Check the directory's AuthzURL to see if the server supports pre-authorization. If it's empty, then the server is not advertising support and we can encounter other bugs. Better to return early and give a clear error to the caller. From https://www.rfc-editor.org/rfc/rfc8555#section-7.4.1 If a CA wishes to allow pre-authorization within ACME, it can offer a "new authorization" resource in its directory by adding the field "newAuthz" with a URL for the newAuthz resource. Fixes golang/go#40839 Change-Id: Id3e92e8e2ae3c57285183d37544dd59b4988b3be Reviewed-on: https://go-review.googlesource.com/c/crypto/+/661675 Reviewed-by: Dmitri Shuralyov <dmitshur@google.com> Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Roland Shoemaker <roland@golang.org> LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>