diff --git a/server/crates/arbiter-server/src/actors/keyholder.rs b/server/crates/arbiter-server/src/actors/keyholder.rs
index 2b21327..eeded0d 100644
--- a/server/crates/arbiter-server/src/actors/keyholder.rs
+++ b/server/crates/arbiter-server/src/actors/keyholder.rs
@@ -68,13 +68,13 @@ pub enum Error {
 /// Provides API for encrypting and decrypting data using the vault root key.
 /// Abstraction over database to make sure nonces are never reused and encryption keys are never exposed in plaintext outside of this actor.
 #[derive(Actor)]
-pub struct KeyHolderActor {
+pub struct KeyHolder {
     db: db::DatabasePool,
     state: State,
 }
 
 #[messages]
-impl KeyHolderActor {
+impl KeyHolder {
     pub async fn new(db: db::DatabasePool) -> Result<Self, Error> {
         let state = {
             let mut conn = db.get().await?;
@@ -206,14 +206,16 @@ impl KeyHolderActor {
             return Err(Error::NotBootstrapped);
         };
 
-        let mut conn = self.db.get().await?;
-
-        let current_key = schema::root_key_history::table
-            .filter(schema::root_key_history::id.eq(*root_key_history_id))
-            .select((schema::root_key_history::data_encryption_nonce))
-            .select((RootKeyHistory::as_select()))
-            .first(&mut conn)
-            .await?;
+        // We don't want to hold connection while doing expensive KDF work
+        let current_key = {
+            let mut conn = self.db.get().await?;
+            schema::root_key_history::table
+                .filter(schema::root_key_history::id.eq(*root_key_history_id))
+                .select((schema::root_key_history::data_encryption_nonce))
+                .select((RootKeyHistory::as_select()))
+                .first(&mut conn)
+                .await?
+        };
 
         let salt = &current_key.salt;
         let salt = v1::Salt::try_from(salt.as_slice()).map_err(|_| {
@@ -257,14 +259,17 @@ impl KeyHolderActor {
         let State::Unsealed { root_key, .. } = &mut self.state else {
             return Err(Error::NotBootstrapped);
         };
-        let mut conn = self.db.get().await?;
-        let row: models::AeadEncrypted = schema::aead_encrypted::table
-            .select(models::AeadEncrypted::as_select())
-            .filter(schema::aead_encrypted::id.eq(aead_id))
-            .first(&mut conn)
-            .await
-            .optional()?
-            .ok_or(Error::NotFound)?;
+
+        let row: models::AeadEncrypted = {
+            let mut conn = self.db.get().await?;
+            schema::aead_encrypted::table
+                .select(models::AeadEncrypted::as_select())
+                .filter(schema::aead_encrypted::id.eq(aead_id))
+                .first(&mut conn)
+                .await
+                .optional()?
+                .ok_or(Error::NotFound)?
+        };
 
         let nonce = v1::Nonce::try_from(row.current_nonce.as_slice()).map_err(|_| {
             error!(
@@ -293,14 +298,13 @@ impl KeyHolderActor {
         // Borrow checker note: &mut borrow a few lines above is disjoint from this field
         let nonce = Self::get_new_nonce(&self.db, *root_key_history_id).await?;
 
-        let mut conn = self.db.get().await?;
-
         let mut ciphertext_buffer = plaintext.write().unwrap();
         let ciphertext_buffer: &mut Vec<u8> = ciphertext_buffer.as_mut();
         root_key.encrypt_in_place(&nonce, v1::TAG, &mut *ciphertext_buffer)?;
 
         let ciphertext = std::mem::take(ciphertext_buffer);
 
+        let mut conn = self.db.get().await?;
         let aead_id: i32 = insert_into(schema::aead_encrypted::table)
             .values(&models::NewAeadEncrypted {
                 ciphertext,
@@ -319,11 +323,16 @@ impl KeyHolderActor {
 
