Separate sign_all_but_one for immediate signing

Author: nickfarrow

schnorr_fun/src/blind.rs

@@ -390,17 +390,63 @@ where
     /// Returns a scalar of the unblinded signature
     pub fn sign_single(&mut self, sig_request: SignatureRequest) -> Option<Scalar<Public, Zero>> {
         let secret_nonce = self.use_secret_nonce(sig_request.public_nonce);
-        match secret_nonce {
+        let signature_response = match secret_nonce {
             Some(secret_nonce) => {
                 let sig = s!(secret_nonce + sig_request.blind_challenge * self.secret).public();
-                Some(sig) //.secret().non_zero()
+                Some(sig)
             }
             // Did not expect this nonce
             None => None,
-        }
+        };
+        // Store this signature
+        self.already_signed
+            .insert(sig_request.public_nonce, signature_response);
+        signature_response
     }
 
-    /// Sign multiple blind schnorr signatures concurrently once enough have been requested
+    /// Sign all the signature requests immediately, except for one
+    ///
+    /// # Returns
+    ///
+    /// A vector of scalar signature options
+    pub fn sign_all_but_one<R: RngCore>(
+        &mut self,
+        rng: &mut R,
+    ) -> Vec<Option<Scalar<Public, Zero>>> {
+        // Choose an index to skip signing request
+        let skip_i = rng.gen_range(0..self.signature_requests.len() as u32);
+
+        // Sign all the signature requests but don't store one (given there is more than one)
+        let signatures = self
+            .signature_requests
+            .clone()
+            .into_iter()
+            .enumerate()
+            .map(|(i, sig_request)| {
+                let sig_response = if self.max_sessions > 1 && i as u32 == skip_i {
+                    // For one out of the N sessions, drop the signature.
+                    // ⚠ IMPORTANT: Overwrite the stored signature for this nonce
+                    self.already_signed.insert(sig_request.public_nonce, None);
+                    assert!(self
+                        .already_signed
+                        .get(&sig_request.public_nonce)
+                        .expect("history has to have None written for this nonce")
+                        .is_none());
+                    None
+                } else {
+                    // Otherwise, sign and store the signature
+                    self.sign_single(sig_request.clone())
+                };
+                sig_response
+            })
+            .collect();
+
+        // Clear our signature requests
+        self.signature_requests = vec![];
+        signatures
+    }
+
+    /// Pass signature request in for signing, batches until max_sessions requests have been made.
     ///
     /// Does not do any signing until max_session number of [`SignatureRequest`]s have been requested.
     /// Then sign them all but randomly disconnect (return None) one of the N sessions.
@@ -421,32 +467,7 @@ where
         if self.max_sessions > 1 && self.signature_requests.len() < self.max_sessions {
             vec![]
         } else {
-            // Choose an index to skip signing request
-            let skip_i = rng.gen_range(0..self.signature_requests.len() as u32);
-
-            // Sign all the signature requests but don't store one (given there is more than one)
-            let signatures = self
-                .signature_requests
-                .clone()
-                .into_iter()
-                .enumerate()
-                .map(|(i, sig_req)| {
-                    let sig = self.sign_single(sig_req.clone());
-                    let response = if self.max_sessions > 1 && i as u32 == skip_i {
-                        // Maybe don't return the signature
-                        None
-                    } else {
-                        sig
-                    };
-                    // Store signature (or None) for this public nonce
-                    self.already_signed.insert(sig_req.public_nonce, response);
-                    response
-                })
-                .collect();
-
-            // Clear our signature requests
-            self.signature_requests = vec![];
-            signatures
+            self.sign_all_but_one(rng)
         }
     }
 }