How to sign HTTP Response and put it into header?

I use FastEndpoints over ASP.NET Core minimal API and want to sign the response body which is in application/json format.

I think instead of calling SendAsync(MyResponse) I need to:

serialize MyResponse to JSON,
get signature of the JSON text,
add it to the Response.Header
and then send the JSON in the body.

I'd like to use the same JSON serializer with exactly the same options as if I call SendAsync(MyResponse). And I don't know how to access the serializer from endpoint.

What I tried to do:

I saw the answer how to replace Body.Stream at the middleware but I don't think it's a good idea. In addition I need to sign only specific endpoints.
I tried to use IPostProcessor but I can't read from response Body.
I tried to use IResponseInterceptor but it seems to just add

I use custom json options like this:

var app = bld.Build();
app
    .UseAuthentication()
    .UseAuthorization()
    .UseFastEndpoints(c =>
    {
        c.Endpoints.RoutePrefix = "api/v1";
        c.Serializer.Options.Converters.Add(new JsonStringEnumConverter(JsonNamingPolicy.SnakeCaseUpper));
    })
    .UseSwaggerGen()
    ;

Solution

a custom send extension method should get the job done nicely. something like the following:

var bld = WebApplication.CreateBuilder(args);
bld.Services
   .Configure<JsonOptions>(o => o.SerializerOptions.PropertyNamingPolicy = null)
   .AddFastEndpoints()
   .SwaggerDocument();

var app = bld.Build();
app.UseFastEndpoints()
   .UseSwaggerGen();
app.Run();

sealed class MyResponse
{
    public string Name { get; set; }
}

sealed class MyEndpoint : Ep.NoReq.Res<MyResponse>
{
    public override void Configure()
    {
        Get("test");
        AllowAnonymous();
    }

    public override async Task HandleAsync(CancellationToken c)
    {
        await this.SendSignedResponse(new MyResponse { Name = "test" });
    }
}

static class SendExtensions
{
    public static Task SendSignedResponse<TResponse>(this IEndpoint ep, TResponse dto)
    {
        var jsonOptions = ep.HttpContext.Resolve<IOptions<JsonOptions>>().Value.SerializerOptions;
        var json = JsonSerializer.Serialize(dto, jsonOptions);
        var md5 = BitConverter.ToString(MD5.HashData(Encoding.UTF8.GetBytes(json))).Replace("-", "").ToLower();

        ep.HttpContext.MarkResponseStart();
        ep.HttpContext.Response.Headers.Append("x-signature", md5);

        return ep.HttpContext.Response.SendStringAsync(json);
    }
}