 #[cfg(test)]
 mod tests {
-    use std::collections::HashSet;
+    use std::collections::{HashMap, HashSet};
+    use std::sync::Arc;
 
-    use diesel::dsl::insert_into;
+    use diesel::dsl::{insert_into, sql_query, update};
     use diesel_async::RunQueryDsl;
+    use futures::stream::TryUnfold;
+    use kameo::actor::{ActorRef, Spawn as _};
     use memsafe::MemSafe;
+    use tokio::sync::Mutex;
+    use tokio::task::JoinSet;
 
     use crate::db::{self, models::ArbiterSetting};
 
@@ -343,20 +352,49 @@ mod tests {
             .unwrap();
     }
 
-    async fn bootstrapped_actor(db: &db::DatabasePool) -> KeyHolderActor {
+    async fn bootstrapped_actor(db: &db::DatabasePool) -> KeyHolder {
         seed_settings(db).await;
-        let mut actor = KeyHolderActor::new(db.clone()).await.unwrap();
+        let mut actor = KeyHolder::new(db.clone()).await.unwrap();
         let seal_key = MemSafe::new(b"test-seal-key".to_vec()).unwrap();
         actor.bootstrap(seal_key).await.unwrap();
         actor
     }
 
+    async fn write_concurrently(
+        actor: ActorRef<KeyHolder>,
+        prefix: &'static str,
+        count: usize,
+    ) -> Vec<(i32, Vec<u8>)> {
+        let mut set = JoinSet::new();
+        for i in 0..count {
+            let actor = actor.clone();
+            set.spawn(async move {
+                let plaintext = format!("{prefix}-{i}").into_bytes();
+                let id = {
+                    actor
+                        .ask(CreateNew {
+                            plaintext: MemSafe::new(plaintext.clone()).unwrap(),
+                        })
+                        .await
+                        .unwrap()
+                };
+                (id, plaintext)
+            });
+        }
+
+        let mut out = Vec::with_capacity(count);
+        while let Some(res) = set.join_next().await {
+            out.push(res.unwrap());
+        }
+        out
+    }
+
     #[tokio::test]
     #[test_log::test]
     async fn test_bootstrap() {
         let db = db::create_test_pool().await;
         seed_settings(&db).await;
-        let mut actor = KeyHolderActor::new(db.clone()).await.unwrap();
+        let mut actor = KeyHolder::new(db.clone()).await.unwrap();
 
         assert!(matches!(actor.state, State::Unbootstrapped));
 
@@ -412,7 +450,7 @@ mod tests {
     async fn test_create_new_before_bootstrap_fails() {
         let db = db::create_test_pool().await;
         seed_settings(&db).await;
-        let mut actor = KeyHolderActor::new(db).await.unwrap();
+        let mut actor = KeyHolder::new(db).await.unwrap();
 
         let err = actor
             .create_new(MemSafe::new(b"data".to_vec()).unwrap())
@@ -426,7 +464,7 @@ mod tests {
     async fn test_decrypt_before_bootstrap_fails() {
         let db = db::create_test_pool().await;
         seed_settings(&db).await;
-        let mut actor = KeyHolderActor::new(db).await.unwrap();
+        let mut actor = KeyHolder::new(db).await.unwrap();
 
         let err = actor.decrypt(1).await.unwrap_err();
         assert!(matches!(err, Error::NotBootstrapped));
@@ -449,7 +487,7 @@ mod tests {
         let actor = bootstrapped_actor(&db).await;
         drop(actor);
 
-        let actor2 = KeyHolderActor::new(db).await.unwrap();
+        let actor2 = KeyHolder::new(db).await.unwrap();
         assert!(matches!(actor2.state, State::Sealed { .. }));
     }
 
@@ -518,7 +556,7 @@ mod tests {
             .unwrap();
         drop(actor);
 
