Implement a fee on every input to prevent prior described economic attacks

Completes #297.

processor/src/multisigs/mod.rs

@@ -57,9 +57,10 @@ fn instruction_from_output<N: Network>(output: &N::Output) -> Option<InInstructi
   let Ok(shorthand) = Shorthand::decode(&mut data) else { None? };
   let Ok(instruction) = RefundableInInstruction::try_from(shorthand) else { None? };
 
-  let balance = output.balance();
-  // TODO: Decrease amount by
-  // `2 * (the estimation of an input-merging transaction fee) / max_inputs_per_tx`
+  let mut balance = output.balance();
+  // Deduct twice the cost to aggregate to prevent economic attacks by malicious miners against
+  // other users
+  balance.amount.0 -= 2 * N::COST_TO_AGGREGATE;
 
   // TODO2: Set instruction.origin if not set (and handle refunds in general)
   Some(InInstructionWithBalance { instruction: instruction.instruction, balance })

processor/src/networks/bitcoin.rs

@@ -451,6 +451,14 @@ impl Network for Bitcoin {
   */
   const DUST: u64 = 10_000;
 
+  // 2 inputs should be 2 * 230 = 460 weight units
+  // The output should be ~36 bytes, or 144 weight units
+  // The overhead should be ~20 bytes at most, or 80 weight units
+  // 684 weight units, 171 vbytes, round up to 200
+  // 200 vbytes at 1 sat/weight (our current minumum fee, 4 sat/vbyte) = 800 sat fee for the
+  // aggregation TX
+  const COST_TO_AGGREGATE: u64 = 800;
+
   // Bitcoin has a max weight of 400,000 (MAX_STANDARD_TX_WEIGHT)
   // A non-SegWit TX will have 4 weight units per byte, leaving a max size of 100,000 bytes
   // While our inputs are entirely SegWit, such fine tuning is not necessary and could create

processor/src/networks/mod.rs

@@ -278,6 +278,9 @@ pub trait Network: 'static + Send + Sync + Clone + PartialEq + Eq + Debug {
   /// magnitude).
   const DUST: u64;
 
+  /// The cost to perform input aggregation with a 2-input 1-output TX.
+  const COST_TO_AGGREGATE: u64;
+
   /// Tweak keys for this network.
   fn tweak_keys(key: &mut ThresholdKeys<Self::Curve>);
 

processor/src/networks/monero.rs

@@ -397,6 +397,9 @@ impl Network for Monero {
   // TODO: Set a sane dust
   const DUST: u64 = 10000000000;
 
+  // TODO
+  const COST_TO_AGGREGATE: u64 = 0;
+
   // Monero doesn't require/benefit from tweaking
   fn tweak_keys(_: &mut ThresholdKeys<Self::Curve>) {}