refactor: renaming nullifier secret key as hiding key

As Nico proposed on [slack](https://aztecprotocol.slack.com/archives/C060PU5R327/p1769534402985839) makes sense to rename the key to indicate what's its purpose is - purpose of nullifier secret key is not to authenticate who is able to nullify a given note (that is expected to be guarded by e.g. checking a  `msg_sender`) but instead its purpose is to prevent anyone else but the people who know the key from figuring out that a note has been nullified.

## Followup Work

Mike pointed out on [slack](https://aztecprotocol.slack.com/archives/C060PU5R327/p1769681367559339?thread_ts=1769534402.985839&cid=C060PU5R327) that it's a good idea to super clarify the `request_nhk_app` function. I agree and plan on doing that in a followup PR.

Author: benesjan

barretenberg/cpp/src/barretenberg/vm2/testing/minimal_tx.testdata.bin

@@ -1,7 +1,7 @@
 module.exports = {
   "\\sk": "{\\color{red}{sk}}",
   "\\seed": "{\\color{red}\\text{{seed}}}",
-  "\\nskm": "{\\color{red}{nsk_m}}",
+  "\\nhkm": "{\\color{red}{nhk_m}}",
   "\\tskm": "{\\color{red}{tsk_m}}",
   "\\ivskm": "{\\color{red}{ivsk_m}}",
   "\\ovskm": "{\\color{red}{ovsk_m}}",
@@ -16,7 +16,7 @@ module.exports = {
   "\\constructorhash": "{\\color{green}{constructor_hash}}",
   "\\classid": "{\\color{green}{classid}}",
 
-  "\\nskapp": "{\\color{red}{nsk_{app}}}",
+  "\\nhkapp": "{\\color{red}{nhk_{app}}}",
   "\\tskapp": "{\\color{red}{tsk_{app}}}",
   "\\ivsk": "{\\color{red}{ivsk}}",
   "\\ovskapp": "{\\color{red}{ovsk_{app}}}",

barretenberg/docs/src/katex-macros.js

@@ -38,7 +38,7 @@ pub contract PrivateVoting {
     fn cast_vote(candidate: Field) {
         let msg_sender_npk_m_hash = get_public_keys(self.msg_sender()).npk_m.hash();
 
-        let secret = self.context.request_nsk_app(msg_sender_npk_m_hash); // get secret key of caller of function
+        let secret = self.context.request_nhk_app(msg_sender_npk_m_hash); // get nullifier hiding key of caller of function
         let nullifier = poseidon2_hash([self.msg_sender().to_field(), secret]); // derive nullifier from sender and secret
         self.context.push_nullifier(nullifier);
         self.enqueue_self.add_to_tally_public(candidate);

boxes/boxes/vanilla/contracts/src/main.nr

@@ -160,9 +160,9 @@ impl NoteHash for CustomHashNote {
         owner: AztecAddress,
         note_hash_for_nullification: Field,
     ) -> Field {
-        // Standard nullifier using owner's nullifier secret key
+        // Standard nullifier using owner's nullifier hiding key
         let owner_npk_m = aztec::keys::getters::get_public_keys(owner).npk_m;
-        let secret = context.request_nsk_app(owner_npk_m.hash());
+        let secret = context.request_nhk_app(owner_npk_m.hash());
         poseidon2_hash_with_separator(
             [note_hash_for_nullification, secret],
             DOM_SEP__NOTE_NULLIFIER,
@@ -175,7 +175,7 @@ impl NoteHash for CustomHashNote {
         note_hash_for_nullification: Field,
     ) -> Field {
         let owner_npk_m = aztec::keys::getters::get_public_keys(owner).npk_m;
-        let secret = aztec::keys::getters::get_nsk_app(owner_npk_m.hash());
+        let secret = aztec::keys::getters::get_nhk_app(owner_npk_m.hash());
         poseidon2_hash_with_separator(
             [note_hash_for_nullification, secret],
             DOM_SEP__NOTE_NULLIFIER,

docs/docs-developers/docs/aztec-nr/framework-description/how_to_implement_custom_notes.md

@@ -43,8 +43,8 @@ Nullifier keys enable spending private notes. When using a note (like spending a
 
