Add constant for length of Share.

All of the `{ KEY_LEN + 1 }` was a bit much,

src/hsm.rs

@@ -39,6 +39,7 @@ const DOMAIN: Domain = Domain::all();
 const ID: Id = 0x1;
 const SEED_LEN: usize = 32;
 const KEY_LEN: usize = 32;
+const SHARE_LEN: usize = KEY_LEN + 1;
 const LABEL: &str = "backup";
 
 const SHARES: usize = 5;
@@ -229,7 +230,7 @@ impl Hsm {
             Scalar,
             ProjectivePoint,
             ChaCha20Rng,
-            { KEY_LEN + 1 },
+            SHARE_LEN,
         >(*nzs.as_ref(), None, &mut rng)
         .map_err(|e| HsmError::SplitKeyFailed { e })?;
 
@@ -424,7 +425,7 @@ impl Hsm {
         info!("Restoring HSM from backup");
         info!("Restoring backup / wrap key from shares");
         // vector used to collect shares
-        let mut shares: Vec<Share<{ KEY_LEN + 1 }>> = Vec::new();
+        let mut shares: Vec<Share<SHARE_LEN>> = Vec::new();
 
         // deserialize verifier:
         // verifier was serialized to output/verifier.json in the provisioning ceremony
@@ -434,7 +435,7 @@ impl Hsm {
         let verifier: FeldmanVerifier<
             Scalar,
             ProjectivePoint,
-            { KEY_LEN + 1 },
+            SHARE_LEN,
         > = serde_json::from_str(&verifier)?;
 
         // get enough shares to recover backup key
@@ -499,7 +500,7 @@ impl Hsm {
                 };
 
                 // construct a Share from the decoded hex string
-                let share: Share<{ KEY_LEN + 1 }> =
+                let share: Share<SHARE_LEN> =
                     match Share::try_from(&share_vec[..]) {
                         Ok(share) => share,
                         Err(_) => {
@@ -536,7 +537,7 @@ impl Hsm {
 
         let scalar = Feldman::<THRESHOLD, SHARES>::combine_shares::<
             Scalar,
-            { KEY_LEN + 1 },
+            SHARE_LEN,
         >(&shares)
         .map_err(|e| HsmError::CombineKeyFailed { e })?;
 
@@ -936,7 +937,7 @@ mod tests {
         secret
     }
 
-    fn deserialize_share(share: &str) -> Result<Share<{ KEY_LEN + 1 }>> {
+    fn deserialize_share(share: &str) -> Result<Share<SHARE_LEN>> {
         // filter out whitespace to keep hex::decode happy
         let share: String =
             share.chars().filter(|c| !c.is_whitespace()).collect();
@@ -960,7 +961,7 @@ mod tests {
             Scalar,
             ProjectivePoint,
             ThreadRng,
-            { KEY_LEN + 1 },
+            SHARE_LEN,
         >(*nzs.as_ref(), None, &mut rng)
         .map_err(|e| anyhow::anyhow!("failed to split secret: {}", e))?;
 
@@ -970,7 +971,7 @@ mod tests {
 
         let scalar = Feldman::<THRESHOLD, SHARES>::combine_shares::<
             Scalar,
-            { KEY_LEN + 1 },
+            SHARE_LEN,
         >(&shares)
         .map_err(|e| anyhow::anyhow!("failed to combine secret: {}", e))?;
 
@@ -989,7 +990,7 @@ mod tests {
         let verifier: FeldmanVerifier<
             Scalar,
             ProjectivePoint,
-            { KEY_LEN + 1 },
+            SHARE_LEN,
         > = serde_json::from_str(VERIFIER)
             .context("Failed to deserialize FeldmanVerifier from JSON.")?;
 
@@ -1006,12 +1007,12 @@ mod tests {
         let verifier: FeldmanVerifier<
             Scalar,
             ProjectivePoint,
-            { KEY_LEN + 1 },
+            SHARE_LEN,
         > = serde_json::from_str(VERIFIER)
             .context("Failed to deserialize FeldmanVerifier from JSON.")?;
 
-        let share: Share<{ KEY_LEN + 1 }> =
-            Share::try_from([0u8; KEY_LEN + 1].as_ref())
+        let share: Share<SHARE_LEN> =
+            Share::try_from([0u8; SHARE_LEN].as_ref())
             .context("Failed to create Share from static array.")?;
 
         assert!(!verifier.verify(&share));
@@ -1026,7 +1027,7 @@ mod tests {
         let verifier: FeldmanVerifier<
             Scalar,
             ProjectivePoint,
-            { KEY_LEN + 1 },
+            SHARE_LEN,
         > = serde_json::from_str(VERIFIER)
             .context("Failed to deserialize FeldmanVerifier from JSON.")?;
 
@@ -1047,14 +1048,14 @@ mod tests {
 
     #[test]
     fn recover_secret() -> Result<()> {
-        let mut shares: Vec<Share<{ KEY_LEN + 1 }>> = Vec::new();
+        let mut shares: Vec<Share<SHARE_LEN>> = Vec::new();
         for share in SHARE_ARRAY {
             shares.push(deserialize_share(share)?);
         }
 
         let scalar = Feldman::<THRESHOLD, SHARES>::combine_shares::<
             Scalar,
-            { KEY_LEN + 1 },
+            SHARE_LEN,
         >(&shares)
         .map_err(|e| anyhow::anyhow!("failed to combine secret: {}", e))?;