ECDSA - verify with existing key

[rohintoncollins]

I have an existing app where the app is storing an ECDSA private key and the public key has been registered with the server. We use this key pair for authentication.

(I want to replace the client apps with a flutter app in due course and the current C++ (Botan) cryptography library with one based on dint-dev/cryptography).

I want to be able to use the Ecdsa.p256 class to sign some data with this stored private key, which I think I can do with the following code:

class MyEcdsa {
  KeyPair keyPair;  
  MyEcdsa(this.keyPair);
  
  Future<Signature> sign(List<int> data) async {
    final ecdsa = Ecdsa.p256(Sha256());
    final signatureWand = await ecdsa.newSignatureWandFromKeyPair(keyPair);
    return await signatureWand.sign(data);
 }

However, a problem arises if I want to verify a signature with an ECDSA public key which I have access to. I don't see a way to feed this public key into Ecdsa.p256 in order to verify the signature.

The example uses a randomly generated key pair, which isn't my use case.

[rohintoncollins]

Looks like DartEcdsa is currently unimplemented:

/// [Ecdsa] (P256, P384, P521) implementation in pure Dart. Currently it throws
/// [UnimplementedError] if you try to use it.
///
/// For examples and more information about the algorithm, see documentation for
/// the class [Ecdsa].

Does this mean that if you include the flutter library it will use the iOS/macOS implementations (via FlutterEcdsa), but for other OSes, this functionality is not available and will result in DartEcdsa being used which will throw an UnimplementedError?