Fix DigestTranscript to be secure

Collisions were possible depending on static label substrings. Now, 
labels are prefixed by their length to prevent this from being possible. 
All variables are also flagged by their type, preventing other potential 
conflicts.

crypto/transcript/src/lib.rs

@@ -14,6 +14,26 @@ pub trait Transcript {
   fn rng_seed(&mut self, label: &'static [u8]) -> [u8; 32];
 }
 
+enum DigestTranscriptMember {
+  Name,
+  Domain,
+  Label,
+  Value,
+  Challenge
+}
+
+impl DigestTranscriptMember {
+  fn as_u8(&self) -> u8 {
+    match self {
+      DigestTranscriptMember::Name => 0,
+      DigestTranscriptMember::Domain => 1,
+      DigestTranscriptMember::Label => 2,
+      DigestTranscriptMember::Value => 3,
+      DigestTranscriptMember::Challenge => 4
+    }
+  }
+}
+
 #[derive(Clone, Debug)]
 pub struct DigestTranscript<D: Digest>(Vec<u8>, PhantomData<D>);
 
@@ -24,25 +44,32 @@ impl<D: Digest> PartialEq for DigestTranscript<D> {
 }
 
 impl<D: Digest> DigestTranscript<D> {
-  pub fn new(label: &'static [u8]) -> Self {
-    DigestTranscript(label.to_vec(), PhantomData)
+  fn append(&mut self, kind: DigestTranscriptMember, value: &[u8]) {
+    self.0.push(kind.as_u8());
+    // Assumes messages don't exceed 16 exabytes
+    self.0.extend(u64::try_from(value.len()).unwrap().to_le_bytes());
+    self.0.extend(value);
+  }
+
+  pub fn new(name: &'static [u8]) -> Self {
+    let mut res = DigestTranscript(vec![], PhantomData);
+    res.append(DigestTranscriptMember::Name, name);
+    res
   }
 }
 
 impl<D: Digest> Transcript for DigestTranscript<D> {
   fn domain_separate(&mut self, label: &[u8]) {
-    self.append_message(b"domain", label);
+    self.append(DigestTranscriptMember::Domain, label);
   }
 
   fn append_message(&mut self, label: &'static [u8], message: &[u8]) {
-    self.0.extend(label);
-    // Assumes messages don't exceed 16 exabytes
-    self.0.extend(u64::try_from(message.len()).unwrap().to_le_bytes());
-    self.0.extend(message);
+    self.append(DigestTranscriptMember::Label, label);
+    self.append(DigestTranscriptMember::Value, message);
   }
 
   fn challenge(&mut self, label: &'static [u8]) -> Vec<u8> {
-    self.0.extend(label);
+    self.append(DigestTranscriptMember::Challenge, label);
     D::new().chain_update(&self.0).finalize().to_vec()
   }