fix(challenge/preact): ensure that the client waits as long as it needs to

Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/challenge/preact/preact.go

@@ -8,6 +8,7 @@ import (
 	"io"
 	"log/slog"
 	"net/http"
+	"time"
 
 	"github.com/TecharoHQ/anubis"
 	"github.com/TecharoHQ/anubis/internal"
@@ -56,6 +57,12 @@ func (i *impl) Issue(r *http.Request, lg *slog.Logger, in *challenge.IssueInput)
 }
 
 func (i *impl) Validate(r *http.Request, lg *slog.Logger, in *challenge.ValidateInput) error {
+	wantTime := in.Challenge.IssuedAt.Add(time.Duration(in.Rule.Challenge.Difficulty) * 95 * time.Millisecond)
+
+	if time.Now().Before(wantTime) {
+		return challenge.NewError("validate", "insufficent time", fmt.Errorf("%w: wanted user to wait until at least %s", wantTime.Format(time.RFC3339)))
+	}
+
 	got := r.FormValue("result")
 	want := internal.SHA256sum(in.Challenge.RandomData)