diff --git a/examples/native-fungible/tests/transfers.rs b/examples/native-fungible/tests/transfers.rs
index a4347a703cc4..b5ecac6eb045 100644
--- a/examples/native-fungible/tests/transfers.rs
+++ b/examples/native-fungible/tests/transfers.rs
@@ -33,7 +33,7 @@ async fn chain_balance_transfers() {
 
     let transfer_certificate = funding_chain
         .add_block(|block| {
-            block.with_native_token_transfer(None, recipient, transfer_amount);
+            block.with_native_token_transfer(AccountOwner::Chain, recipient, transfer_amount);
         })
         .await;
 
@@ -69,7 +69,7 @@ async fn transfer_to_owner() {
 
     let transfer_certificate = funding_chain
         .add_block(|block| {
-            block.with_native_token_transfer(None, recipient, transfer_amount);
+            block.with_native_token_transfer(AccountOwner::Chain, recipient, transfer_amount);
         })
         .await;
 
@@ -114,7 +114,7 @@ async fn transfer_to_multiple_owners() {
     let transfer_certificate = funding_chain
         .add_block(|block| {
             for (recipient, transfer_amount) in recipients.zip(transfer_amounts.clone()) {
-                block.with_native_token_transfer(None, recipient, transfer_amount);
+                block.with_native_token_transfer(AccountOwner::Chain, recipient, transfer_amount);
             }
         })
         .await;
@@ -155,7 +155,7 @@ async fn emptied_account_disappears_from_queries() {
 
     let transfer_certificate = funding_chain
         .add_block(|block| {
-            block.with_native_token_transfer(None, recipient, transfer_amount);
+            block.with_native_token_transfer(AccountOwner::Chain, recipient, transfer_amount);
         })
         .await;
 
@@ -167,7 +167,11 @@ async fn emptied_account_disappears_from_queries() {
 
     recipient_chain
         .add_block(|block| {
-            block.with_native_token_transfer(Some(owner), Recipient::Burn, transfer_amount);
+            block.with_native_token_transfer(
+                AccountOwner::User(owner),
+                Recipient::Burn,
+                transfer_amount,
+            );
         })
         .await;
 
diff --git a/linera-chain/src/test.rs b/linera-chain/src/test.rs
index 121672d09c3c..a270e1c4bdd1 100644
--- a/linera-chain/src/test.rs
+++ b/linera-chain/src/test.rs
@@ -7,7 +7,7 @@ use linera_base::{
     crypto::{AccountPublicKey, AccountSecretKey},
     data_types::{Amount, BlockHeight, Round, Timestamp},
     hashed::Hashed,
-    identifiers::{ChainId, Owner},
+    identifiers::{AccountOwner, ChainId, Owner},
 };
 use linera_execution::{
     committee::{Committee, Epoch, ValidatorState},
@@ -62,7 +62,7 @@ pub trait BlockTestExt: Sized {
     fn with_operation(self, operation: impl Into<Operation>) -> Self;
 
     /// Returns the block with a transfer operation appended at the end.
-    fn with_transfer(self, owner: Option<Owner>, recipient: Recipient, amount: Amount) -> Self;
+    fn with_transfer(self, owner: AccountOwner, recipient: Recipient, amount: Amount) -> Self;
 
     /// Returns the block with a simple transfer operation appended at the end.
     fn with_simple_transfer(self, chain_id: ChainId, amount: Amount) -> Self;
@@ -97,7 +97,7 @@ impl BlockTestExt for ProposedBlock {
         self
     }
 
-    fn with_transfer(self, owner: Option<Owner>, recipient: Recipient, amount: Amount) -> Self {
+    fn with_transfer(self, owner: AccountOwner, recipient: Recipient, amount: Amount) -> Self {
         self.with_operation(SystemOperation::Transfer {
             owner,
             recipient,
@@ -106,7 +106,7 @@ impl BlockTestExt for ProposedBlock {
     }
 
     fn with_simple_transfer(self, chain_id: ChainId, amount: Amount) -> Self {
-        self.with_transfer(None, Recipient::chain(chain_id), amount)
+        self.with_transfer(AccountOwner::Chain, Recipient::chain(chain_id), amount)
     }
 
