remove NonePod and use dummy signed pods (#272)

* remove NonePod and use dummy signed pods

* apply suggestion by @arnaucube

src/backends/plonky2/circuits/mainpod.rs

@@ -1464,12 +1464,9 @@ impl InnerCircuit for MainPodVerifyTarget {
         }
         // Padding
         if input.signed_pods.len() != self.params.max_input_signed_pods {
-            // TODO: Instead of using an input for padding, use a canonical minimal SignedPod,
-            // without it a MainPod configured to support input signed pods must have at least one
-            // input signed pod :(
-            let pad_pod = &input.signed_pods[0];
+            let dummy = SignedPod::dummy();
             for i in input.signed_pods.len()..self.params.max_input_signed_pods {
-                self.signed_pods[i].set_targets(pw, pad_pod)?;
+                self.signed_pods[i].set_targets(pw, &dummy)?;
             }
         }
 

src/backends/plonky2/emptypod.rs

@@ -1,4 +1,7 @@
-use std::{collections::HashMap, sync::Mutex};
+use std::{
+    collections::HashMap,
+    sync::{LazyLock, Mutex},
+};
 
 use itertools::Itertools;
 use plonky2::{
@@ -23,7 +26,7 @@ use crate::{
         error::{Error, Result},
         mainpod::{self, calculate_id},
         recursion::pad_circuit,
-        serialize_proof, LazyLock, DEFAULT_PARAMS, STANDARD_REC_MAIN_POD_CIRCUIT_DATA,
+        serialize_proof, DEFAULT_PARAMS, STANDARD_REC_MAIN_POD_CIRCUIT_DATA,
     },
     middleware::{
         self, AnchoredKey, DynError, Hash, Params, Pod, PodId, PodType, RecursivePod, Statement,

src/backends/plonky2/mainpod/mod.rs

@@ -18,7 +18,7 @@ use crate::{
         deserialize_proof,
         emptypod::EmptyPod,
         error::{Error, Result},
-        mock::emptypod::MockEmptyPod,
+        mock::{emptypod::MockEmptyPod, signedpod::MockSignedPod},
         primitives::merkletree::MerkleClaimAndProof,
         recursion::{RecursiveCircuit, RecursiveParams},
         serialize_proof,
@@ -27,8 +27,8 @@ use crate::{
     },
     middleware::{
         self, resolve_wildcard_values, AnchoredKey, CustomPredicateBatch, DynError, Hash,
-        MainPodInputs, NativeOperation, NonePod, OperationType, Params, Pod, PodId, PodProver,
-        PodType, RecursivePod, StatementArg, ToFields, VDSet, F, KEY_TYPE, SELF,
+        MainPodInputs, NativeOperation, OperationType, Params, Pod, PodId, PodProver, PodType,
+        RecursivePod, StatementArg, ToFields, VDSet, F, KEY_TYPE, SELF,
     },
 };
 
@@ -257,13 +257,16 @@ pub(crate) fn layout_statements(
     statements.push(middleware::Statement::None.into());
 
     // Input signed pods region
-    // TODO: Replace this with a dumb signed pod
-    // https://github.com/0xPARC/pod2/issues/246
-    let none_sig_pod_box: Box<dyn Pod> = Box::new(NonePod {});
-    let none_sig_pod = none_sig_pod_box.as_ref();
+    let dummy_signed_pod_box: Box<dyn Pod> =
+        if mock || inputs.signed_pods.len() == params.max_input_signed_pods {
+            Box::new(MockSignedPod::dummy())
+        } else {
+            Box::new(SignedPod::dummy())
+        };
+    let dummy_signed_pod = dummy_signed_pod_box.as_ref();
     assert!(inputs.signed_pods.len() <= params.max_input_signed_pods);
     for i in 0..params.max_input_signed_pods {
-        let pod = inputs.signed_pods.get(i).unwrap_or(&none_sig_pod);
+        let pod = inputs.signed_pods.get(i).unwrap_or(&dummy_signed_pod);
         let sts = pod.pub_statements();
         assert!(sts.len() <= params.max_signed_pod_values);
         for j in 0..params.max_signed_pod_values {

src/backends/plonky2/mock/signedpod.rs

@@ -120,6 +120,15 @@ impl MockSignedPod {
             kvs: data.kvs,
         }))
     }
+    /// Generate a valid MockSignedPod with a public deterministic public key and no other
+    /// key-values than the default ones.  This is used for padding.
+    pub fn dummy() -> MockSignedPod {
+        MockSigner {
+            pk: "dummy".to_string(),
+        }
+        ._sign(&Params::default(), &HashMap::new())
+        .expect("valid")
+    }
 }
 
 impl Pod for MockSignedPod {

src/backends/plonky2/signedpod.rs

@@ -1,7 +1,7 @@
-use std::collections::HashMap;
+use std::{collections::HashMap, sync::LazyLock};
 
 use itertools::Itertools;
-use num_bigint::RandBigInt;
+use num_bigint::{BigUint, RandBigInt};
 use rand::rngs::OsRng;
 use serde::{Deserialize, Serialize};
 
@@ -28,7 +28,12 @@ use crate::{
 pub struct Signer(pub SecretKey);
 
 impl Signer {
-    fn _sign(&mut self, params: &Params, kvs: &HashMap<Key, Value>) -> Result<SignedPod> {
+    fn sign_with_nonce(
+        &mut self,
+        params: &Params,
+        nonce: BigUint,
+        kvs: &HashMap<Key, Value>,
+    ) -> Result<SignedPod> {
         let mut kvs = kvs.clone();
         let pubkey = self.0.public_key();
         kvs.insert(Key::from(KEY_SIGNER), Value::from(pubkey));
@@ -37,7 +42,6 @@ impl Signer {
         let dict = Dictionary::new(params.max_depth_mt_containers, kvs)?;
         let id = RawValue::from(dict.commitment()); // PodId as Value
 
-        let nonce = OsRng.gen_biguint_below(&GROUP_ORDER);
         let signature: Signature = self.0.sign(id, &nonce);
         Ok(SignedPod {
             id: PodId(Hash::from(id)),
@@ -46,6 +50,10 @@ impl Signer {
             dict,
         })
     }
+    fn _sign(&mut self, params: &Params, kvs: &HashMap<Key, Value>) -> Result<SignedPod> {
+        let nonce = OsRng.gen_biguint_below(&GROUP_ORDER);
+        self.sign_with_nonce(params, nonce, kvs)
+    }
 
     pub fn public_key(&self) -> Point {
         self.0.public_key()
@@ -77,6 +85,15 @@ struct Data {
     kvs: Dictionary,
 }
 
+static DUMMY_POD: LazyLock<SignedPod> = LazyLock::new(dummy);
+
+fn dummy() -> SignedPod {
+    let nonce = BigUint::from(2u32);
+    Signer(SecretKey(BigUint::from(1u32)))
+        .sign_with_nonce(&Params::default(), nonce, &HashMap::new())
+        .expect("valid")
+}
+
 impl SignedPod {
     fn _verify(&self) -> Result<()> {
         // 1. Verify type
@@ -142,6 +159,12 @@ impl SignedPod {
             dict: data.kvs,
         }))
     }
+
+    /// Generate a valid SignedPod with a public deterministic secret key and nonce and no other
+    /// key-values than the default ones.  This is used for padding.
+    pub fn dummy() -> SignedPod {
+        DUMMY_POD.clone()
+    }
 }
 
 impl Pod for SignedPod {