update from_resharing conversion

Author: alxiong

timeboost-types/src/decryption.rs

@@ -6,8 +6,12 @@ use serde::{Deserialize, Serialize};
 use std::collections::{BTreeMap, btree_map};
 use timeboost_crypto::{
     DecryptionScheme,
+    feldman::FeldmanVssPublicParam,
     prelude::{DkgEncKey, Vess, Vss},
-    traits::{dkg::VerifiableSecretSharing, threshold_enc::ThresholdEncScheme},
+    traits::{
+        dkg::{KeyResharing, VerifiableSecretSharing},
+        threshold_enc::ThresholdEncScheme,
+    },
     vess::VessError,
 };
 
@@ -63,6 +67,29 @@ impl DecryptionKey {
         Self::from_single_dkg(committee_size, node_idx, agg_key_share, &agg_comm)
     }
 
+    /// Construct all key material for the threshold decryption from Key resharing.
+    pub fn from_resharing<I>(
+        old_committee: &Committee,
+        new_committee: &Committee,
+        recv_node_idx: usize,
+        dealings: I,
+    ) -> anyhow::Result<Self>
+    where
+        I: ExactSizeIterator<
+                Item = (
+                    usize,
+                    <Vss as VerifiableSecretSharing>::SecretShare,
+                    <Vss as VerifiableSecretSharing>::Commitment,
+                ),
+            > + Clone,
+    {
+        let old_pp = FeldmanVssPublicParam::from(old_committee);
+        let new_pp = FeldmanVssPublicParam::from(new_committee);
+
+        let (new_share, new_comm) = Vss::combine(&old_pp, &new_pp, recv_node_idx, dealings)?;
+        Self::from_single_dkg(new_pp.num_nodes(), recv_node_idx, new_share, &new_comm)
+    }
+
     /// inner routine to construct from a single (aggregated or interpolated) DKG output,
     /// shared in both DKG and resharing logic.
     fn from_single_dkg(
@@ -89,10 +116,6 @@ impl DecryptionKey {
         Ok(Self::new(pk, combkey, prikey))
     }
 
-    pub fn from_resharing() -> anyhow::Result<Self> {
-        todo!("after #406 merged, invoked FeldmanVss.combine(), then from_single_dkg()")
-    }
-
     pub fn pubkey(&self) -> &PublicKey {
         &self.pubkey
     }