WebAuthnぽいことができるWinデスクトップアプリ用ライブラリ　WebAuthnModokiDesktopβ ref: https://qiita.com/gebo/items/f6d3024f7e164ac0a195

file0.txt

var devParam = DevParam.GetDefaultParams();
byte[] challenge = System.Text.Encoding.ASCII.GetBytes("this is challenge");
string pin = "xxxx";
string json =
    "{" +
        "rp : {" +
            "id : 'demo.WebauthnMODOKI.gebogebo.com'," +
        "}," +
        "user : {" +
            "id : 'userid'," +
            "name :'name_name'," +
            "displayName :'my name is gebogebo'," +
        "}," +
        "pubKeyCredParams: [{type: 'public-key',alg: -7}]," +
        "timeout: 60000," +
        "authenticatorSelection : {" +
            "requireResidentKey : false," +
        "}," +
        string.Format($"challenge:[{string.Join(",", challenge)}],") +
     "}";
var response = await Credentials.Create(devParam,json, pin);
if (response.isSuccess == true) {
    // 成功
    // response.attestation ...
}

file1.txt

var devParam = DevParam.GetDefaultParams();
byte[] challenge = System.Text.Encoding.ASCII.GetBytes("this is challenge");
string pin = "xxxx";
byte[] CredentialId= // createでGETしたCredentialIdを指定してください(response.attestation.CredentialId);

string json =
   "{" +
        "timeout : 60000," +
        string.Format($"challenge:[{string.Join(",", challenge)}],") +
        "rpId : 'demo.WebauthnMODOKI.gebogebo.com'," +
        "allowCredentials : [{" +
           string.Format($"id : [{string.Join(",", CredentialId)}],") +
           "type : 'public-key'," +
       "}]," +
       "requireUserPresence : 'true'," +
       "userVerification : 'discouraged'," +
    "}";

var response = await Credentials.Get(devParam ,json, pin);
if (response.isSuccess == true) {
    // 成功
    // response.assertions[0] ...
}