     fn with_incoming_bundle(mut self, incoming_bundle: IncomingBundle) -> Self {
diff --git a/linera-chain/src/unit_tests/chain_tests.rs b/linera-chain/src/unit_tests/chain_tests.rs
index e1a2b181427e..695babb0ec03 100644
--- a/linera-chain/src/unit_tests/chain_tests.rs
+++ b/linera-chain/src/unit_tests/chain_tests.rs
@@ -17,7 +17,7 @@ use linera_base::{
         UserApplicationDescription,
     },
     hashed::Hashed,
-    identifiers::{ApplicationId, ChainId, MessageId, ModuleId},
+    identifiers::{AccountOwner, ApplicationId, ChainId, MessageId, ModuleId},
     ownership::ChainOwnership,
     vm::VmRuntime,
 };
@@ -168,7 +168,7 @@ async fn test_block_size_limit() {
     let invalid_block = valid_block
         .clone()
         .with_operation(SystemOperation::Transfer {
-            owner: None,
+            owner: AccountOwner::Chain,
             recipient: Recipient::root(0),
             amount: Amount::ONE,
         });
diff --git a/linera-client/src/benchmark.rs b/linera-client/src/benchmark.rs
index e84f47a8164e..1dc51c9dd7b6 100644
--- a/linera-client/src/benchmark.rs
+++ b/linera-client/src/benchmark.rs
@@ -594,7 +594,7 @@ where
                     amount,
                 ),
                 None => Operation::System(SystemOperation::Transfer {
-                    owner: None,
+                    owner: AccountOwner::Chain,
                     recipient: Recipient::chain(previous_chain_id),
                     amount,
                 }),
diff --git a/linera-client/src/unit_tests/chain_listener.rs b/linera-client/src/unit_tests/chain_listener.rs
index b25a28ea8e8d..cd6e3d9259b2 100644
--- a/linera-client/src/unit_tests/chain_listener.rs
+++ b/linera-client/src/unit_tests/chain_listener.rs
@@ -10,7 +10,7 @@ use futures::{lock::Mutex, FutureExt as _};
 use linera_base::{
     crypto::{AccountPublicKey, AccountSecretKey, Secp256k1SecretKey},
     data_types::{Amount, BlockHeight, TimeDelta, Timestamp},
-    identifiers::ChainId,
+    identifiers::{AccountOwner, ChainId},
     ownership::{ChainOwnership, TimeoutConfig},
 };
 use linera_core::{
@@ -160,7 +160,9 @@ async fn test_chain_listener() -> anyhow::Result<()> {
     // Transfer one token to chain 0. The listener should eventually become leader and receive
     // the message.
     let recipient0 = Recipient::chain(chain_id0);
-    client1.transfer(None, Amount::ONE, recipient0).await?;
+    client1
+        .transfer(AccountOwner::Chain, Amount::ONE, recipient0)
+        .await?;
     for i in 0.. {
         client0.synchronize_from_validators().boxed().await?;
         let balance = client0.local_balance().await?;
diff --git a/linera-core/benches/client_benchmarks.rs b/linera-core/benches/client_benchmarks.rs
index 47b36b0a9194..d17ba1910e90 100644
--- a/linera-core/benches/client_benchmarks.rs
+++ b/linera-core/benches/client_benchmarks.rs
@@ -2,7 +2,11 @@
 // SPDX-License-Identifier: Apache-2.0
 
 use criterion::{criterion_group, criterion_main, measurement::Measurement, BatchSize, Criterion};
-use linera_base::{data_types::Amount, identifiers::Account, time::Duration};
+use linera_base::{
+    data_types::Amount,
+    identifiers::{Account, AccountOwner},
+    time::Duration,
+};
 use linera_core::{
     client,
     test_utils::{MemoryStorageBuilder, NodeProvider, StorageBuilder, TestBuilder},
@@ -55,7 +59,7 @@ where
 
     let account = Account::owner(chain2.chain_id(), owner1);
     let cert = chain1
-        .transfer_to_account(None, amt, account)
+        .transfer_to_account(AccountOwner::Chain, amt, account)
         .await
         .unwrap()
         .unwrap();
diff --git a/linera-core/src/client/mod.rs b/linera-core/src/client/mod.rs
index a70366b074d0..9c8e74a9551e 100644
--- a/linera-core/src/client/mod.rs
+++ b/linera-core/src/client/mod.rs
@@ -1610,7 +1610,7 @@ where
     #[instrument(level = "trace")]
     pub async fn transfer(
         &self,
-        owner: Option<Owner>,
+        owner: AccountOwner,
         amount: Amount,
         recipient: Recipient,
     ) -> Result<ClientOutcome<ConfirmedBlockCertificate>, ChainClientError> {
@@ -2432,7 +2432,7 @@ where
     #[instrument(level = "trace")]
     pub async fn transfer_to_account(
         &self,
-        owner: Option<Owner>,
+        owner: AccountOwner,
         amount: Amount,
         account: Account,
     ) -> Result<ClientOutcome<ConfirmedBlockCertificate>, ChainClientError> {
@@ -2444,7 +2444,7 @@ where
     #[instrument(level = "trace")]
     pub async fn burn(
         &self,
-        owner: Option<Owner>,
+        owner: AccountOwner,
         amount: Amount,
     ) -> Result<ClientOutcome<ConfirmedBlockCertificate>, ChainClientError> {
         self.transfer(owner, amount, Recipient::Burn).await
@@ -3159,7 +3159,7 @@ where
     #[instrument(level = "trace")]
     pub async fn transfer_to_account_unsafe_unconfirmed(
         &self,
-        owner: Option<Owner>,
+        owner: AccountOwner,
         amount: Amount,
         account: Account,
     ) -> Result<ClientOutcome<ConfirmedBlockCertificate>, ChainClientError> {
diff --git a/linera-core/src/unit_tests/client_tests.rs b/linera-core/src/unit_tests/client_tests.rs
index b7684dc224c5..ad4732b85943 100644
--- a/linera-core/src/unit_tests/client_tests.rs
+++ b/linera-core/src/unit_tests/client_tests.rs
@@ -104,7 +104,7 @@ where
     {
         let certificate = sender
             .transfer_to_account(
-                None,
+                AccountOwner::Chain,
                 Amount::from_tokens(3),
                 Account::chain(ChainId::root(2)),
             )
@@ -156,7 +156,7 @@ where
     let friend = receiver.identity().await?;
     sender
         .transfer_to_account(
-            None,
+            AccountOwner::Chain,
             Amount::from_tokens(3),
             Account::owner(receiver_id, owner),
         )
@@ -165,7 +165,7 @@ where
         .unwrap();
     let cert = sender
         .transfer_to_account(
-            None,
+            AccountOwner::Chain,
             Amount::from_millis(100),
             Account::owner(receiver_id, friend),
         )
@@ -291,7 +291,10 @@ where
     );
     sender.synchronize_from_validators().await.unwrap();
     // Can still use the chain.
-    sender.burn(None, Amount::from_tokens(3)).await.unwrap();
+    sender
+        .burn(AccountOwner::Chain, Amount::from_tokens(3))
+        .await
+        .unwrap();
     Ok(())
 }
 
