acme: return error from pre-authorization when unsupported

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>

acme/acme.go

@@ -353,6 +353,10 @@ func (c *Client) authorize(ctx context.Context, typ, val string) (*Authorization
 	if _, err := c.Discover(ctx); err != nil {
 		return nil, err
 	}
+	if c.dir.AuthzURL == "" {
+		// Pre-Authorization is unsupported
+		return nil, errPreAuthorizationNotSupported
+	}
 
 	type authzID struct {
 		Type  string `json:"type"`

acme/acme_test.go

@@ -15,6 +15,7 @@ import (
 	"encoding/base64"
 	"encoding/hex"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"math/big"
@@ -254,6 +255,57 @@ func TestAuthorizeValid(t *testing.T) {
 	}
 }
 
+func TestAuthorizeUnsupported(t *testing.T) {
+	const (
+		nonce       = "https://example.com/acme/new-nonce"
+		reg         = "https://example.com/acme/new-acct"
+		order       = "https://example.com/acme/new-order"
+		revoke      = "https://example.com/acme/revoke-cert"
+		keychange   = "https://example.com/acme/key-change"
+		metaTerms   = "https://example.com/acme/terms/2017-5-30"
+		metaWebsite = "https://www.example.com/"
+		metaCAA     = "example.com"
+	)
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Replay-Nonce", "nonce")
+		if r.Method == http.MethodHead {
+			return
+		}
+		switch r.URL.Path {
+		case "/": // Directory
+			w.Header().Set("Content-Type", "application/json")
+			fmt.Fprintf(w, `{
+                "newNonce": %q,
+                "newAccount": %q,
+                "newOrder": %q,
+                "revokeCert": %q,
+                "keyChange": %q,
+                "meta": {
+                    "termsOfService": %q,
+                    "website": %q,
+                    "caaIdentities": [%q],
+                    "externalAccountRequired": true
+                }
+            }`, nonce, reg, order, revoke, keychange, metaTerms, metaWebsite, metaCAA)
+			w.WriteHeader(http.StatusOK)
+		case "/acme/new-authz":
+			w.WriteHeader(http.StatusBadRequest)
+		}
+	}))
+	defer ts.Close()
+	client := &Client{Key: testKey, DirectoryURL: ts.URL}
+	dir, err := client.Discover(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if dir.AuthzURL != "" {
+		t.Fatalf("expected AuthzURL to be empty, got %q", dir.AuthzURL)
+	}
+	if _, err := client.Authorize(context.Background(), "example.com"); !errors.Is(err, errPreAuthorizationNotSupported) {
+		t.Errorf("expected err to indicate pre-authorization is unsupported, got %+v", err)
+	}
+}
+
 func TestWaitAuthorization(t *testing.T) {
 	t.Run("wait loop", func(t *testing.T) {
 		var count int

acme/types.go

@@ -56,6 +56,10 @@ var (
 
 	// ErrNoAccount indicates that the Client's key has not been registered with the CA.
 	ErrNoAccount = errors.New("acme: account does not exist")
+
+	// errPreAuthorizationNotSupported indicates that the server does not
+	// support pre-authorization of identifiers.
+	errPreAuthorizationNotSupported = errors.New("acme: pre-authorization is not supported")
 )
 
 // A Subproblem describes an ACME subproblem as reported in an Error.