Allow modifications to get sum inputs == sum outputs

crypto/fcmps/src/prover/blinds.rs

@@ -179,6 +179,14 @@ where
     let R = *self.i_blind_blind.0.scalar_mul_and_divisor.point - self.i_blind.v.point.deref();
     Input::new(O_tilde, I_tilde, R, C_tilde)
   }
+
+  /// Update the existing c_blind
+  ///
+  /// This allows recalculating a c_blind at tx construction time when making sure
+  /// sum of inputs == sum of outputs.
+  pub fn set_c_blind(&mut self, c_blind: CBlind<G>) {
+    self.c_blind = c_blind;
+  }
 }
 
 /// A blind for a branch.

networks/monero/ringct/fcmp++/src/sal/mod.rs

@@ -105,6 +105,17 @@ impl RerandomizedOutput {
     -self.r_c
   }
 
+  /// The scalar used to re-randomize the amount commitment
+  pub fn r_c(&self) -> <Ed25519 as Ciphersuite>::F {
+    self.r_c
+  }
+
+  /// Allow changing r_c and C_tilde (the pseudo out) to balance the tx
+  pub fn set_r_c(&mut self, C: <Ed25519 as Ciphersuite>::G, r_c: <Ed25519 as Ciphersuite>::F) {
+    self.r_c = r_c;
+    self.input.C_tilde = C + (<Ed25519 as Ciphersuite>::generator() * r_c);
+  }
+
   /// The input tuple produced by this output and set of rerandomizations.
   pub fn input(&self) -> Input {
     self.input