@@ -335,7 +338,9 @@ where
     sender.synchronize_from_validators().await.unwrap();
     // Cannot use the chain any more.
     assert_matches!(
-        sender.burn(None, Amount::from_tokens(3)).await,
+        sender
+            .burn(AccountOwner::Chain, Amount::from_tokens(3))
+            .await,
         Err(ChainClientError::CannotFindKeyForChain(_))
     );
     Ok(())
@@ -376,7 +381,10 @@ where
     );
     sender.synchronize_from_validators().await.unwrap();
     // Can still use the chain with the old client.
-    sender.burn(None, Amount::from_tokens(2)).await.unwrap();
+    sender
+        .burn(AccountOwner::Chain, Amount::from_tokens(2))
+        .await
+        .unwrap();
     assert_eq!(sender.next_block_height(), BlockHeight::from(2));
     // Make a client to try the new key.
     let client = builder
@@ -401,7 +409,7 @@ where
 
     // We need at least three validators for making an operation.
     builder.set_fault_type([0, 1], FaultType::Offline).await;
-    let result = client.burn(None, Amount::ONE).await;
+    let result = client.burn(AccountOwner::Chain, Amount::ONE).await;
     assert_matches!(
         result,
         Err(ChainClientError::CommunicationError(
@@ -411,7 +419,7 @@ where
     builder.set_fault_type([0, 1], FaultType::Honest).await;
     builder.set_fault_type([2, 3], FaultType::Offline).await;
     assert_matches!(
-        sender.burn(None, Amount::ONE).await,
+        sender.burn(AccountOwner::Chain, Amount::ONE).await,
         Err(ChainClientError::CommunicationError(
             CommunicationError::Trusted(ClientIoError { .. })
         ))
@@ -429,14 +437,18 @@ where
         Amount::from_tokens(2)
     );
     client.clear_pending_proposal();
-    client.burn(None, Amount::ONE).await.unwrap().unwrap();
+    client
+        .burn(AccountOwner::Chain, Amount::ONE)
+        .await
+        .unwrap()
+        .unwrap();
 
     // The other client doesn't know the new round number yet:
     sender.synchronize_from_validators().await.unwrap();
     sender.process_inbox().await.unwrap();
     assert_eq!(sender.local_balance().await.unwrap(), Amount::ONE);
     sender.clear_pending_proposal();
-    sender.burn(None, Amount::ONE).await.unwrap();
+    sender.burn(AccountOwner::Chain, Amount::ONE).await.unwrap();
 
     // That's it, we spent all our money on this test!
     assert_eq!(sender.local_balance().await.unwrap(), Amount::ZERO);
@@ -522,7 +534,11 @@ where
     });
     // Transfer before creating the chain. The validators will ignore the cross-chain messages.
     sender
-        .transfer_to_account(None, Amount::from_tokens(2), Account::chain(new_id))
+        .transfer_to_account(
+            AccountOwner::Chain,
+            Amount::from_tokens(2),
+            Account::chain(new_id),
+        )
         .await
         .unwrap();
     // Open the new chain.
@@ -564,7 +580,11 @@ where
     // Make another block on top of the one that sent the two tokens, so that the validators
     // process the cross-chain messages.
     let certificate2 = sender
-        .transfer_to_account(None, Amount::from_tokens(1), Account::chain(new_id))
+        .transfer_to_account(
+            AccountOwner::Chain,
+            Amount::from_tokens(1),
+            Account::chain(new_id),
+        )
         .await
         .unwrap()
         .unwrap();
@@ -576,7 +596,10 @@ where
         client.query_balance().await.unwrap(),
         Amount::from_tokens(3)
     );
-    client.burn(None, Amount::from_tokens(3)).await.unwrap();
+    client
+        .burn(AccountOwner::Chain, Amount::from_tokens(3))
+        .await
+        .unwrap();
     Ok(())
 }
 