 **How it works:**
 
-1. Each account has a master nullifier key pair (`Npk_m`, `nsk_m`)
-2. For each application, an **app-siloed** key is derived: `nsk_app = hash(nsk_m, app_contract_address)`
+1. Each account has a master nullifier key pair (`Npk_m`, `nhk_m`)
+2. For each application, an **app-siloed** key is derived: `nhk_app = hash(nhk_m, app_contract_address)`
 3. To spend a note, compute its nullifier using the note hash and app-siloed key
 4. The protocol verifies the app-siloed key comes from your master key and that your master public key is in your address
 
@@ -150,7 +150,7 @@ Nullifier keys are **app-siloed** - scoped to each contract that uses them. This
 **How it works:**
 
 ```text
-nsk_app = hash(nsk_m, app_contract_address)
+nhk_app = hash(nhk_m, app_contract_address)
 ```
 
 **Security benefits:**

docs/docs-developers/docs/foundational-topics/accounts/keys.md

@@ -29,6 +29,33 @@ These methods were not used by PXE and returned a subset of the information alre
 
 The membership witness methods return both the sibling path and additional context (leaf index, preimage data) needed for proofs.
 
+### [Protocol] "Nullifier secret key" renamed to "nullifier hiding key" (nsk → nhk)
+
+The nullifier secret key (`nsk_m` / `nsk_app`) has been renamed to nullifier hiding key (`nhk_m` / `nhk_app`). This is a protocol-breaking change: the domain separator string changes from `"az_nsk_m"` to `"az_nhk_m"`, producing a different constant value.
+
+**Noir changes:**
+```diff
+- context.request_nsk_app(npk_m_hash)
++ context.request_nhk_app(npk_m_hash)
+
+- get_nsk_app(npk_m_hash)
++ get_nhk_app(npk_m_hash)
+```
+
+**TypeScript changes:**
+```diff
+- import { computeAppNullifierSecretKey, deriveMasterNullifierSecretKey } from '@aztec/stdlib/keys';
++ import { computeAppNullifierHidingKey, deriveMasterNullifierHidingKey } from '@aztec/stdlib/keys';
+
+- const masterNullifierSecretKey = deriveMasterNullifierSecretKey(secret);
++ const masterNullifierHidingKey = deriveMasterNullifierHidingKey(secret);
+
+- const nskApp = await computeAppNullifierSecretKey(masterNullifierSecretKey, contractAddress);
++ const nhkApp = await computeAppNullifierHidingKey(masterNullifierHidingKey, contractAddress);
+```
+
+The `GeneratorIndex.NSK_M` enum member is now `GeneratorIndex.NHK_M`.
+
 ### [Aztec.nr] `protocol_types` renamed to `protocol`
 
 The `protocol_types` re-export from the `aztec` crate has been renamed to `protocol`. Update all imports accordingly:

docs/docs-developers/docs/resources/migration_notes.md

@@ -1,7 +1,7 @@
 module.exports = {
   "\\sk": "{\\color{red}{sk}}",
   "\\seed": "{\\color{red}\\text{{seed}}}",
-  "\\nskm": "{\\color{red}{nsk_m}}",
+  "\\nhkm": "{\\color{red}{nhk_m}}",
   "\\tskm": "{\\color{red}{tsk_m}}",
   "\\ivskm": "{\\color{red}{ivsk_m}}",
   "\\ovskm": "{\\color{red}{ovsk_m}}",
@@ -16,7 +16,7 @@ module.exports = {
   "\\constructorhash": "{\\color{green}{constructor_hash}}",
   "\\classid": "{\\color{green}{classid}}",
 
-  "\\nskapp": "{\\color{red}{nsk_{app}}}",
+  "\\nhkapp": "{\\color{red}{nhk_{app}}}",
   "\\tskapp": "{\\color{red}{tsk_{app}}}",
   "\\ivsk": "{\\color{red}{ivsk}}",
   "\\ovskapp": "{\\color{red}{ovsk_{app}}}",

docs/src/katex-macros.js

@@ -704,30 +704,30 @@ impl PrivateContext {
         self.nullifier_read_requests.push(request);
     }
 