-        let mut actor = KeyHolderActor::new(db.clone()).await.unwrap();
+        let mut actor = KeyHolder::new(db.clone()).await.unwrap();
         assert!(matches!(actor.state, State::Sealed { .. }));
 
         let seal_key = MemSafe::new(b"test-seal-key".to_vec()).unwrap();
@@ -543,7 +581,7 @@ mod tests {
             .unwrap();
         drop(actor);
 
-        let mut actor = KeyHolderActor::new(db.clone()).await.unwrap();
+        let mut actor = KeyHolder::new(db.clone()).await.unwrap();
         assert!(matches!(actor.state, State::Sealed { .. }));
 
         // wrong password
@@ -603,4 +641,291 @@ mod tests {
         assert_eq!(*d1.read().unwrap(), plaintext);
         assert_eq!(*d2.read().unwrap(), plaintext);
     }
+
+    #[tokio::test]
+    #[test_log::test]
+    async fn concurrent_create_new_no_duplicate_nonces_() {
+        let db = db::create_test_pool().await;
+        let actor = KeyHolder::spawn(bootstrapped_actor(&db).await);
+
+        let writes = write_concurrently(actor, "nonce-unique", 32).await;
+        assert_eq!(writes.len(), 32);
+
+        let mut conn = db.get().await.unwrap();
+        let rows: Vec<models::AeadEncrypted> = schema::aead_encrypted::table
+            .select(models::AeadEncrypted::as_select())
+            .load(&mut conn)
+            .await
+            .unwrap();
+        assert_eq!(rows.len(), 32);
+
+        let nonces: Vec<&Vec<u8>> = rows.iter().map(|r| &r.current_nonce).collect();
+        let unique: HashSet<&Vec<u8>> = nonces.iter().copied().collect();
+        assert_eq!(nonces.len(), unique.len(), "all nonces must be unique");
+    }
+
+    #[tokio::test]
+    #[test_log::test]
+    async fn concurrent_create_new_root_nonce_never_moves_backward() {
+        let db = db::create_test_pool().await;
+        let actor = KeyHolder::spawn(bootstrapped_actor(&db).await);
+
+        write_concurrently(actor, "root-max", 24).await;
+
+        let mut conn = db.get().await.unwrap();
+        let rows: Vec<models::AeadEncrypted> = schema::aead_encrypted::table
+            .select(models::AeadEncrypted::as_select())
+            .load(&mut conn)
+            .await
+            .unwrap();
+        let max_nonce = rows
+            .iter()
+            .map(|r| r.current_nonce.clone())
+            .max()
+            .expect("at least one row");
+
+        let root_row: models::RootKeyHistory = schema::root_key_history::table
+            .select(models::RootKeyHistory::as_select())
+            .first(&mut conn)
+            .await
+            .unwrap();
+        assert_eq!(root_row.data_encryption_nonce, max_nonce);
+    }
+
+    #[tokio::test]
+    #[test_log::test]
+    async fn nonce_monotonic_even_when_nonce_allocation_interleaves() {
+        let db = db::create_test_pool().await;
+        let mut actor = bootstrapped_actor(&db).await;
+        let root_key_history_id = match actor.state {
+            State::Unsealed {
+                root_key_history_id,
+                ..
+            } => root_key_history_id,
+            _ => panic!("expected unsealed state"),
+        };
+
+        let n1 = KeyHolder::get_new_nonce(&db, root_key_history_id)
+            .await
+            .unwrap();
+        let n2 = KeyHolder::get_new_nonce(&db, root_key_history_id)
+            .await
+            .unwrap();
+        assert!(n2.to_vec() > n1.to_vec(), "nonce must increase");
+
+        let mut conn = db.get().await.unwrap();
+        let root_row: models::RootKeyHistory = schema::root_key_history::table
+            .select(models::RootKeyHistory::as_select())
+            .first(&mut conn)
+            .await
+            .unwrap();
+        assert_eq!(root_row.data_encryption_nonce, n2.to_vec());
+
+        let id = actor
