fix(noir-contracts): change variable and migrate to poseidon

This is part of addressing Zac's feedback on Slack.

Co-authored-by: Alejo Amiras <[email protected]>

Author: AztecBot

noir-projects/noir-contracts/contracts/app/private_voting_contract/src/main.nr

@@ -10,7 +10,11 @@ pub contract PrivateVoting {
         keys::getters::get_public_keys,
         macros::{functions::{initializer, internal, private, public, utility}, storage::storage},
     };
-    use dep::aztec::protocol_types::{address::AztecAddress, traits::{Hash, ToField}};
+    use dep::aztec::protocol_types::{
+        address::AztecAddress,
+        hash::poseidon2_hash,
+        traits::{Hash, ToField},
+    };
     use dep::aztec::state_vars::{Map, PublicImmutable, PublicMutable};
 
     // docs:end:imports
@@ -39,10 +43,11 @@ pub contract PrivateVoting {
     #[private]
     // annotation to mark function as private and expose private context
     fn cast_vote(candidate: Field) {
-        let msg_sender_npk_m_hash = get_public_keys(context.msg_sender()).npk_m.hash();
+        let msg_sender_nullifier_public_key_message_hash =
+            get_public_keys(context.msg_sender()).npk_m.hash();
 
-        let secret = context.request_nsk_app(msg_sender_npk_m_hash); // get secret key of caller of function
-        let nullifier = std::hash::pedersen_hash([context.msg_sender().to_field(), secret]); // derive nullifier from sender and secret
+        let secret = context.request_nsk_app(msg_sender_nullifier_public_key_message_hash); // get secret key of caller of function
+        let nullifier = poseidon2_hash([context.msg_sender().to_field(), secret]); // derive nullifier from sender and secret
         context.push_nullifier(nullifier);
         PrivateVoting::at(context.this_address()).add_to_tally_public(candidate).enqueue(
             &mut context,