Subject: bug#57956: 29.0.50; Add minimal authorization support to sasl-scram-rfc

From: "J.P." <[email-masked]>
Date: Tue, 20 Sep 2022 06:06:37 -0700

Tags: patch

Hi people,

ERC plans on basing its SASL library on sasl.el and friends. Although
rare, authorization (or "authz") support is sometimes needed by IRC
administrators wanting to authenticate as other users. It's also
expected by at least one IRC-compliance test suite [1]. The PLAIN
implementation in sasl.el currently offers implicit support via the
`authenticator-name' client property (see `sasl-plain-response'). This
patch proposes we do much the same with sasl-scram-rfc.el.

As for specifics, I've encapsulated the actual prop-lookup and
header-construction details in a new function that's called indirectly
via a new top-level variable (although perhaps that's just unnecessary
or more suited to a user option). The only other change appears in
`sasl-scram--client-final-message'. It concerns the base64 encoding of
the GS2 header and the client proof, both of which currently suffer from
occasional whitespace complications [2].

Anyway, ERC would benefit greatly from these (or superior) changes
because we'd like to introduce `erc-compat' analogs in an upcoming
release (probably ERC 5.6). If anyone out there can spare the time,
your feedback would be greatly appreciated.

Thanks,
J.P.

P.S. Tests covering these changes appear in the patch sets for bug#29108
and bug#49860.

[1] Authz support for PLAIN, with SCRAM possibly on the way:

    https://github.com/progval/irctest/blob/master/irctest/client_tests/sasl.py

[2] Calling `base64-encode-string' with NO-LINE-BREAK set to t seems to
    solve the issue, which is likely related to this excerpt from
    https://www.rfc-editor.org/rfc/rfc5802#section-2.1:
 
     "The use of base64 in SCRAM is restricted to the canonical form
      with no whitespace."
 
    FWIW, I tried advising `base64-encode-string' to avoid having to
    submit a patch, but it seems the byte compiler precomputes the
    result for certain constant params, like `cbind-input' in
    `sasl-scram--client-final-message'.

In GNU Emacs 29.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version
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>From 91e33541457a55e2e509d800cd8b9f97702e706d Mon Sep 17 00:00:00 2001
From: "F. Jason Park" <[email-masked]>
Date: Mon, 19 Sep 2022 21:28:52 -0700
Subject: [PATCH 1/4] Add GS2 authorization to sasl-scram-rfc

* lisp/net/sasl-scram-rfc.el (sasl-scram-fs2-header-function,
sasl-scram-construct-gs2-header): Add new variable and default
function for determining a SCRAM GSS-API message header.
(sasl-scram-client-first-message): Use gs2-header function.
(sasl-scram--client-final-message): Use dedicated gs2-header function.
Also remove whitespace when base64-encoding, as per RFC 5802.
---
 lisp/net/sasl-scram-rfc.el | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/lisp/net/sasl-scram-rfc.el b/lisp/net/sasl-scram-rfc.el
index ee52ed6e07..f7a2e42541 100644
--- a/lisp/net/sasl-scram-rfc.el
+++ b/lisp/net/sasl-scram-rfc.el
@@ -45,14 +45,21 @@
 
 ;;; Generic for SCRAM-*
 
+(defvar sasl-scram-gs2-header-function 'sasl-scram-construct-gs2-header
+  "Function to create GS2 header.
+See https://www.rfc-editor.org/rfc/rfc5801#section-4.")
+
+(defun sasl-scram-construct-gs2-header (client)
+  ;; The "n," means the client doesn't support channel binding, and
+  ;; the trailing comma is included as per RFC 5801.
+  (let ((authzid (sasl-client-property client 'authenticator-name)))
+    (concat "n," (and authzid "a=") authzid ",")))
+
 (defun sasl-scram-client-first-message (client _step)
   (let ((c-nonce (sasl-unique-id)))
     (sasl-client-set-property client 'c-nonce c-nonce))
   (concat
-   ;; n = client doesn't support channel binding
-   "n,"
-   ;; TODO: where would we get authorization id from?
-   ","
+   (funcall sasl-scram-gs2-header-function client)
    (sasl-scram--client-first-message-bare client)))
 
 (defun sasl-scram--client-first-message-bare (client)
@@ -77,11 +84,11 @@ sasl-scram--client-final-message
 
 	 (c-nonce (sasl-client-property client 'c-nonce))
 	 ;; no channel binding, no authorization id
-	 (cbind-input "n,,"))
+         (cbind-input (funcall sasl-scram-gs2-header-function client)))
     (unless (string-prefix-p c-nonce nonce)
       (sasl-error "Invalid nonce from server"))
     (let* ((client-final-message-without-proof
-	    (concat "c=" (base64-encode-string cbind-input) ","
+            (concat "c=" (base64-encode-string cbind-input t) ","
 		    "r=" nonce))
 	   (password
 	    ;; TODO: either apply saslprep or disallow non-ASCII characters
@@ -113,7 +120,7 @@ sasl-scram--client-final-message
 	   (client-proof (funcall string-xor client-key client-signature))
 	   (client-final-message
 	    (concat client-final-message-without-proof ","
-		    "p=" (base64-encode-string client-proof))))
+                    "p=" (base64-encode-string client-proof t))))
       (sasl-client-set-property client 'auth-message auth-message)
       (sasl-client-set-property client 'salted-password salted-password)
       client-final-message)))
-- 
2.37.2