Transcript crate with both a merlin backend and a basic label len value backend

Moves binding factor/seeded RNGs over to the transcripts.

crypto/transcript/src/merlin.rs

@@ -0,0 +1,42 @@
+use core::{marker::PhantomData, fmt::{Debug, Formatter}};
+
+use rand_core::{RngCore, CryptoRng, SeedableRng};
+use rand_chacha::ChaCha12Rng;
+
+use digest::Digest;
+
+#[derive(Clone)]
+pub struct MerlinTranscript(merlin::Transcript);
+// Merlin doesn't implement Debug so provide a stub which won't panic
+impl Debug for MerlinTranscript {
+  fn fmt(&self, _: &mut Formatter<'_>) -> Result<(), std::fmt::Error> { Ok(()) }
+}
+
+impl Transcript for MerlinTranscript {
+  type SeededRng = ChaCha12Rng;
+
+  fn new(label: &'static [u8]) -> Self {
+    MerlinTranscript(merlin::Transcript::new(label))
+  }
+
+  fn append_message(&mut self, label: &'static [u8], message: &[u8]) {
+    self.0.append_message(label, message);
+  }
+
+  fn challenge(&mut self, label: &'static [u8], len: usize) -> Vec<u8> {
+    let mut challenge = vec![];
+    challenge.resize(len, 0);
+    self.0.challenge_bytes(label, &mut challenge);
+    challenge
+  }
+
+  fn seeded_rng(&self, label: &'static [u8], additional_entropy: Option<[u8; 32]>) -> ChaCha12Rng {
+    let mut transcript = self.0.clone();
+    if additional_entropy.is_some() {
+      transcript.append_message(b"additional_entropy", &additional_entropy.unwrap());
+    }
+    let mut seed = [0; 32];
+    transcript.challenge_bytes(label, &mut seed);
+    ChaCha12Rng::from_seed(seed)
+  }
+}