@@ -602,7 +625,11 @@ where
     });
     // Transfer before creating the chain.
     sender
-        .transfer_to_account(None, Amount::from_tokens(3), Account::chain(new_id))
+        .transfer_to_account(
+            AccountOwner::Chain,
+            Amount::from_tokens(3),
+            Account::chain(new_id),
+        )
         .await
         .unwrap();
     // Open the new chain.
@@ -640,7 +667,9 @@ where
         .receive_certificate_and_update_validators(certificate)
         .await
         .unwrap();
-    let result = client.burn(None, Amount::from_tokens(3)).await;
+    let result = client
+        .burn(AccountOwner::Chain, Amount::from_tokens(3))
+        .await;
     assert_matches!(
         result,
         Err(ChainClientError::LocalNodeError(
@@ -680,7 +709,11 @@ where
     let new_id = ChainId::child(message_id);
     // Transfer after creating the chain.
     let transfer_certificate = sender
-        .transfer_to_account(None, Amount::from_tokens(3), Account::chain(new_id))
+        .transfer_to_account(
+            AccountOwner::Chain,
+            Amount::from_tokens(3),
+            Account::chain(new_id),
+        )
         .await
         .unwrap()
         .unwrap();
@@ -705,7 +738,10 @@ where
         client.query_balance().await.unwrap(),
         Amount::from_tokens(3)
     );
-    client.burn(None, Amount::from_tokens(3)).await.unwrap();
+    client
+        .burn(AccountOwner::Chain, Amount::from_tokens(3))
+        .await
+        .unwrap();
     assert_eq!(client.local_balance().await.unwrap(), Amount::ZERO);
     Ok(())
 }
@@ -743,7 +779,9 @@ where
         certificate
     );
     // Cannot use the chain for operations any more.
-    let result = client1.burn(None, Amount::from_tokens(3)).await;
+    let result = client1
+        .burn(AccountOwner::Chain, Amount::from_tokens(3))
+        .await;
     assert!(
         matches!(
             &result,
@@ -758,7 +796,7 @@ where
     // Incoming messages now get rejected.
     client2
         .transfer_to_account(
-            None,
+            AccountOwner::Chain,
             Amount::from_tokens(3),
             Account::chain(client1.chain_id()),
         )
@@ -816,7 +854,7 @@ where
     let sender = builder.add_root_chain(1, Amount::from_tokens(4)).await?;
     let result = sender
         .transfer_to_account_unsafe_unconfirmed(
-            None,
+            AccountOwner::Chain,
             Amount::from_tokens(3),
             Account::chain(ChainId::root(2)),
         )
@@ -866,7 +904,7 @@ where
     );
     let certificate = client1
         .transfer_to_account(
-            None,
+            AccountOwner::Chain,
             Amount::from_tokens(3),
             Account::chain(client2.chain_id),
         )
@@ -914,7 +952,11 @@ where
     // Process the inbox and send back some money.
     assert_eq!(client2.next_block_height(), BlockHeight::ZERO);
     client2
-        .transfer_to_account(None, Amount::ONE, Account::chain(client1.chain_id))
+        .transfer_to_account(
+            AccountOwner::Chain,
+            Amount::ONE,
+            Account::chain(client1.chain_id),
+        )
         .await
         .unwrap();
     assert_eq!(client2.next_block_height(), BlockHeight::from(1));
@@ -957,7 +999,7 @@ where
     let client2 = builder.add_root_chain(2, Amount::ZERO).await?;
     let certificate = client1
         .transfer_to_account_unsafe_unconfirmed(
-            None,
+            AccountOwner::Chain,
             Amount::from_tokens(2),
             Account::chain(client2.chain_id),
         )
@@ -1006,11 +1048,19 @@ where
     // Transferring funds from client1 to client2.
     // Confirming to a quorum of nodes only at the end.
     client1
-        .transfer_to_account_unsafe_unconfirmed(None, Amount::ONE, Account::chain(client2.chain_id))
+        .transfer_to_account_unsafe_unconfirmed(
+            AccountOwner::Chain,
+            Amount::ONE,
+            Account::chain(client2.chain_id),
+        )
         .await
         .unwrap();
     client1
-        .transfer_to_account_unsafe_unconfirmed(None, Amount::ONE, Account::chain(client2.chain_id))
+        .transfer_to_account_unsafe_unconfirmed(
+            AccountOwner::Chain,
+            Amount::ONE,
+            Account::chain(client2.chain_id),
+        )
         .await
         .unwrap();
     client1
@@ -1027,7 +1077,7 @@ where
     // Sending money from client2 fails, as a consequence.
     let obtained_error = client2
         .transfer_to_account_unsafe_unconfirmed(
-            None,
+            AccountOwner::Chain,
             Amount::from_tokens(2),
             Account::chain(client3.chain_id),
         )
@@ -1044,7 +1094,7 @@ where
     client2.synchronize_from_validators().await.unwrap();
     let certificate = client2
         .transfer_to_account(
-            None,
+            AccountOwner::Chain,
             Amount::from_tokens(2),
             Account::chain(client3.chain_id),
         )
@@ -1107,7 +1157,7 @@ where
     // Sending money from the admin chain is supported.
     let cert = admin
         .transfer_to_account(
-            None,
+            AccountOwner::Chain,
             Amount::from_tokens(2),
             Account::chain(user.chain_id()),
         )
@@ -1115,7 +1165,11 @@ where
         .unwrap()
         .unwrap();
     admin
-        .transfer_to_account(None, Amount::ONE, Account::chain(user.chain_id()))
+        .transfer_to_account(
+            AccountOwner::Chain,
+            Amount::ONE,
+            Account::chain(user.chain_id()),
+        )
         .await
         .unwrap()
         .unwrap();
@@ -1138,7 +1192,7 @@ where
     // Try to make a transfer back to the admin chain.
     let cert = user
         .transfer_to_account(
-            None,
+            AccountOwner::Chain,
             Amount::from_tokens(2),
             Account::chain(admin.chain_id()),
         )
@@ -1153,7 +1207,11 @@ where
 
     // Try again to make a transfer back to the admin chain.
     let cert = user
-        .transfer_to_account(None, Amount::ONE, Account::chain(admin.chain_id()))
+        .transfer_to_account(
+            AccountOwner::Chain,
+            Amount::ONE,
+            Account::chain(admin.chain_id()),
+        )
         .await
         .unwrap()
         .unwrap();
@@ -1179,10 +1237,14 @@ where
         .with_policy(ResourceControlPolicy::fuel_and_block());
     let sender = builder.add_root_chain(1, Amount::from_tokens(3)).await?;
 
-    let obtained_error = sender.burn(None, Amount::from_tokens(4)).await;
+    let obtained_error = sender
+        .burn(AccountOwner::Chain, Amount::from_tokens(4))
+        .await;
     assert_insufficient_funding_during_operation(obtained_error, 0);
 
-    let obtained_error = sender.burn(None, Amount::from_tokens(3)).await;
+    let obtained_error = sender
+        .burn(AccountOwner::Chain, Amount::from_tokens(3))
+        .await;
     assert_insufficient_funding_fees(obtained_error);
     Ok(())
 }
@@ -1346,7 +1408,7 @@ where
         *info2_b.manager.requested_locking.unwrap()
     );
     let bt_certificate = client2_b
-        .burn(None, Amount::from_tokens(1))
+        .burn(AccountOwner::Chain, Amount::from_tokens(1))
         .await
         .unwrap()
         .unwrap();
@@ -1363,7 +1425,7 @@ where
         .body
         .operations
         .contains(&Operation::System(SystemOperation::Transfer {
-            owner: None,
+            owner: AccountOwner::Chain,
             recipient: Recipient::Burn,
             amount: Amount::from_tokens(1),
         })));
@@ -1473,7 +1535,7 @@ where
 
     client2_b.prepare_chain().await.unwrap();
     let bt_certificate = client2_b
-        .burn(None, Amount::from_tokens(1))
+        .burn(AccountOwner::Chain, Amount::from_tokens(1))
         .await
         .unwrap()
         .unwrap();
@@ -1490,7 +1552,7 @@ where
         .body
         .operations
         .contains(&Operation::System(SystemOperation::Transfer {
-            owner: None,
+            owner: AccountOwner::Chain,
             recipient: Recipient::Burn,
             amount: Amount::from_tokens(1),
         })));
@@ -1822,7 +1884,7 @@ where
 
