flexible_transcript/
merlin.rs

1use core::fmt::{Debug, Formatter};
2
3use crate::Transcript;
4
5/// A wrapper around a Merlin transcript which satisfiees the Transcript API.
6///
7/// Challenges are fixed to 64 bytes, despite Merlin supporting variable length challenges.
8///
9/// This implementation is intended to remain in the spirit of Merlin more than it's intended to be
10/// in the spirit of the provided DigestTranscript. While DigestTranscript uses flags for each of
11/// its different field types, the domain_separate function simply appends a message with a label
12/// of "dom-sep", Merlin's preferred domain separation label. Since this could introduce transcript
13/// conflicts between a domain separation and a message with a label of "dom-sep", the
14/// append_message function uses an assertion to prevent such labels.
15#[derive(Clone)]
16pub struct MerlinTranscript(merlin::Transcript);
17// Merlin doesn't implement Debug so provide a stub which won't panic
18impl Debug for MerlinTranscript {
19  fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), core::fmt::Error> {
20    fmt.debug_struct("MerlinTranscript").finish_non_exhaustive()
21  }
22}
23
24impl Transcript for MerlinTranscript {
25  // Uses a challenge length of 64 bytes to support wide reduction on commonly used EC scalars
26  // From a security level standpoint (Merlin targets 128-bits), this should just be 32 bytes
27  // From a Merlin standpoint, this should be variable per call
28  // From a practical standpoint, this should be practical
29  type Challenge = [u8; 64];
30
31  fn new(name: &'static [u8]) -> Self {
32    MerlinTranscript(merlin::Transcript::new(name))
33  }
34
35  fn domain_separate(&mut self, label: &'static [u8]) {
36    self.0.append_message(b"dom-sep", label);
37  }
38
39  fn append_message<M: AsRef<[u8]>>(&mut self, label: &'static [u8], message: M) {
40    assert!(
41      label != "dom-sep".as_bytes(),
42      "\"dom-sep\" is reserved for the domain_separate function",
43    );
44    self.0.append_message(label, message.as_ref());
45  }
46
47  fn challenge(&mut self, label: &'static [u8]) -> Self::Challenge {
48    let mut challenge = [0; 64];
49    self.0.challenge_bytes(label, &mut challenge);
50    challenge
51  }
52
53  fn rng_seed(&mut self, label: &'static [u8]) -> [u8; 32] {
54    let mut seed = [0; 32];
55    seed.copy_from_slice(&self.challenge(label)[.. 32]);
56    seed
57  }
58}
flexible_transcript/merlin.rs

flexible_transcript/
merlin.rs
