        assert_ne!(compute_hash(&a).unwrap(), compute_hash(&b).unwrap());
+    }
+
+    #[test]
+    fn actor_and_object_id_have_distinct_boundaries() {
+        let mut a = sample_entry();
+        a.actor_pubkey = Some(vec![1; 32]);
+        let mut b = a.clone();
+        b.actor_pubkey = Some(vec![1; 31]);
+        b.object_id = Some(format!("\u{1}{}", a.object_id.as_deref().unwrap()));
+        // Moving the old presence byte into the object ID preserved its hash.
+        assert_ne!(compute_hash(&a).unwrap(), compute_hash(&b).unwrap());
     }
 
     #[test]

crates/buzz-audit/src/lib.rs

@@ -14,8 +14,8 @@
 //! advisory lock, so the chain stays consistent across relay processes without one
 //! global lock serializing (and timing-coupling) every tenant.
 //!
-//! The `audit_log` table is owned by the consolidated `0001` migration — this crate
-//! is pure chain logic and ships no DDL.
+//! The `audit_log` table is owned by `migrations/` — this crate is pure chain
+//! logic and ships no DDL.
 
 /// Audit action types recorded in the log.
 pub mod action;

crates/buzz-audit/src/service.rs

@@ -11,7 +11,7 @@ use crate::{
     action::AuditAction,
     entry::{AuditEntry, NewAuditEntry},
     error::AuditError,
-    hash::{compute_hash, to_storage_precision},
+    hash::{compute_hash, to_storage_precision, CURRENT_HASH_VERSION},
 };
 
 /// The `created_at` stamped on a new entry.
@@ -119,6 +119,7 @@ impl AuditService {
         let mut audit_entry = AuditEntry {
             community_id,
             seq,
+            hash_version: CURRENT_HASH_VERSION,
             hash: Vec::new(),
             prev_hash,
             action: entry.action,
@@ -135,8 +136,8 @@ impl AuditService {
         sqlx::query(
             r#"
             INSERT INTO audit_log
-                (community_id, seq, hash, prev_hash, action, actor_pubkey, object_id, detail, created_at)
-            VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9)
+                (community_id, seq, hash, prev_hash, action, actor_pubkey, object_id, detail, created_at, hash_version)
+            VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10)
             "#,
         )
         .bind(audit_entry.community_id)
@@ -148,6 +149,7 @@ impl AuditService {
         .bind(audit_entry.object_id.as_deref())
         .bind(&audit_entry.detail)
         .bind(audit_entry.created_at)
+        .bind(audit_entry.hash_version)
         .execute(&mut *tx)
         .await?;
 
@@ -175,7 +177,7 @@ impl AuditService {
         let rows = sqlx::query(
             r#"
             SELECT community_id, seq, hash, prev_hash, action, actor_pubkey,
-                   object_id, detail, created_at
+                   object_id, detail, created_at, hash_version
             FROM audit_log
             WHERE community_id = $1 AND seq BETWEEN $2 AND $3
             ORDER BY seq ASC
@@ -231,7 +233,7 @@ impl AuditService {
         let rows = sqlx::query(
             r#"
             SELECT community_id, seq, hash, prev_hash, action, actor_pubkey,
-                   object_id, detail, created_at
+                   object_id, detail, created_at, hash_version
             FROM audit_log
             WHERE community_id = $1 AND seq >= $2
             ORDER BY seq ASC
@@ -258,6 +260,7 @@ fn row_to_audit_entry(row: &sqlx::postgres::PgRow) -> Result<AuditEntry, AuditEr
     Ok(AuditEntry {
         community_id: row.get::<Uuid, _>("community_id"),
         seq: row.get("seq"),
+        hash_version: row.get("hash_version"),
         hash: row.get("hash"),
         prev_hash: row.get("prev_hash"),
         action,
@@ -343,6 +346,7 @@ mod postgres_tests {
             .await
             .unwrap();
         assert_eq!(e.seq, 1, "first entry in a community starts at seq 1");
+        assert_eq!(e.hash_version, CURRENT_HASH_VERSION);
         assert!(e.prev_hash.is_none(), "genesis entry has NULL prev_hash");
         assert_eq!(e.hash.len(), 32);
         assert_eq!(e.community_id, c);
@@ -358,10 +362,20 @@ mod postgres_tests {
         let svc = AuditService::new(pool.clone());
         let c = make_community(&pool).await;
 