     // The other owner is leader now. Trying to submit a block should return `WaitForTimeout`.
     let result = client
-        .transfer(None, Amount::ONE, Recipient::root(2))
+        .transfer(AccountOwner::Chain, Amount::ONE, Recipient::root(2))
         .await
         .unwrap();
     let timeout = match result {
@@ -1849,7 +1911,7 @@ where
 
     // Now we are the leader, and the transfer should succeed.
     let _certificate = client
-        .transfer(None, Amount::ONE, Recipient::root(2))
+        .transfer(AccountOwner::Chain, Amount::ONE, Recipient::root(2))
         .await
         .unwrap()
         .unwrap();
@@ -1906,7 +1968,9 @@ where
     builder
         .set_fault_type([2], FaultType::OfflineWithInfo)
         .await;
-    let result = client0.burn(None, Amount::from_tokens(3)).await;
+    let result = client0
+        .burn(AccountOwner::Chain, Amount::from_tokens(3))
+        .await;
     assert!(result.is_err());
 
     // Client 1 thinks it is madness to burn 3 tokens! They want to publish a blob instead.
@@ -1947,7 +2011,10 @@ where
 
     // Client 0 now only tries to burn 1 token. Before that, they automatically finalize the
     // pending block, which publishes the blob, leaving 10 - 1 = 9.
-    client0.burn(None, Amount::from_tokens(1)).await.unwrap();
+    client0
+        .burn(AccountOwner::Chain, Amount::from_tokens(1))
+        .await
+        .unwrap();
     client0.synchronize_from_validators().await.unwrap();
     client0.process_inbox().await.unwrap();
     assert_eq!(
@@ -1958,7 +2025,10 @@ where
 
     // Burn another token so Client 1 sees that the blob is already published
     client1.prepare_chain().await.unwrap();
-    client1.burn(None, Amount::from_tokens(1)).await.unwrap();
+    client1
+        .burn(AccountOwner::Chain, Amount::from_tokens(1))
+        .await
+        .unwrap();
     client1.synchronize_from_validators().await.unwrap();
     client1.process_inbox().await.unwrap();
     assert_eq!(
@@ -1987,7 +2057,9 @@ where
     builder
         .set_fault_type([2], FaultType::OfflineWithInfo)
         .await;
-    let result = client.burn(None, Amount::from_tokens(3)).await;
+    let result = client
+        .burn(AccountOwner::Chain, Amount::from_tokens(3))
+        .await;
     assert!(result.is_err());
 
     // Now three validators are online again.
@@ -1995,7 +2067,7 @@ where
 
     // The client tries to burn another token. Before that, they automatically finalize the
     // pending block, which burns 3 tokens, leaving 10 - 3 - 1 = 6.
-    client.burn(None, Amount::ONE).await.unwrap();
+    client.burn(AccountOwner::Chain, Amount::ONE).await.unwrap();
     client.synchronize_from_validators().await.unwrap();
     client.process_inbox().await.unwrap();
     assert_eq!(
@@ -2045,7 +2117,9 @@ where
         .set_fault_type([1, 2], FaultType::DontProcessValidated)
         .await;
     builder.set_fault_type([3], FaultType::Offline).await;
-    let result = client0.burn(None, Amount::from_tokens(3)).await;
+    let result = client0
+        .burn(AccountOwner::Chain, Amount::from_tokens(3))
+        .await;
     assert!(result.is_err());
     let manager = client0
         .chain_info_with_manager_values()
@@ -2081,7 +2155,9 @@ where
     assert!(manager.requested_proposed.is_some());
     assert!(manager.requested_locking.is_none());
     assert_eq!(manager.current_round, Round::MultiLeader(0));
-    let result = client1.burn(None, Amount::from_tokens(2)).await;
+    let result = client1
+        .burn(AccountOwner::Chain, Amount::from_tokens(2))
+        .await;
     assert!(result.is_err());
 
     // Finally, three validators are online and honest again. Client 1 realizes there has been a
@@ -2100,7 +2176,10 @@ where
     );
     assert_eq!(manager.current_round, Round::MultiLeader(1));
     assert!(client1.pending_proposal().is_some());
-    client1.burn(None, Amount::from_tokens(4)).await.unwrap();
+    client1
+        .burn(AccountOwner::Chain, Amount::from_tokens(4))
+        .await
+        .unwrap();
 
     // Burning 3 and 4 tokens got finalized; the pending 2 tokens got skipped.
     client0.synchronize_from_validators().await.unwrap();
@@ -2125,7 +2204,7 @@ where
     let mut receiver = builder.add_root_chain(2, Amount::ZERO).await?;
     let recipient = Recipient::chain(receiver.chain_id());
     let cert = sender
-        .transfer(None, Amount::ONE, recipient)
+        .transfer(AccountOwner::Chain, Amount::ONE, recipient)
         .await
         .unwrap()
         .unwrap();
@@ -2210,7 +2289,11 @@ where
 
