diff --git a/Cargo.lock b/Cargo.lock
index d8568bf685e..3256db6e14c 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -12879,7 +12879,9 @@ dependencies = [
  "apollo_config",
  "async-recursion",
  "async-trait",
+ "blake2",
  "derive_more",
+ "digest 0.10.7",
  "ethnum",
  "expect-test",
  "hex",
diff --git a/crates/starknet_committer/Cargo.toml b/crates/starknet_committer/Cargo.toml
index 906ede3d156..4de6c991c6f 100644
--- a/crates/starknet_committer/Cargo.toml
+++ b/crates/starknet_committer/Cargo.toml
@@ -7,12 +7,15 @@ license.workspace = true
 description = "Computes and manages Starknet state."
 
 [features]
+os_input = ["dep:blake2", "dep:digest"]
 testing = ["starknet_patricia/testing"]
 
 [dependencies]
 apollo_config.workspace = true
 async-trait.workspace = true
+blake2 = { workspace = true, optional = true }
 derive_more = { workspace = true, features = ["as_ref", "from", "into"] }
+digest = { workspace = true, optional = true }
 ethnum.workspace = true
 pretty_assertions.workspace = true
 rand.workspace = true
diff --git a/crates/starknet_committer/src/db/serde_db_utils.rs b/crates/starknet_committer/src/db/serde_db_utils.rs
index 67babb06e9d..613ef870d60 100644
--- a/crates/starknet_committer/src/db/serde_db_utils.rs
+++ b/crates/starknet_committer/src/db/serde_db_utils.rs
@@ -1,8 +1,17 @@
+#[cfg(feature = "os_input")]
+use blake2::Blake2s256;
+#[cfg(feature = "os_input")]
+use digest::Digest;
 use serde::{Deserialize, Serialize};
 use starknet_api::block::BlockNumber;
+#[cfg(feature = "os_input")]
+use starknet_patricia::patricia_merkle_tree::types::NodeIndex;
 use starknet_patricia_storage::storage_trait::DbValue;
 use starknet_types_core::felt::Felt;
 
+#[cfg(feature = "os_input")]
+use crate::patricia_merkle_tree::tree::SortedLeavesRequest;
+
 #[derive(Clone, Debug, Default, Deserialize, Eq, PartialEq, Hash, Serialize)]
 pub struct DbBlockNumber(pub BlockNumber);
 
@@ -23,3 +32,51 @@ pub fn serialize_felt_no_packing(felt: Felt) -> DbValue {
 pub fn deserialize_felt_no_packing(value: &DbValue) -> Felt {
     Felt::from_bytes_be_slice(&value.0)
 }
+
+/// BLAKE2s-256 digest over a deterministic encoding of [`SortedLeavesRequest`].
+///
+/// The hashed payload is a concatenation length-prefixed big-endian 32-byte
+/// [`NodeIndex`] values (`idx.0.to_be_bytes()`), in this order:
+///
+/// 1. Classes trie — `len(classes_sorted)` then each class leaf index (already sorted).
+/// 2. Contracts trie — `len(contracts_sorted)` then each contract leaf index (already sorted).
+/// 3. Storage tries — `len(storage_sorted)` then, for each contract index in ascending order: the
+///    contract index, `len(storage slot indices)` for that contract, then each storage slot index
+///    (already sorted within the contract).
+#[cfg(feature = "os_input")]
+pub fn accessed_keys_digest(sorted: &SortedLeavesRequest<'_>) -> [u8; 32] {
+    let mut payload = Vec::new();
+
+    let class = sorted.class_sorted.get_indices();
+    payload.extend_from_slice(&encode_usize(class.len()));
+    for idx in class {
+        payload.extend_from_slice(&idx.0.to_be_bytes());
+    }
+
+    let contract = sorted.contract_sorted.get_indices();
+    payload.extend_from_slice(&encode_usize(contract.len()));
+    for idx in contract {
+        payload.extend_from_slice(&idx.0.to_be_bytes());
+    }
+
+    let mut contract_indices: Vec<NodeIndex> = sorted.storage_sorted.keys().copied().collect();
+    contract_indices.sort_unstable();
+
+    payload.extend_from_slice(&encode_usize(contract_indices.len()));
+    for contract_idx in contract_indices {
+        let sorted_slots = sorted.storage_sorted.get(&contract_idx).unwrap();
+        payload.extend_from_slice(&contract_idx.0.to_be_bytes());
+        let slot_indices = sorted_slots.get_indices();
+        payload.extend_from_slice(&encode_usize(slot_indices.len()));
+        for slot in slot_indices {
+            payload.extend_from_slice(&slot.0.to_be_bytes());
+        }
+    }
+
+    Blake2s256::digest(&payload).into()
+}
+
+#[cfg(feature = "os_input")]
+fn encode_usize(n: usize) -> [u8; 8] {
+    u64::try_from(n).expect("accessed leaf count exceeds u64::MAX").to_be_bytes()
+}
diff --git a/crates/starknet_committer/src/patricia_merkle_tree/tree.rs b/crates/starknet_committer/src/patricia_merkle_tree/tree.rs
index f43c2624281..dcfade54499 100644
--- a/crates/starknet_committer/src/patricia_merkle_tree/tree.rs
+++ b/crates/starknet_committer/src/patricia_merkle_tree/tree.rs
@@ -63,6 +63,7 @@ pub struct LeavesRequest {
 pub struct SortedLeavesRequest<'a> {
     pub class_sorted: SortedLeafIndices<'a>,
     pub contract_sorted: SortedLeafIndices<'a>,
+    // TODO(Ariel): use BTreeMap here and in fetch_all_patricia_paths.
     pub storage_sorted: HashMap<NodeIndex, SortedLeafIndices<'a>>,
 }