consuming oAuth bearer issued by OWIN from asp.net core

You could set the OAuthValidation AccessTokenFormat to use a MachineKey DataProtectionProvider and DataProtector which will protect and unprotect your bearer tokens. You will need to implement the MachineKey DataProtector. This guy already did it https://github.com/daixinkai/AspNetCore.Owin/blob/master/src/DataProtection/src/AspNetCore.DataProtection.MachineKey/MachineKeyDataProtectionProvider.cs.

public void ConfigureServices(IServiceCollection services){
   services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
   ConfigureAuth(services);

  string machineKey = @"<?xml version=""1.0"" encoding=""utf-8"" ?>
        <machineKey decryption=""Auto"" decryptionKey =""DEC_KEY"" validation=""HMACSHA256"" validationKey=""VAL_KEY"" />";
        var machineKeyConfig = new XmlMachineKeyConfig(machineKey);
        MachineKeyDataProtectionOptions machinekeyOptions = new MachineKeyDataProtectionOptions();
        machinekeyOptions.MachineKey = new MachineKey(machineKeyConfig);
        MachineKeyDataProtectionProvider machineKeyDataProtectionProvider = new MachineKeyDataProtectionProvider(machinekeyOptions);
        MachineKeyDataProtector machineKeyDataProtector = new MachineKeyDataProtector(machinekeyOptions.MachineKey);

   //purposes from owin middleware
   IDataProtector dataProtector = 
   machineKeyDataProtector.CreateProtector("Microsoft.Owin.Security.OAuth",
               "Access_Token", "v1"); 

   services.AddAuthentication(options =>
   {
       options.DefaultScheme = JwtBearerDefaults.AuthenticationScheme;
   })
   .AddOAuthValidation(option=> {
            option.AccessTokenFormat = new OwinTicketDataFormat(new OwinTicketSerializer(), dataProtector); })

It's important to keep the same DataProtector "purposes" Owin uses in the OAuthAuthorizationServerMiddleware so the data is encrypted/decrypted correctly. Those are "Microsoft.Owin.Security.OAuth", "Access_Token" and "v1". (see https://stackoverflow.com/a/29454816/2734166).

And finally you will have to migrate the Owin TicketSerializer (and maybe the TicketFormat too) since the one in NetCore is slightly different. You can grab it from here:

https://github.com/aspnet/AspNetKatana/blob/e2b18ec84ceab7ffa29d80d89429c9988ab40144/src/Microsoft.Owin.Security/DataHandler/Serializer/TicketSerializer.cs

I got this working recently. Basically authenticating to a .NET 4.5 Owin API and running a resource API in NET Core using the same token. I'll try to share the code in github as soon as I clean it up.

As far as I know it's not recommended to keep the old machine key data protector, but to migrate to the new ones from NET Core. Sometimes this is not possible. In my case I have too many APIs already in production, so I'm trying some new NET Core APIs to work with the legacy ones.

You should try this Owin.Token.AspNetCore nuget package instead. By following the code example provided in the README file I'm able to decode legacy tokens using the machine keys on .NET Core 3.1. Note: there's also an option to specify encryption method and validation method if the defaults are not working for you.