     // Send a message from chain 2 to chain 3.
     let certificate = client2
-        .transfer(None, Amount::from_millis(1), Recipient::chain(chain_id3))
+        .transfer(
+            AccountOwner::Chain,
+            Amount::from_millis(1),
+            Recipient::chain(chain_id3),
+        )
         .await
         .unwrap()
         .unwrap();
diff --git a/linera-core/src/unit_tests/worker_tests.rs b/linera-core/src/unit_tests/worker_tests.rs
index 98e50ac30c26..5ca104437b34 100644
--- a/linera-core/src/unit_tests/worker_tests.rs
+++ b/linera-core/src/unit_tests/worker_tests.rs
@@ -203,7 +203,7 @@ where
         chain_description,
         key_pair,
         Some(key_pair.public().into()),
-        None,
+        AccountOwner::Chain,
         Recipient::chain(target_id),
         amount,
         incoming_bundles,
@@ -221,7 +221,8 @@ where
 async fn make_transfer_certificate<S>(
     chain_description: ChainDescription,
     key_pair: &AccountSecretKey,
-    source: Option<Owner>,
+    authenticated_signer: Option<Owner>,
+    source: AccountOwner,
     recipient: Recipient,
     amount: Amount,
     incoming_bundles: Vec<IncomingBundle>,
@@ -237,7 +238,7 @@ where
     make_transfer_certificate_for_epoch(
         chain_description,
         key_pair,
-        source.or_else(|| Some(key_pair.public().into())),
+        authenticated_signer,
         source,
         recipient,
         amount,
@@ -257,7 +258,7 @@ async fn make_transfer_certificate_for_epoch<S>(
     chain_description: ChainDescription,
     key_pair: &AccountSecretKey,
     authenticated_signer: Option<Owner>,
-    source: Option<Owner>,
+    source: AccountOwner,
     recipient: Recipient,
     amount: Amount,
     incoming_bundles: Vec<IncomingBundle>,
@@ -323,9 +324,7 @@ where
                 account.chain_id,
                 MessageKind::Tracked,
                 SystemMessage::Credit {
-                    source: source
-                        .map(AccountOwner::User)
-                        .unwrap_or(AccountOwner::Chain),
+                    source,
                     target: account.owner,
                     amount,
                 },
@@ -1391,7 +1390,7 @@ where
         ChainDescription::Root(2),
         &sender_key_pair,
         Some(chain_key_pair.public().into()),
-        None,
+        AccountOwner::Chain,
         Recipient::chain(ChainId::root(2)),
         Amount::from_tokens(5),
         Vec::new(),
@@ -2124,7 +2123,8 @@ where
     let certificate00 = make_transfer_certificate(
         ChainDescription::Root(1),
         &sender_key_pair,
-        None,
+        Some(Owner::from(sender_key_pair.public())),
+        AccountOwner::Chain,
         Recipient::Account(sender_account),
         Amount::from_tokens(5),
         Vec::new(),
@@ -2143,7 +2143,8 @@ where
     let certificate01 = make_transfer_certificate(
         ChainDescription::Root(1),
         &sender_key_pair,
-        None,
+        Some(Owner::from(sender_key_pair.public())),
+        AccountOwner::Chain,
         Recipient::Burn,
         Amount::ONE,
         vec![IncomingBundle {
@@ -2185,6 +2186,7 @@ where
         ChainDescription::Root(1),
         &sender_key_pair,
         Some(sender),
+        AccountOwner::User(sender),
         Recipient::Account(recipient_account),
         Amount::from_tokens(3),
         Vec::new(),
@@ -2204,6 +2206,7 @@ where
         ChainDescription::Root(1),
         &sender_key_pair,
         Some(sender),
+        AccountOwner::User(sender),
         Recipient::Account(recipient_account),
         Amount::from_tokens(2),
         Vec::new(),
@@ -2224,6 +2227,7 @@ where
         ChainDescription::Root(2),
         &recipient_key_pair,
         Some(recipient),
+        AccountOwner::User(recipient),
         Recipient::Burn,
         Amount::ONE,
         vec![
@@ -2283,6 +2287,7 @@ where
         ChainDescription::Root(1),
         &sender_key_pair,
         Some(sender),
+        AccountOwner::User(sender),
         Recipient::Burn,
         Amount::from_tokens(3),
         vec![IncomingBundle {
@@ -2978,7 +2983,7 @@ async fn test_cross_chain_helper() -> anyhow::Result<()> {
         ChainDescription::Root(0),
         &key_pair0,
         Some(key_pair0.public().into()),
-        None,
+        AccountOwner::Chain,
         Recipient::chain(id1),
         Amount::ONE,
         Vec::new(),
@@ -2994,7 +2999,7 @@ async fn test_cross_chain_helper() -> anyhow::Result<()> {
         ChainDescription::Root(0),
         &key_pair0,
         Some(key_pair0.public().into()),
-        None,
+        AccountOwner::Chain,
         Recipient::chain(id1),
         Amount::ONE,
         Vec::new(),
@@ -3010,7 +3015,7 @@ async fn test_cross_chain_helper() -> anyhow::Result<()> {
         ChainDescription::Root(0),
         &key_pair0,
         Some(key_pair0.public().into()),
-        None,
+        AccountOwner::Chain,
         Recipient::chain(id1),
         Amount::ONE,
         Vec::new(),
@@ -3027,7 +3032,7 @@ async fn test_cross_chain_helper() -> anyhow::Result<()> {
         ChainDescription::Root(0),
         &key_pair0,
         Some(key_pair0.public().into()),
-        None,
+        AccountOwner::Chain,
         Recipient::chain(id1),
         Amount::ONE,
         Vec::new(),
@@ -3506,7 +3511,7 @@ where
     // The first round is the multi-leader round 0. Anyone is allowed to propose.
     // But non-owners are not allowed to transfer the chain's funds.
     let proposal = make_child_block(&change_ownership_value)
-        .with_transfer(None, Recipient::Burn, Amount::from_tokens(1))
+        .with_transfer(AccountOwner::Chain, Recipient::Burn, Amount::from_tokens(1))
         .into_proposal_with_round(&AccountSecretKey::generate(), Round::MultiLeader(0));
     let result = worker.handle_block_proposal(proposal).await;
     assert_matches!(result, Err(WorkerError::ChainError(error)) if matches!(&*error,
diff --git a/linera-execution/src/system.rs b/linera-execution/src/system.rs
index 3af1132eb566..1e8e3aa71c99 100644
--- a/linera-execution/src/system.rs
+++ b/linera-execution/src/system.rs
@@ -114,8 +114,7 @@ pub enum SystemOperation {
     /// Transfers `amount` units of value from the given owner's account to the recipient.
     /// If no owner is given, try to take the units out of the unattributed account.
     Transfer {
-        #[debug(skip_if = Option::is_none)]
-        owner: Option<Owner>,
+        owner: AccountOwner,
         recipient: Recipient,
         amount: Amount,
     },
@@ -414,13 +413,7 @@ where
                 ..
             } => {
                 let message = self
-                    .transfer(
-                        context.authenticated_signer,
-                        None,
-                        owner.map(AccountOwner::User).unwrap_or(AccountOwner::Chain),
-                        recipient,
-                        amount,
-                    )
+                    .transfer(context.authenticated_signer, None, owner, recipient, amount)
                     .await?;
 
