Implement optional cipher in --data-ciphers prefixed with ?

This allows to use the same configuration multiple platforms/ssl libraries
and include optional algorithms that are not available on all platforms

For example "AES-256-GCM:AES-128-GCM:?CHACHA20-POLY1305" can be used to
emulate the default behaviour of OpenVPN 2.6.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20211201180727.2496903-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23279.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 766044507497c41f0319159c37992788ecb681e6)

diff --git a/Changes.rst b/Changes.rst
index cafb1f23fd854ae6974fd6344856989754489007..ec7045dae9b757d03e394bd4d408b0155023a80d 100644 (file)
--- a/Changes.rst
+++ b/Changes.rst
@@ -1,3 +1,12 @@
+Overview of changes in 2.5.9
+============================
+
+New features
+------------
+- Optional ciphers in ``--data-ciphers``
+  Ciphers in ``--data-ciphers`` can now be prefixed with a ``?`` to mark
+  those as optional and only use them if the SSL library supports them.
+
 Overview of changes in 2.5.8
 ============================
 

diff --git a/doc/man-sections/protocol-options.rst b/doc/man-sections/protocol-options.rst
index e9d5d63d43b8e7edf22755112e879160755530ec..25f8db12d95e594809e79e47b69f8d604a41b964 100644 (file)
--- a/doc/man-sections/protocol-options.rst
+++ b/doc/man-sections/protocol-options.rst
@@ -184,6 +184,13 @@ configured in a compatible way between both the local and remote side.
   supported by the client will be pushed to clients that support cipher
   negotiation.
 
+  Starting with OpenVPN 2.5.9 a cipher can be prefixed with a :code:`?` to mark
+  it as optional. This allows including ciphers in the list that may not be
+  available on all platforms.
+  E.g. :code:`AES-256-GCM:AES-128-GCM:?CHACHA20-POLY1305` would only enable
+  Chacha20-Poly1305 if the underlying SSL library (and its configuration)
+  supports it.
+
   Cipher negotiation is enabled in client-server mode only. I.e. if
   ``--mode`` is set to 'server' (server-side, implied by setting
   ``--server`` ), or if ``--pull`` is specified (client-side, implied by

diff --git a/src/openvpn/ssl_ncp.c b/src/openvpn/ssl_ncp.c
index b94c786ee55ed578bc58357fe7853895cf2ee365..de7efa40793a90a01028e7282c73d9b0d2890d98 100644 (file)
--- a/src/openvpn/ssl_ncp.c
+++ b/src/openvpn/ssl_ncp.c
@@ -108,7 +108,18 @@ mutate_ncp_cipher_list(const char *list, struct gc_arena *gc)
          * (and translate_cipher_name_from_openvpn/
          * translate_cipher_name_to_openvpn) also normalises the cipher name,
          * e.g. replacing AeS-128-gCm with AES-128-GCM
+         *
+         * ciphers that have ? in front of them are considered optional and
+         * OpenVPN will only warn if they are not found (and remove them from
+         * the list)
          */
+
+        bool optional = false;
+        if (token[0] == '?')
+        {
+            token++;
+            optional = true;
+        }
         const cipher_kt_t *ktc = cipher_kt_get(token);
         if (strcmp(token, "none") == 0)
         {
@@ -120,8 +131,9 @@ mutate_ncp_cipher_list(const char *list, struct gc_arena *gc)
         }
         if (!ktc && strcmp(token, "none") != 0)
         {
-            msg(M_WARN, "Unsupported cipher in --data-ciphers: %s", token);
-            error_found = true;
+            const char* optstr = optional ? "optional ": "";
+            msg(M_WARN, "Unsupported %scipher in --data-ciphers: %s", optstr, token);
+            error_found = !optional;
         }
         else
         {

diff --git a/tests/unit_tests/openvpn/test_ncp.c b/tests/unit_tests/openvpn/test_ncp.c
index 4337f6df739c665ffdb9de1f2d76f0e4dbb0df08..134a58ab6f7e0a8ad4bc2bf6d684243761be2a51 100644 (file)
--- a/tests/unit_tests/openvpn/test_ncp.c
+++ b/tests/unit_tests/openvpn/test_ncp.c
@@ -74,6 +74,17 @@ test_check_ncp_ciphers_list(void **state)
         assert_ptr_equal(mutate_ncp_cipher_list(bf_chacha, &gc), NULL);
     }
 
+    /* Check that optional ciphers work */
+    assert_string_equal(mutate_ncp_cipher_list("AES-256-GCM:?vollbit:AES-128-GCM", &gc),
+                        aes_ciphers);
+
+    /* Check that optional ciphers work */
+    assert_string_equal(mutate_ncp_cipher_list("?AES-256-GCM:?AES-128-GCM", &gc),
+                        aes_ciphers);
+
+    /* All unsupported should still yield an empty list */
+    assert_ptr_equal(mutate_ncp_cipher_list("?kugelfisch:?grasshopper", &gc), NULL);
+
     /* For testing that with OpenSSL 1.1.0+ that also accepts ciphers in
      * a different spelling the normalised cipher output is the same */
     bool have_chacha_mixed_case = cipher_kt_get("ChaCha20-Poly1305");