Overflow Risk Mitigation in Merkle Sum Trees Using keccak256

backend/src/tests.rs

@@ -262,7 +262,7 @@ mod test {
             liability_commitment_logs[0],
             LiabilitiesCommitmentSubmittedFilter {
                 timestamp: U256::from(1),
-                mst_root: "0x18d6ab953235a811edffa4cead74ea045e7cd2085771a2269d59dca054c955b1"
+                mst_root: "0x177bf452ad139f067a64fe09fdc30aae46144d60abfa2ad9f0c70928e29a26d1"
                     .parse()
                     .unwrap(),
                 root_balances: vec![U256::from(556862), U256::from(556862)],

zk_prover/src/circuits/tests.rs

@@ -339,17 +339,17 @@ mod test {
                     cell_values: vec![
                         (
                             ((Any::advice(), 0).into(), 0).into(),
-                            "0xe113acd03b98f0bab0ef6f577245d5d008cbcc19ef2dab3608aa4f37f72a407"
+                            "0x167505f45c4ef4a0b051c30e881d2e8f881f26f5edb231396198a2cc1712f5ad"
                                 .to_string()
                         ),
                         (
                             ((Any::advice(), 0).into(), 1).into(),
-                            "0x17ef9d8ee0e2c8470814651413b71009a607a020214f749687384a7b7a7eb67a"
+                            "0x2c688f624d2bca741a1c2ad1ad2880721fbfd1613bbc5fe3d2ba66eb672e3aab"
                                 .to_string()
                         ),
                         (
                             ((Any::advice(), 1).into(), 0).into(),
-                            "0x17ef9d8ee0e2c8470814651413b71009a607a020214f749687384a7b7a7eb67a"
+                            "0x2c688f624d2bca741a1c2ad1ad2880721fbfd1613bbc5fe3d2ba66eb672e3aab"
                                 .to_string()
                         ),
                         (((Any::advice(), 2).into(), 0).into(), "0x2".to_string()),
@@ -364,17 +364,17 @@ mod test {
                     cell_values: vec![
                         (
                             ((Any::advice(), 0).into(), 0).into(),
-                            "0xe113acd03b98f0bab0ef6f577245d5d008cbcc19ef2dab3608aa4f37f72a407"
+                            "0x167505f45c4ef4a0b051c30e881d2e8f881f26f5edb231396198a2cc1712f5ad"
                                 .to_string()
                         ),
                         (
                             ((Any::advice(), 1).into(), 0).into(),
-                            "0x17ef9d8ee0e2c8470814651413b71009a607a020214f749687384a7b7a7eb67a"
+                            "0x2c688f624d2bca741a1c2ad1ad2880721fbfd1613bbc5fe3d2ba66eb672e3aab"
                                 .to_string()
                         ),
                         (
                             ((Any::advice(), 1).into(), 1).into(),
-                            "0xe113acd03b98f0bab0ef6f577245d5d008cbcc19ef2dab3608aa4f37f72a407"
+                            "0x167505f45c4ef4a0b051c30e881d2e8f881f26f5edb231396198a2cc1712f5ad"
                                 .to_string()
                         ),
                         (((Any::advice(), 2).into(), 0).into(), "0x2".to_string()),

zk_prover/src/merkle_sum_tree/entry.rs

@@ -1,20 +1,26 @@
-use crate::merkle_sum_tree::utils::big_intify_username;
 use crate::merkle_sum_tree::Node;
+use ethers::utils::keccak256;
 use num_bigint::BigUint;
 
 /// An entry in the Merkle Sum Tree from the database of the CEX.
 /// It contains the username and the balances of the user.
 #[derive(Clone, Debug, std::cmp::PartialEq)]
 pub struct Entry<const N_CURRENCIES: usize> {
-    username_as_big_uint: BigUint,
+    hashed_username: BigUint,
     balances: [BigUint; N_CURRENCIES],
     username: String,
 }
 
 impl<const N_CURRENCIES: usize> Entry<N_CURRENCIES> {
     pub fn new(username: String, balances: [BigUint; N_CURRENCIES]) -> Result<Self, &'static str> {
+        // Security Assumptions:
+        // Using `keccak256` for `hashed_username` ensures high collision resistance,
+        // appropriate for the assumed userbase of $2^{30}$.
+        // The `hashed_username` utilizes the full 256 bits produced by `keccak256`,
+        // but is adjusted to the field size through the Poseidon hash function's modulo operation.
+        let hashed_username: BigUint = BigUint::from_bytes_be(&keccak256(username.as_bytes()));
         Ok(Entry {
-            username_as_big_uint: big_intify_username(&username),
+            hashed_username,
             balances,
             username,
         })
@@ -25,7 +31,7 @@ impl<const N_CURRENCIES: usize> Entry<N_CURRENCIES> {
         let empty_balances: [BigUint; N_CURRENCIES] = std::array::from_fn(|_| BigUint::from(0u32));
 
         Entry {
-            username_as_big_uint: BigUint::from(0u32),
+            hashed_username: BigUint::from(0u32),
             balances: empty_balances,
             username: "0".to_string(),
         }
@@ -35,7 +41,7 @@ impl<const N_CURRENCIES: usize> Entry<N_CURRENCIES> {
     where
         [usize; N_CURRENCIES + 1]: Sized,
     {
-        Node::leaf(&self.username_as_big_uint, &self.balances)
+        Node::leaf(&self.hashed_username, &self.balances)
     }
 
     /// Stores the new balance values
@@ -49,15 +55,15 @@ impl<const N_CURRENCIES: usize> Entry<N_CURRENCIES> {
         [usize; N_CURRENCIES + 1]: Sized,
     {
         self.balances = updated_balances.clone();
-        Node::leaf(&self.username_as_big_uint, updated_balances)
+        Node::leaf(&self.hashed_username, updated_balances)
     }
 
     pub fn balances(&self) -> &[BigUint; N_CURRENCIES] {
         &self.balances
     }
 
     pub fn username_as_big_uint(&self) -> &BigUint {
-        &self.username_as_big_uint
+        &self.hashed_username
     }
 
     pub fn username(&self) -> &str {