convert der sig to compact

Cargo.lock

···
 dependencies = [
  "base58",
  "hex-literal",
+ "k256",
  "postcard",
  "serde",
  "serde_ipld_dagcbor",

Cargo.toml

···
 stack_size = 65536
 
 [dependencies]
+k256 = { version = "0.13.4", default-features = false, features = ["alloc", "ecdsa-core"] }
 postcard = { version = "1.1.1", features = ["alloc"] }
 serde_ipld_dagcbor = { git = "http://github.com/edouardparis/serde_ipld_dagcbor", branch = "scopeguard-no-default-features", default-features = false }
 sdk = { package = "vanadium-app-sdk", git = "https://github.com/LedgerHQ/vanadium"}

src/main.rs

···
     hasher.digest(&mut digest);
 
     let sig = privkey.ecdsa_sign_hash(&digest).unwrap();
-
-    Ok(Response::Signature(sig))
+    let sig = k256::ecdsa::Signature::from_der(&sig).unwrap();
+    Ok(Response::Signature(sig.to_bytes().to_vec()))
 }
 
 fn process(_app: &mut App, request: &[u8]) -> Vec<u8> {