Solve the fact that in some cases the built transaction is discarded by Bitcoin nodes.

In `sec1_to_der`, I used to say that we need mock signing and assume the largest signature size that is reachable by providing `vec![1; 64]` in `mock_signer` in order to be in the if case for both conditions of `sec1_to_der`. However as far as I know Bitcoin uses sometimes variable length integer so `r` and `s` absolute values encoded by `vec![1; 64]` are less than `r` and `s` values encoded by `vec![255; 64]` and so the latter may take more signature size than the former. So in addition to assume that `r` and `s` are negative, we also have to assume that their value are the most negative one.

So the solution is to return `vec![255; 64]` in mock_signer and not `vec![1; 64]`. It solves the problem as for a signed transaction size of 223 bytes it now considers a fee of 224 satoshis.

Author: Benjamin-Loison

rust/src/transaction_management.rs

@@ -546,7 +546,7 @@ async fn mock_signer(
     _derivation_path: Vec<Vec<u8>>,
     _message_hash: Vec<u8>,
 ) -> Result<Vec<u8>, ManagementCanisterReject> {
-    Ok(vec![1; 64])
+    Ok(vec![255; 64])
 }
 
 // Converts a SEC1 ECDSA signature to the DER format.