From 963d9eab10b8d5e93f13975f48dc3d9d04c4e6d0 Mon Sep 17 00:00:00 2001
From: Luke Parker <lukeparker5132@gmail.com>
Date: Fri, 24 Jun 2022 18:49:04 -0400
Subject: [PATCH] Have DigestTranscript require a 32-byte hash

Needed to generate RNG seeds without panicking. Mandates at least a
128-bit security level.
---
 crypto/transcript/src/lib.rs | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/crypto/transcript/src/lib.rs b/crypto/transcript/src/lib.rs
index 54299c94..1ea4a7a8 100644
--- a/crypto/transcript/src/lib.rs
+++ b/crypto/transcript/src/lib.rs
@@ -1,11 +1,11 @@
-use core::{marker::PhantomData, fmt::Debug};
+use core::fmt::Debug;
 
 #[cfg(features = "merlin")]
 mod merlin;
 #[cfg(features = "merlin")]
 pub use merlin::MerlinTranscript;
 
-use digest::Digest;
+use digest::{typenum::type_operators::IsGreaterOrEqual, consts::U256, Digest};
 
 pub trait Transcript {
   fn domain_separate(&mut self, label: &'static [u8]);
@@ -35,9 +35,9 @@ impl DigestTranscriptMember {
 }
 
 #[derive(Clone, Debug)]
-pub struct DigestTranscript<D: Clone + Digest>(D, PhantomData<D>);
+pub struct DigestTranscript<D: Clone + Digest>(D) where D::OutputSize: IsGreaterOrEqual<U256>;
 
-impl<D: Clone + Digest> DigestTranscript<D> {
+impl<D: Clone + Digest> DigestTranscript<D> where D::OutputSize: IsGreaterOrEqual<U256> {
   fn append(&mut self, kind: DigestTranscriptMember, value: &[u8]) {
     self.0.update(&[kind.as_u8()]);
     // Assumes messages don't exceed 16 exabytes
@@ -46,13 +46,14 @@ impl<D: Clone + Digest> DigestTranscript<D> {
   }
 
   pub fn new(name: &'static [u8]) -> Self {
-    let mut res = DigestTranscript(D::new(), PhantomData);
+    let mut res = DigestTranscript(D::new());
     res.append(DigestTranscriptMember::Name, name);
     res
   }
 }
 
-impl<D: Digest + Clone> Transcript for DigestTranscript<D> {
+impl<D: Digest + Clone> Transcript for DigestTranscript<D>
+  where D::OutputSize: IsGreaterOrEqual<U256> {
   fn domain_separate(&mut self, label: &[u8]) {
     self.append(DigestTranscriptMember::Domain, label);
   }