remove NonePod and use dummy signed pods (#272)

* remove NonePod and use dummy signed pods

* apply suggestion by @arnaucube

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 {