add RandomizedParams::from_randomizer() (#421)

frost-rerandomized/src/lib.rs

@@ -9,7 +9,7 @@ pub use frost_core;
 
 use frost_core::{
     frost::{self, keys::PublicKeyPackage},
-    Ciphersuite, Error, Field, Group, VerifyingKey,
+    Ciphersuite, Error, Field, Group, Scalar, VerifyingKey,
 };
 
 // When pulled into `reddsa`, that has its own sibling `rand_core` import.
@@ -191,6 +191,18 @@ where
         mut rng: R,
     ) -> Self {
         let randomizer = <<C::Group as Group>::Field as Field>::random(&mut rng);
+        Self::from_randomizer(public_key_package, randomizer)
+    }
+
+    /// Create a new RandomizedParams for the given [`PublicKeyPackage`]
+    /// with the given `randomizer`. The `randomizer` MUST be generated uniformly
+    /// at random! Use [`RandomizedParams::new()`] which generates a fresh
+    /// randomizer, unless your application requires generating a randomizer
+    /// outside.
+    pub fn from_randomizer(
+        public_key_package: &PublicKeyPackage<C>,
+        randomizer: Scalar<C>,
+    ) -> Self {
         let randomizer_point = <C::Group as Group>::generator() * randomizer;
 
         let group_public_point = public_key_package.group_public().to_element();

frost-rerandomized/src/tests.rs

@@ -3,7 +3,7 @@
 use std::collections::HashMap;
 
 use crate::{frost_core::frost, frost_core::Ciphersuite, RandomizedParams};
-use frost_core::{Signature, VerifyingKey};
+use frost_core::{Field, Group, Signature, VerifyingKey};
 use rand_core::{CryptoRng, RngCore};
 
 /// Test re-randomized FROST signing with trusted dealer with a Ciphersuite.
@@ -33,6 +33,7 @@ pub fn check_randomized_sign_with_dealer<C: Ciphersuite, R: RngCore + CryptoRng>
     let mut commitments: HashMap<frost::Identifier<C>, frost::round1::SigningCommitments<C>> =
         HashMap::new();
 
+    check_from_randomizer(&pubkeys, &mut rng);
     let randomizer_params = RandomizedParams::new(&pubkeys, &mut rng);
 
     ////////////////////////////////////////////////////////////////////////////
@@ -116,3 +117,14 @@ pub fn check_randomized_sign_with_dealer<C: Ciphersuite, R: RngCore + CryptoRng>
         *randomizer_params.randomized_group_public_key(),
     )
 }
+
+fn check_from_randomizer<C: Ciphersuite, R: RngCore + CryptoRng>(
+    pubkeys: &frost::keys::PublicKeyPackage<C>,
+    mut rng: &mut R,
+) {
+    let randomizer = <<C::Group as Group>::Field as Field>::random(&mut rng);
+
+    let randomizer_params = RandomizedParams::from_randomizer(pubkeys, randomizer);
+
+    assert!(*randomizer_params.randomizer() == randomizer);
+}