                 if let Some(message) = message {
diff --git a/linera-execution/tests/test_system_execution.rs b/linera-execution/tests/test_system_execution.rs
index 9a7319dc0e06..04182ed3fca1 100644
--- a/linera-execution/tests/test_system_execution.rs
+++ b/linera-execution/tests/test_system_execution.rs
@@ -30,7 +30,7 @@ async fn test_simple_system_operation() -> anyhow::Result<()> {
     };
     let mut view = state.into_view().await;
     let operation = SystemOperation::Transfer {
-        owner: None,
+        owner: AccountOwner::Chain,
         amount: Amount::from_tokens(4),
         recipient: Recipient::Burn,
     };
diff --git a/linera-indexer/example/tests/test.rs b/linera-indexer/example/tests/test.rs
index 3823611c31c1..716a5f189d38 100644
--- a/linera-indexer/example/tests/test.rs
+++ b/linera-indexer/example/tests/test.rs
@@ -12,7 +12,7 @@ use std::{str::FromStr, sync::LazyLock, time::Duration};
 use linera_base::{
     command::resolve_binary,
     data_types::Amount,
-    identifiers::{Account, ChainId},
+    identifiers::{Account, AccountOwner, ChainId},
 };
 use linera_indexer_graphql_client::{
     indexer::{plugins, state, Plugins, State},
@@ -76,6 +76,7 @@ fn indexer_running(child: &mut Child) {
 async fn transfer(client: &reqwest::Client, from: ChainId, to: Account, amount: &str) {
     let variables = transfer::Variables {
         chain_id: from,
+        owner: AccountOwner::Chain,
         recipient_chain: to.chain_id,
         recipient_account: to.owner,
         amount: Amount::from_str(amount).unwrap(),
diff --git a/linera-rpc/tests/snapshots/format__format.yaml.snap b/linera-rpc/tests/snapshots/format__format.yaml.snap
index bdc284b7644f..8a4a22860f3f 100644
--- a/linera-rpc/tests/snapshots/format__format.yaml.snap
+++ b/linera-rpc/tests/snapshots/format__format.yaml.snap
@@ -1090,8 +1090,7 @@ SystemOperation:
       Transfer:
         STRUCT:
           - owner:
-              OPTION:
-                TYPENAME: Owner
+              TYPENAME: AccountOwner
           - recipient:
               TYPENAME: Recipient
           - amount:
diff --git a/linera-sdk/src/test/block.rs b/linera-sdk/src/test/block.rs
index e9f127f8cec0..3820bb770e43 100644
--- a/linera-sdk/src/test/block.rs
+++ b/linera-sdk/src/test/block.rs
@@ -9,7 +9,9 @@ use linera_base::{
     abi::ContractAbi,
     data_types::{Amount, ApplicationPermissions, Round, Timestamp},
     hashed::Hashed,
-    identifiers::{ApplicationId, ChainId, ChannelFullName, GenericApplicationId, Owner},
+    identifiers::{
+        AccountOwner, ApplicationId, ChainId, ChannelFullName, GenericApplicationId, Owner,
+    },
     ownership::TimeoutConfig,
 };
 use linera_chain::{
@@ -89,7 +91,7 @@ impl BlockBuilder {
     /// Adds a native token transfer to this block.
     pub fn with_native_token_transfer(
         &mut self,
-        sender: Option<Owner>,
+        sender: AccountOwner,
         recipient: Recipient,
         amount: Amount,
     ) -> &mut Self {
diff --git a/linera-service-graphql-client/gql/service_requests.graphql b/linera-service-graphql-client/gql/service_requests.graphql
index d6374ff1a44a..8ddc8285cf2d 100644
--- a/linera-service-graphql-client/gql/service_requests.graphql
+++ b/linera-service-graphql-client/gql/service_requests.graphql
@@ -358,6 +358,6 @@ subscription Notifications($chainId: ChainId!) {
   notifications(chainId: $chainId)
 }
 