+            .create_new(MemSafe::new(b"post-interleave".to_vec()).unwrap())
+            .await
+            .unwrap();
+        let row: models::AeadEncrypted = schema::aead_encrypted::table
+            .filter(schema::aead_encrypted::id.eq(id))
+            .select(models::AeadEncrypted::as_select())
+            .first(&mut conn)
+            .await
+            .unwrap();
+        assert!(
+            row.current_nonce > n2.to_vec(),
+            "next write must advance nonce"
+        );
+    }
+
+    #[tokio::test]
+    #[test_log::test]
+    async fn insert_failure_does_not_create_partial_row() {
+        let db = db::create_test_pool().await;
+        let mut actor = bootstrapped_actor(&db).await;
+        let root_key_history_id = match actor.state {
+            State::Unsealed {
+                root_key_history_id,
+                ..
+            } => root_key_history_id,
+            _ => panic!("expected unsealed state"),
+        };
+
+        let mut conn = db.get().await.unwrap();
+        let before_count: i64 = schema::aead_encrypted::table
+            .count()
+            .get_result(&mut conn)
+            .await
+            .unwrap();
+        let before_root_nonce: Vec<u8> = schema::root_key_history::table
+            .filter(schema::root_key_history::id.eq(root_key_history_id))
+            .select(schema::root_key_history::data_encryption_nonce)
+            .first(&mut conn)
+            .await
+            .unwrap();
+
+        sql_query(
+            "CREATE TRIGGER fail_aead_insert BEFORE INSERT ON aead_encrypted BEGIN SELECT RAISE(ABORT, 'forced test failure'); END;",
+        )
+        .execute(&mut conn)
+        .await
+        .unwrap();
+        drop(conn);
+
+        let err = actor
+            .create_new(MemSafe::new(b"should fail".to_vec()).unwrap())
+            .await
+            .unwrap_err();
+        assert!(matches!(err, Error::DatabaseTransaction(_)));
+
+        let mut conn = db.get().await.unwrap();
+        sql_query("DROP TRIGGER fail_aead_insert;")
+            .execute(&mut conn)
+            .await
+            .unwrap();
+
+        let after_count: i64 = schema::aead_encrypted::table
+            .count()
+            .get_result(&mut conn)
+            .await
+            .unwrap();
+        assert_eq!(
+            before_count, after_count,
+            "failed insert must not create row"
+        );
+
+        let after_root_nonce: Vec<u8> = schema::root_key_history::table
+            .filter(schema::root_key_history::id.eq(root_key_history_id))
+            .select(schema::root_key_history::data_encryption_nonce)
+            .first(&mut conn)
+            .await
+            .unwrap();
+        assert!(
+            after_root_nonce > before_root_nonce,
+            "current behavior allows nonce gap on failed insert"
+        );
+    }
+
+    #[tokio::test]
+    #[test_log::test]
+    async fn decrypt_roundtrip_after_high_concurrency() {
+        let db = db::create_test_pool().await;
+        let actor = KeyHolder::spawn(bootstrapped_actor(&db).await);
+
+        let writes = write_concurrently(actor, "roundtrip", 40).await;
+        let expected: HashMap<i32, Vec<u8>> = writes.into_iter().collect();
+
+        let mut decryptor = KeyHolder::new(db.clone()).await.unwrap();
+        decryptor
+            .try_unseal(MemSafe::new(b"test-seal-key".to_vec()).unwrap())
+            .await
+            .unwrap();
+
+        for (id, plaintext) in expected {
+            let mut decrypted = decryptor.decrypt(id).await.unwrap();
+            assert_eq!(*decrypted.read().unwrap(), plaintext);
+        }
+    }
+
+    #[tokio::test]
+    #[test_log::test]
+    async fn swapping_ciphertext_and_nonce_between_rows_changes_logical_binding() {
+        let db = db::create_test_pool().await;
