New Native Operation PublicKeyOf (#355)

* wrote some initial code

* added way to input private key into circuit

* TypedValue::SecretKey hashed as 10 32-bit limbs

* Check PublicKeyOf in Frontend and Middleware

* Diff review

* PR review

* Finish utest

* Fix bounds check

* added giving secret key witness to circuit

* Test & doc improvements

* added private key comparison to circuit and added test cases

* cargo fmt

* Add frontend tests for PublicKeyOf

* Add public_key_of and hash_of to op! macro

* Add ownership check to ticket example

* Group order checking in tests

* More negative test cases at circuit level

* Cleanups after self review

* clippy fixes

* Fixes after merge.  Temporarily remove plonky2 commit hash

* Add a nullifier to the ticket test example

* Test PublicKeyOf with a real prover (not mock)

* plonky-u32 dependency

* feat: optimize operation checks

Skip the circuits that verify operation checks other than None, Copy or
NewEntry for the public statements.  This works because public
statements are created by copying private statements, so we never use
the other operation checks in those slots.

---------

Co-authored-by: Andrew Twyman <artwyman@gmail.com>
Co-authored-by: Eduard S. <eduardsanou@posteo.net>

src/middleware/mod.rs

@@ -34,7 +34,8 @@ use serialization::*;
 pub use statement::*;
 
 use crate::backends::plonky2::primitives::{
-    ec::curve::Point as PublicKey, merkletree::MerkleProof,
+    ec::{curve::Point as PublicKey, schnorr::SecretKey},
+    merkletree::MerkleProof,
 };
 
 pub const SELF: PodId = PodId(SELF_ID_HASH);
@@ -62,8 +63,10 @@ pub enum TypedValue {
     ),
     // Uses the serialization for middleware::Value:
     Raw(RawValue),
-    // Public key variant
+    // Schnorr public key variant (EC point)
     PublicKey(PublicKey),
+    // Schnorr secret key variant (scalar)
+    SecretKey(SecretKey),
     PodId(PodId),
     // UNTAGGED TYPES:
     #[serde(untagged)]
@@ -114,6 +117,12 @@ impl From<PublicKey> for TypedValue {
     }
 }
 
+impl From<SecretKey> for TypedValue {
+    fn from(sk: SecretKey) -> Self {
+        TypedValue::SecretKey(sk)
+    }
+}
+
 impl From<PodId> for TypedValue {
     fn from(id: PodId) -> Self {
         TypedValue::PodId(id)
@@ -188,6 +197,28 @@ impl TryFrom<&TypedValue> for PodId {
     }
 }
 
+impl TryFrom<&TypedValue> for PublicKey {
+    type Error = Error;
+    fn try_from(v: &TypedValue) -> std::result::Result<Self, Self::Error> {
+        if let TypedValue::PublicKey(pk) = v {
+            Ok(*pk)
+        } else {
+            Err(Error::custom("Value not a public key".to_string()))
+        }
+    }
+}
+
+impl TryFrom<&TypedValue> for SecretKey {
+    type Error = Error;
+    fn try_from(v: &TypedValue) -> std::result::Result<Self, Self::Error> {
+        if let TypedValue::SecretKey(sk) = v {
+            Ok(sk.clone())
+        } else {
+            Err(Error::custom("Value not a secret key".to_string()))
+        }
+    }
+}
+
 impl fmt::Display for TypedValue {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
@@ -233,6 +264,7 @@ impl fmt::Display for TypedValue {
                 write!(f, "]")
             }
             TypedValue::PublicKey(p) => write!(f, "PublicKey({})", p),
+            TypedValue::SecretKey(p) => write!(f, "SecretKey({})", p),
             TypedValue::PodId(p) => {
                 write!(f, "0x{}", p.0.encode_hex::<String>())
             }
@@ -254,6 +286,7 @@ impl From<&TypedValue> for RawValue {
             TypedValue::Array(a) => RawValue::from(a.commitment()),
             TypedValue::Raw(v) => *v,
             TypedValue::PublicKey(p) => RawValue::from(hash_fields(&p.as_fields())),
+            TypedValue::SecretKey(sk) => RawValue::from(hash_fields(&sk.to_limbs())),
             TypedValue::PodId(id) => RawValue::from(id.0),
         }
     }
@@ -321,6 +354,19 @@ impl JsonSchema for TypedValue {
             ..Default::default()
         };
 
+        let secret_key_schema = schemars::schema::SchemaObject {
+            instance_type: Some(SingleOrVec::Single(Box::new(InstanceType::Object))),
+            object: Some(Box::new(schemars::schema::ObjectValidation {
+                // SecretKey is serialized as a string
+                properties: [("SecretKey".to_string(), gen.subschema_for::<String>())]
+                    .into_iter()
+                    .collect(),
+                required: ["SecretKey".to_string()].into_iter().collect(),
+                ..Default::default()
+            })),
+            ..Default::default()
+        };
+
         // This is the part that Schemars can't generate automatically:
         let untagged_array_schema = gen.subschema_for::<Array>();
         let untagged_set_schema = gen.subschema_for::<Set>();
@@ -334,6 +380,7 @@ impl JsonSchema for TypedValue {
                     Schema::Object(int_schema),
                     Schema::Object(raw_schema),
                     Schema::Object(public_key_schema),
+                    Schema::Object(secret_key_schema),
                     untagged_array_schema,
                     untagged_dictionary_schema,
                     untagged_string_schema,