-    /// Requests the app-siloed nullifier secret key (nsk_app) for the given (hashed) master nullifier public key
+    /// Requests the app-siloed nullifier hiding key (nhk_app) for the given (hashed) master nullifier public key
     /// (npk_m), from the user's PXE.
     ///
     /// Advanced function: Only needed if you're designing your own notes and/or nullifiers.
     ///
     /// Contracts are not allowed to compute nullifiers for other contracts, as that would let them read parts of their
-    /// private state. Because of this, a contract is only given an "app-siloed secret key", which is constructed by
-    /// hashing the user's master nullifier secret key with the contract's address. However, because contracts cannot
-    /// be trusted with a user's master nullifier secret key (because we don't know which contracts are honest or
+    /// private state. Because of this, a contract is only given an "app-siloed key", which is constructed by
+    /// hashing the user's master nullifier hiding key with the contract's address. However, because contracts cannot
+    /// be trusted with a user's master nullifier hiding key (because we don't know which contracts are honest or
     /// malicious), the PXE refuses to provide any master secret keys to any app smart contract function. This means
-    /// app functions are unable to prove that the derivation of an app-siloed nullifier secret key has been computed
-    /// correctly. Instead, an app function can request to the kernel (via `request_nsk_app`) that it validates the
+    /// app functions are unable to prove that the derivation of an app-siloed nullifier hiding key has been computed
+    /// correctly. Instead, an app function can request to the kernel (via `request_nhk_app`) that it validates the
     /// siloed derivation, since the kernel has been vetted to not leak any master secret keys.
     ///
-    /// A common nullification scheme is to inject a nullifier secret key into the preimage of a nullifier, to make the
+    /// A common nullification scheme is to inject a nullifier hiding key into the preimage of a nullifier, to make the
     /// nullifier deterministic but random-looking. This function enables that flow.
     ///
     /// # Arguments
     /// * `npk_m_hash` - A hash of the master nullifier public key of the user whose PXE is executing this function.
     ///
     /// # Returns
-    /// * The app-siloed nullifier secret key that corresponds to the given `npk_m_hash`.
+    /// * The app-siloed nullifier hiding key that corresponds to the given `npk_m_hash`.
     ///
-    pub fn request_nsk_app(&mut self, npk_m_hash: Field) -> Field {
+    pub fn request_nhk_app(&mut self, npk_m_hash: Field) -> Field {
         self.request_sk_app(npk_m_hash, NULLIFIER_INDEX)
     }
 

noir-projects/aztec-nr/aztec/src/context/private_context.nr

@@ -1,4 +1,4 @@
-use crate::protocol::constants::{DOM_SEP__IVSK_M, DOM_SEP__NSK_M, DOM_SEP__OVSK_M, DOM_SEP__TSK_M};
+use crate::protocol::constants::{DOM_SEP__IVSK_M, DOM_SEP__NHK_M, DOM_SEP__OVSK_M, DOM_SEP__TSK_M};
 
 pub global NULLIFIER_INDEX: Field = 0;
 pub global INCOMING_INDEX: Field = 1;
@@ -8,4 +8,4 @@ pub global TAGGING_INDEX: Field = 3;
 pub global NUM_KEY_TYPES: u32 = 4;
 
 pub global sk_generators: [Field; 4] =
-    [DOM_SEP__NSK_M as Field, DOM_SEP__IVSK_M as Field, DOM_SEP__OVSK_M as Field, DOM_SEP__TSK_M as Field];
+    [DOM_SEP__NHK_M as Field, DOM_SEP__IVSK_M as Field, DOM_SEP__OVSK_M as Field, DOM_SEP__TSK_M as Field];

noir-projects/aztec-nr/aztec/src/keys/constants.nr

@@ -4,7 +4,7 @@ use crate::{
 };
 use crate::protocol::{address::AztecAddress, public_keys::PublicKeys};
 
-pub unconstrained fn get_nsk_app(npk_m_hash: Field) -> Field {
+pub unconstrained fn get_nhk_app(npk_m_hash: Field) -> Field {
     get_key_validation_request(npk_m_hash, NULLIFIER_INDEX).sk_app
 }