-        let e1 = svc
+        let mut e1 = svc
             .log(new_entry(c, AuditAction::EventCreated))
             .await
             .unwrap();
+        // Model a historical row, including the migration's version default.
+        e1.hash_version = 1;
+        e1.hash = compute_hash(&e1).unwrap().to_vec();
+        sqlx::query("UPDATE audit_log SET hash_version = DEFAULT, hash = $1 WHERE community_id = $2 AND seq = $3")
+            .bind(&e1.hash)
+            .bind(c)
+            .bind(e1.seq)
+            .execute(&pool)
+            .await
+            .unwrap();
         let e2 = svc
             .log(new_entry(c, AuditAction::ChannelCreated))
             .await
@@ -374,13 +388,22 @@ mod postgres_tests {
         assert_eq!(e1.seq, 1);
         assert_eq!(e2.seq, 2);
         assert_eq!(e3.seq, 3);
+        assert_eq!(e2.hash_version, CURRENT_HASH_VERSION);
+        assert_eq!(e3.hash_version, CURRENT_HASH_VERSION);
         assert!(e1.prev_hash.is_none());
         assert_eq!(e2.prev_hash.as_deref(), Some(e1.hash.as_slice()));
         assert_eq!(e3.prev_hash.as_deref(), Some(e2.hash.as_slice()));
         assert!(svc
             .verify_chain(CommunityId::from_uuid(c), 1, 3)
             .await
             .unwrap());
+        let rows = svc
+            .get_entries(CommunityId::from_uuid(c), 1, 3)
+            .await
+            .unwrap();
+        assert_eq!(rows[0].hash_version, 1);
+        assert_eq!(rows[0].hash, e1.hash);
+        assert_eq!(rows[1].hash_version, CURRENT_HASH_VERSION);
     }
 
     /// THE isolation property: two communities keep independent chains. Each
@@ -460,20 +483,19 @@ mod postgres_tests {
         svc.log(new_entry(c, AuditAction::EventCreated))
             .await
             .unwrap();
-        let e2 = svc
-            .log(new_entry(c, AuditAction::EventDeleted))
-            .await
-            .unwrap();
+        let mut input = new_entry(c, AuditAction::EventDeleted);
+        input.actor_pubkey = Some(vec![1; 32]);
+        let e2 = svc.log(input).await.unwrap();
         svc.log(new_entry(c, AuditAction::ChannelDeleted))
             .await
             .unwrap();
 
-        // Tamper with e2's stored actor_pubkey.
-        let tampered: Vec<u8> = vec![0xff; 32];
-        sqlx::query("UPDATE audit_log SET actor_pubkey = $1 WHERE community_id = $2 AND seq = $3")
-            .bind(tampered)
+        // This shift preserved the old concatenated bytes, but changes TLV.
+        sqlx::query("UPDATE audit_log SET actor_pubkey = $1, object_id = $4 WHERE community_id = $2 AND seq = $3")
+            .bind(vec![1_u8; 31])
             .bind(c)
             .bind(e2.seq)
+            .bind(format!("\u{1}{}", e2.object_id.as_deref().unwrap()))
             .execute(&pool)
             .await
             .unwrap();
@@ -503,15 +525,17 @@ mod postgres_tests {
 
         // Forge: copy A's seq-1 row's hash into B's chain at seq 1.
         sqlx::query(
-            "INSERT INTO audit_log (community_id, seq, hash, prev_hash, action, actor_pubkey, object_id, detail, created_at)
-             VALUES ($1, 1, $2, NULL, $3, $4, $5, $6, NOW())",
+            "INSERT INTO audit_log (community_id, seq, hash, prev_hash, action, actor_pubkey, object_id, detail, created_at, hash_version)
+             VALUES ($1, 1, $2, NULL, $3, $4, $5, $6, $7, $8)",
         )
         .bind(b)
         .bind(&a1.hash) // A's hash, which was computed over community_id = A
         .bind(a1.action.as_str())
         .bind(a1.actor_pubkey.as_deref())
         .bind(a1.object_id.as_deref())
         .bind(&a1.detail)
+        .bind(a1.created_at)
+        .bind(a1.hash_version)
         .execute(&pool)
         .await
         .unwrap();

crates/buzz-db/src/runtime/migration.rs

@@ -703,7 +703,7 @@ mod postgres_tests {
         let mut migrations: Vec<_> = MIGRATOR.iter().collect();
         migrations.sort_by_key(|migration| migration.version);
 
-        assert_eq!(migrations.len(), 47);
+        assert_eq!(migrations.len(), 48);
         assert_eq!(migrations[0].version, 1);
         assert_eq!(&*migrations[0].description, "initial schema");
         assert!(migrations[0]

migrations/0048_audit_hash_version.sql

@@ -0,0 +1,4 @@
+-- Preserve existing hashes. New writers explicitly select the TLV encoding.
+ALTER TABLE audit_log
+    ADD COLUMN hash_version SMALLINT NOT NULL DEFAULT 1
+    CHECK (hash_version IN (1, 2));