Fix https://github.com/serai-dex/serai/issues/5

Cargo.toml

@@ -2,6 +2,7 @@
 
 members = [
   "crypto/transcript",
+  "crypto/multiexp",
   "crypto/frost",
   "crypto/dalek-ff-group",
   "coins/monero",

crypto/frost/Cargo.toml

@@ -17,6 +17,8 @@ group = "0.11"
 blake2 = "0.10"
 transcript = { path = "../transcript" }
 
+multiexp = { path = "../multiexp" }
+
 [dev-dependencies]
 rand = "0.8"
 sha2 = "0.10"

crypto/frost/src/lib.rs

@@ -5,6 +5,8 @@ use thiserror::Error;
 use ff::{Field, PrimeField};
 use group::{Group, GroupOps, ScalarMul};
 
+pub use multiexp::multiexp_vartime;
+
 pub mod key_gen;
 pub mod algorithm;
 pub mod sign;
@@ -372,68 +374,3 @@ impl<C: Curve> MultisigKeys<C> {
     )
   }
 }
-
-/*
-An implementation of Straus, which should be more efficient than Pippenger for the expected amount
-of points
-
-Completing key generation from the round 2 messages takes:
-- Naive
-    Completed 33-of-50 in 2.66s
-    Completed 5-of-8 in 11.05ms
-
-- crate Straus
-    Completed 33-of-50 in 730-833ms (extremely notable effects from taking variable time)
-    Completed 5-of-8 in 2.8ms
-
-- dalek VartimeMultiscalarMul
-    Completed 33-of-50 in 266ms
-    Completed 5-of-8 in 1.6ms
-
-This does show this algorithm isn't appropriately tuned (and potentially isn't even the right
-choice), at least with that quantity. Unfortunately, we can't use dalek's multiexp implementation
-everywhere, and this does work
-*/
-pub fn multiexp_vartime<C: Curve>(scalars: &[C::F], points: &[C::G]) -> C::G {
-  let mut tables = vec![];
-  // dalek uses 8 in their impl, along with a carry scheme where values are [-8, 8)
-  // Moving to a similar system here did save a marginal amount, yet not one significant enough for
-  // its pain (as some fields do have scalars which can have their top bit set, a scenario dalek
-  // assumes is never true)
-  tables.resize(points.len(), Vec::with_capacity(15));
-  for p in 0 .. points.len() {
-    let mut accum = C::G::identity();
-    tables[p].push(accum);
-    for _ in 0 .. 15 {
-      accum += points[p];
-      tables[p].push(accum);
-    }
-  }
-
-  let mut nibbles = vec![];
-  nibbles.resize(scalars.len(), vec![]);
-  for s in 0 .. scalars.len() {
-    let bytes = C::F_to_le_bytes(&scalars[s]);
-    nibbles[s].resize(C::F_len() * 2, 0);
-    for i in 0 .. bytes.len() {
-      nibbles[s][i * 2] = bytes[i] & 0b1111;
-      nibbles[s][(i * 2) + 1] = (bytes[i] >> 4) & 0b1111;
-    }
-  }
-
-  let mut res = C::G::identity();
-  for b in (0 .. (C::F_len() * 2)).rev() {
-    for _ in 0 .. 4 {
-      res = res.double();
-    }
-
-    for s in 0 .. scalars.len() {
-      // This creates a 250% performance increase on key gen, which uses a bunch of very low
-      // scalars. This is why this function is now committed to being vartime
-      if nibbles[s][b] != 0 {
-        res += tables[s][nibbles[s][b] as usize];
-      }
-    }
-  }
-  res
-}

crypto/frost/tests/common.rs

@@ -37,7 +37,7 @@ impl Curve for Secp256k1 {
   }
 
   fn multiexp_vartime(scalars: &[Self::F], points: &[Self::G]) -> Self::G {
-    multiexp_vartime::<Secp256k1>(scalars, points)
+    multiexp_vartime(scalars, points, false)
   }
 
   // The IETF draft doesn't specify a secp256k1 ciphersuite

crypto/multiexp/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "multiexp"
+version = "0.1.0"
+description = "Multiexponentation algorithms for ff/group"
+license = "MIT"
+authors = ["Luke Parker <lukeparker5132@gmail.com>"]
+edition = "2021"
+
+[dependencies]
+ff = "0.11"
+group = "0.11"

crypto/multiexp/LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2022 Luke Parker
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

crypto/multiexp/src/lib.rs

@@ -0,0 +1,55 @@
+use ff::PrimeField;
+use group::{Group, GroupEncoding, ScalarMul};
+
+// An implementation of Straus, with a extremely minimal API that lets us add other algorithms in
+// the future. Takes in a list of scalars and points with a boolean for if the scalars are little
+// endian encoded or not
+pub fn multiexp_vartime<F: PrimeField, G: Group + GroupEncoding + ScalarMul<F>>(
+  scalars: &[F],
+  points: &[G],
+  little: bool
+) -> G {
+  let mut tables = vec![];
+  // dalek uses 8 in their impl, along with a carry scheme where values are [-8, 8)
+  // Moving to a similar system here did save a marginal amount, yet not one significant enough for
+  // its pain (as some fields do have scalars which can have their top bit set, a scenario dalek
+  // assumes is never true)
+  tables.resize(points.len(), [G::identity(); 16]);
+  for p in 0 .. points.len() {
+    let mut accum = G::identity();
+    for i in 1 .. 16 {
+      accum += points[p];
+      tables[p][i] = accum;
+    }
+  }
+
+  let mut nibbles = vec![];
+  nibbles.resize(scalars.len(), vec![]);
+  for s in 0 .. scalars.len() {
+    let mut repr = scalars[s].to_repr();
+    let bytes = repr.as_mut();
+    if !little {
+      bytes.reverse();
+    }
+
+    nibbles[s].resize(bytes.len() * 2, 0);
+    for i in 0 .. bytes.len() {
+      nibbles[s][i * 2] = bytes[i] & 0b1111;
+      nibbles[s][(i * 2) + 1] = (bytes[i] >> 4) & 0b1111;
+    }
+  }
+
+  let mut res = G::identity();
+  for b in (0 .. nibbles[0].len()).rev() {
+    for _ in 0 .. 4 {
+      res = res.double();
+    }
+
+    for s in 0 .. scalars.len() {
+      if nibbles[s][b] != 0 {
+        res += tables[s][nibbles[s][b] as usize];
+      }
+    }
+  }
+  res
+}