+        let mut actor = bootstrapped_actor(&db).await;
+
+        let plaintext1 = b"entry-one";
+        let plaintext2 = b"entry-two";
+        let id1 = actor
+            .create_new(MemSafe::new(plaintext1.to_vec()).unwrap())
+            .await
+            .unwrap();
+        let id2 = actor
+            .create_new(MemSafe::new(plaintext2.to_vec()).unwrap())
+            .await
+            .unwrap();
+
+        let mut conn = db.get().await.unwrap();
+        let row1: models::AeadEncrypted = schema::aead_encrypted::table
+            .filter(schema::aead_encrypted::id.eq(id1))
+            .select(models::AeadEncrypted::as_select())
+            .first(&mut conn)
+            .await
+            .unwrap();
+        let row2: models::AeadEncrypted = schema::aead_encrypted::table
+            .filter(schema::aead_encrypted::id.eq(id2))
+            .select(models::AeadEncrypted::as_select())
+            .first(&mut conn)
+            .await
+            .unwrap();
+
+        update(schema::aead_encrypted::table.filter(schema::aead_encrypted::id.eq(id1)))
+            .set((
+                schema::aead_encrypted::ciphertext.eq(row2.ciphertext.clone()),
+                schema::aead_encrypted::current_nonce.eq(row2.current_nonce.clone()),
+            ))
+            .execute(&mut conn)
+            .await
+            .unwrap();
+        update(schema::aead_encrypted::table.filter(schema::aead_encrypted::id.eq(id2)))
+            .set((
+                schema::aead_encrypted::ciphertext.eq(row1.ciphertext.clone()),
+                schema::aead_encrypted::current_nonce.eq(row1.current_nonce.clone()),
+            ))
+            .execute(&mut conn)
+            .await
+            .unwrap();
+
+        let mut d1 = actor.decrypt(id1).await.unwrap();
+        let mut d2 = actor.decrypt(id2).await.unwrap();
+        assert_eq!(*d1.read().unwrap(), plaintext2);
+        assert_eq!(*d2.read().unwrap(), plaintext1);
+    }
+    #[tokio::test]
+    #[test_log::test]
+    async fn broken_db_nonce_format_fails_closed() {
+        // malformed root_key_history nonce must fail create_new
+        let db = db::create_test_pool().await;
+        let mut actor = bootstrapped_actor(&db).await;
+        let root_key_history_id = match actor.state {
+            State::Unsealed {
+                root_key_history_id,
+                ..
+            } => root_key_history_id,
+            _ => panic!("expected unsealed state"),
+        };
+
+        let mut conn = db.get().await.unwrap();
+        update(
+            schema::root_key_history::table
+                .filter(schema::root_key_history::id.eq(root_key_history_id)),
+        )
+        .set(schema::root_key_history::data_encryption_nonce.eq(vec![1, 2, 3]))
+        .execute(&mut conn)
+        .await
+        .unwrap();
+        drop(conn);
+
+        let err = actor
+            .create_new(MemSafe::new(b"must fail".to_vec()).unwrap())
+            .await
+            .unwrap_err();
+        assert!(matches!(err, Error::BrokenDatabase));
+
+        // malformed per-row nonce must fail decrypt
+        let db = db::create_test_pool().await;
+        let mut actor = bootstrapped_actor(&db).await;
+        let id = actor
+            .create_new(MemSafe::new(b"decrypt target".to_vec()).unwrap())
+            .await
+            .unwrap();
+        let mut conn = db.get().await.unwrap();
+        update(schema::aead_encrypted::table.filter(schema::aead_encrypted::id.eq(id)))
+            .set(schema::aead_encrypted::current_nonce.eq(vec![7, 8]))
+            .execute(&mut conn)
+            .await
+            .unwrap();
+        drop(conn);
+
+        let err = actor.decrypt(id).await.unwrap_err();
+        assert!(matches!(err, Error::BrokenDatabase));
+    }
 }