string validationKey = "<value from old web.config/machineKey/validationKey>";
string decryptionKey = "<value from old web.config/machineKey/decryptionKey>";

// Decrypt the token
var ticket = MachineKeyTicketUnprotector.UnprotectOAuthToken(token, decryptionKey, validationKey);

// The ticket is in v3 format and needs to be converted to v5 (ASP.NET Core 2.0).
// Can use whatever you want for AuthScheme
var newTicket = AuthenticationTicketConverter.Convert(ticket, AuthScheme);

// If using a custom AuthenticationHandler, you can return the new ticket 
// in the HandleAuthenticateAsync method.
var result = AuthenticateResult.Success(newTicket);