-mutation Transfer($chainId: ChainId!, $recipient_chain: ChainId!, $recipient_account: AccountOwner!, $amount: Amount!) {
-  transfer(chainId: $chainId, recipient: { Account: { chain_id: $recipient_chain, owner: $recipient_account } }, amount: $amount)
+mutation Transfer($chainId: ChainId!, $owner: AccountOwner!, $recipient_chain: ChainId!, $recipient_account: AccountOwner!, $amount: Amount!) {
+  transfer(chainId: $chainId, owner: $owner, recipient: { Account: { chain_id: $recipient_chain, owner: $recipient_account } }, amount: $amount)
 }
diff --git a/linera-service-graphql-client/gql/service_schema.graphql b/linera-service-graphql-client/gql/service_schema.graphql
index 54ef5675e200..c4cc7f540dbe 100644
--- a/linera-service-graphql-client/gql/service_schema.graphql
+++ b/linera-service-graphql-client/gql/service_schema.graphql
@@ -747,7 +747,7 @@ type MutationRoot {
 	Transfers `amount` units of value from the given owner's account to the recipient.
 	If no owner is given, try to take the units out of the unattributed account.
 	"""
-	transfer(chainId: ChainId!, owner: Owner, recipient: Recipient!, amount: Amount!): CryptoHash!
+	transfer(chainId: ChainId!, owner: AccountOwner!, recipient: Recipient!, amount: Amount!): CryptoHash!
 	"""
 	Claims `amount` units of value from the given owner's account in the remote
 	`target` chain. Depending on its configuration, the `target` chain may refuse to
diff --git a/linera-service-graphql-client/tests/test.rs b/linera-service-graphql-client/tests/test.rs
index 6416fede9cf3..8ade791a809b 100644
--- a/linera-service-graphql-client/tests/test.rs
+++ b/linera-service-graphql-client/tests/test.rs
@@ -12,7 +12,7 @@ use std::{collections::BTreeMap, str::FromStr, sync::LazyLock, time::Duration};
 use fungible::{FungibleTokenAbi, InitialState};
 use linera_base::{
     data_types::Amount,
-    identifiers::{Account, ChainId},
+    identifiers::{Account, AccountOwner, ChainId},
     vm::VmRuntime,
 };
 use linera_service::cli_wrappers::{
@@ -38,6 +38,7 @@ fn reqwest_client() -> reqwest::Client {
 async fn transfer(client: &reqwest::Client, url: &str, from: ChainId, to: Account, amount: &str) {
     let variables = transfer::Variables {
         chain_id: from,
+        owner: AccountOwner::Chain,
         recipient_chain: to.chain_id,
         recipient_account: to.owner,
         amount: Amount::from_str(amount).unwrap(),
diff --git a/linera-service/benches/transfers.rs b/linera-service/benches/transfers.rs
index 483df632184b..f6d25faf3eb7 100644
--- a/linera-service/benches/transfers.rs
+++ b/linera-service/benches/transfers.rs
@@ -9,7 +9,7 @@ use futures::{
 use linera_base::{
     crypto::{AccountSecretKey, Ed25519SecretKey, Secp256k1SecretKey},
     data_types::Amount,
-    identifiers::{Account, ChainId, Owner},
+    identifiers::{Account, AccountOwner, ChainId},
     time::{Duration, Instant},
 };
 use linera_execution::system::Recipient;
@@ -88,7 +88,7 @@ async fn setup_native_token_balances(
         admin_chain
             .add_block(|block| {
                 block.with_native_token_transfer(
-                    None,
+                    AccountOwner::Chain,
                     recipient,
                     Amount::from_tokens(initial_balance),
                 );
@@ -119,7 +119,7 @@ fn prepare_transfers(
         .enumerate()
         .map(|(index, chain)| {
             let chain_id = chain.id();
-            let sender = Some(Owner::from(chain.public_key()));
+            let sender = AccountOwner::from(chain.public_key());
 
             let transfers = accounts
                 .iter()
diff --git a/linera-service/src/linera/main.rs b/linera-service/src/linera/main.rs
index 6e1a7cee036e..b89b516b5cfe 100644
--- a/linera-service/src/linera/main.rs
+++ b/linera-service/src/linera/main.rs
@@ -101,12 +101,8 @@ impl Runnable for Job {
                 amount,
             } => {
                 let chain_client = context.make_chain_client(sender.chain_id)?;
-                let owner = match sender.owner {
-                    AccountOwner::User(owner) => Some(owner),
-                    AccountOwner::Application(_) => {
-                        bail!("Can't transfer from an application account")
-                    }
-                    AccountOwner::Chain => None,
+                if let AccountOwner::Application(_) = sender.owner {
+                    bail!("Can't transfer from an application account")
                 };
                 info!(
                     "Starting transfer of {} native tokens from {} to {}",
@@ -118,7 +114,7 @@ impl Runnable for Job {
                         let chain_client = chain_client.clone();
                         async move {
                             chain_client
-                                .transfer_to_account(owner, amount, recipient)
+                                .transfer_to_account(sender.owner, amount, recipient)
                                 .await
                         }
                     })
diff --git a/linera-service/src/node_service.rs b/linera-service/src/node_service.rs
index 141a479ac8ec..f82766c5aeaa 100644
--- a/linera-service/src/node_service.rs
+++ b/linera-service/src/node_service.rs
@@ -18,7 +18,7 @@ use linera_base::{
     data_types::{Amount, ApplicationPermissions, Bytecode, TimeDelta, UserApplicationDescription},
     ensure,
     hashed::Hashed,
-    identifiers::{ApplicationId, ChainId, ModuleId, Owner, UserApplicationId},
+    identifiers::{AccountOwner, ApplicationId, ChainId, ModuleId, Owner, UserApplicationId},
     ownership::{ChainOwnership, TimeoutConfig},
     vm::VmRuntime,
     BcsHexParseError,
@@ -267,7 +267,7 @@ where
     async fn transfer(
         &self,
         chain_id: ChainId,
-        owner: Option<Owner>,
+        owner: AccountOwner,
         recipient: Recipient,
         amount: Amount,
     ) -> Result<CryptoHash, Error> {