diff --git a/crates/blockifier/src/execution/alias_keys.rs b/crates/blockifier/src/execution/alias_keys.rs
index 58ef8141272..90e8ffb8b9a 100644
--- a/crates/blockifier/src/execution/alias_keys.rs
+++ b/crates/blockifier/src/execution/alias_keys.rs
@@ -5,11 +5,27 @@ use starknet_types_core::felt::Felt;
 use crate::state::cached_state::{CachedState, StorageEntry};
 use crate::state::state_api::{StateReader, StateResult};
 
+/// Keys in contract addresses up to this address don't get aliases.
+const N_SAVED_CONTRACT_ADDRESSES: u8 = 16;
+/// The alias of a felt X up to this number is X.
+/// This trivial mappings don't write to the alias contract.
+const N_SELF_ALIASES: u8 = 128;
+
 /// Returns the number of storage keys charged for new allocated aliases.
+/// Counts keys that were previously empty and are now filled.
 pub fn n_charged_storage_keys<S: StateReader>(
-    _state: &CachedState<S>,
-    _storage_changes: &HashMap<StorageEntry, Felt>,
+    state: &CachedState<S>,
+    storage_changes: &HashMap<StorageEntry, Felt>,
 ) -> StateResult<usize> {
-    // TODO: Implement this function
-    Ok(0)
+    let mut n_storage_keys = 0;
+    for ((contract_address, storage_key), new_value) in storage_changes {
+        if contract_address.0.0 >= N_SAVED_CONTRACT_ADDRESSES.into()
+            && storage_key.0.0 >= N_SELF_ALIASES.into()
+            && state.get_storage_at(*contract_address, *storage_key)? == Felt::ZERO
+            && new_value != &Felt::ZERO
+        {
+            n_storage_keys += 1;
+        }
+    }
+    Ok(n_storage_keys)
 }
diff --git a/crates/starknet_api/src/core.rs b/crates/starknet_api/src/core.rs
index 8296075c8d4..c9bf5413fe9 100644
--- a/crates/starknet_api/src/core.rs
+++ b/crates/starknet_api/src/core.rs
@@ -347,7 +347,7 @@ pub struct StateDiffCommitment(pub PoseidonHash);
     derive_more:: Deref,
 )]
 #[display(fmt = "{}", "_0.to_fixed_hex_string()")]
-pub struct PatriciaKey(StarkHash);
+pub struct PatriciaKey(pub StarkHash);
 
 // 2**251
 pub const PATRICIA_KEY_UPPER_BOUND: &str =