Fix new account tx's not able to verify (#458)

* ProvenTransaction for new accounts uses Digest::default()

* Digest default if new account .hash() if old account

* Add proof_init_hash() to Account

* Change comment

* Improve comment

miden-lib/src/transaction/inputs.rs

@@ -25,7 +25,7 @@ impl ToTransactionKernelInputs for PreparedTransaction {
         let account = self.account();
         let stack_inputs = TransactionKernel::build_input_stack(
             account.id(),
-            if account.is_new() { None } else { Some(account.hash()) },
+            account.proof_init_hash(),
             self.input_notes().commitment(),
             self.block_header().hash(),
         );
@@ -42,7 +42,7 @@ impl ToTransactionKernelInputs for ExecutedTransaction {
         let account = self.initial_account();
         let stack_inputs = TransactionKernel::build_input_stack(
             account.id(),
-            if account.is_new() { None } else { Some(account.hash()) },
+            account.proof_init_hash(),
             self.input_notes().commitment(),
             self.block_header().hash(),
         );
@@ -59,7 +59,7 @@ impl ToTransactionKernelInputs for TransactionWitness {
         let account = self.account();
         let stack_inputs = TransactionKernel::build_input_stack(
             account.id(),
-            if account.is_new() { None } else { Some(account.hash()) },
+            account.proof_init_hash(),
             self.input_notes().commitment(),
             self.block_header().hash(),
         );

miden-lib/src/transaction/mod.rs

@@ -104,13 +104,13 @@ impl TransactionKernel {
     ///   tuples for the notes consumed by the transaction.
     pub fn build_input_stack(
         acct_id: AccountId,
-        init_acct_hash: Option<Digest>,
+        init_acct_hash: Digest,
         input_notes_hash: Digest,
         block_hash: Digest,
     ) -> StackInputs {
         let mut inputs: Vec<Felt> = Vec::with_capacity(13);
         inputs.extend(input_notes_hash);
-        inputs.extend_from_slice(init_acct_hash.unwrap_or_default().as_elements());
+        inputs.extend_from_slice(init_acct_hash.as_elements());
         inputs.push(acct_id.into());
         inputs.extend_from_slice(block_hash.as_elements());
         StackInputs::new(inputs)

miden-tx/src/prover/mod.rs

@@ -5,7 +5,7 @@ use miden_objects::{
 };
 use miden_prover::prove;
 pub use miden_prover::ProvingOptions;
-use vm_processor::MemAdviceProvider;
+use vm_processor::{Digest, MemAdviceProvider};
 
 use super::{TransactionHost, TransactionProverError};
 
@@ -64,7 +64,11 @@ impl TransactionProver {
 
         Ok(ProvenTransaction::new(
             account_id,
-            initial_account_hash,
+            if tx_witness.account().is_new() {
+                Digest::default()
+            } else {
+                initial_account_hash
+            },
             tx_outputs.account.hash(),
             input_notes,
             tx_outputs.output_notes.into(),

miden-tx/src/verifier/mod.rs

@@ -34,7 +34,7 @@ impl TransactionVerifier {
         // build stack inputs and outputs
         let stack_inputs = TransactionKernel::build_input_stack(
             transaction.account_id(),
-            Some(transaction.initial_account_hash()),
+            transaction.initial_account_hash(),
             transaction.input_notes().commitment(),
             transaction.block_ref(),
         );

objects/src/accounts/mod.rs

@@ -110,6 +110,22 @@ impl Account {
         )
     }
 
+    /// Returns hash of this account as used for the initial account state hash in transaction  
+    /// proofs.  
+    ///  
+    /// For existing accounts, this is exactly the same as [Account::hash()], however, for new  
+    /// accounts this value is set to [ZERO; 4]. This is because when a transaction is executed  
+    /// agains a new account, public input for the initial account state is set to [ZERO; 4] to  
+    /// distinguish new accounts from existing accounts. The actual hash of the initial account  
+    /// state (and the initial state itself), are provided to the VM via the advice provider.
+    pub fn proof_init_hash(&self) -> Digest {
+        if self.is_new() {
+            Digest::default()
+        } else {
+            self.hash()
+        }
+    }
+
     /// Returns unique identifier of this account.
     pub fn id(&self) -> AccountId {
         self.id