Implement generating data channel keys via EKM/RFC 5705

OpenVPN currently uses its own (based on TLS 1.0) key derivation
mechanism to generate the 256 bytes key data in key2 struct that
are then used used to generate encryption/hmac/iv vectors. While
this mechanism is still secure, it is not state of the art.

Instead of modernising our own approach, this commit implements
key derivation using the Keying Material Exporters API introduced
by RFC 5705.

We also use an opportunistic approach of negotiating the use of
EKM (exported key material) through an IV_PROTO flag and prefer
EKM to our own PRF if both client and server support it. The
use of EKM is pushed to the client as part of NCP as
key-derivation tls-ekm.

We still exchange the random data (112 bytes from client to server
and 64 byte from server to client) for the OpenVPN PRF but
do not use it. Removing that exchange would break the handshake
and make a key-method 3 or similar necessary.

As a side effect, this makes a little bit easier to have a FIPS compatible
version of OpenVPN since we do not rely on calling MD5 anymore.

Side note: this commit breaks the (not yet merged) WolfSSL support as it
claims to support EKM in the OpenSSL compat API but always returns an error
if you try to use it.

Patch v2: rebase/change to V2 of EKM refactoring

Patch v3: add Changes.rst

Patch v4: Rebase on master.

Patch v5: Refuse internal label to be used with --keying-material-exporter,
          polishing/fixes suggested by Steffan integrated

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Steffan Karger <steffan.karger@foxcrypto.com>
Message-Id: <20201009115453.4279-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21187.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

diff --git a/Changes.rst b/Changes.rst
index f67e1d7619dda80f8986a1ac824b06fe1afd4b1d..2a2829e7c00c1c30a593f4c3c2a9cf98db522416 100644 (file)
--- a/Changes.rst
+++ b/Changes.rst
@@ -1,3 +1,14 @@
+Overview of changes in 2.6
+==========================
+
+
+New features
+------------
+Keying Material Exporters (RFC 5705) based key generation
+    As part of the cipher negotiation OpenVPN will automatically prefer
+    the RFC5705 based key material generation to the current custom
+    OpenVPN PRF. This feature requires OpenSSL or mbed TLS 2.18+.
+
 Overview of changes in 2.5
 ==========================
 

diff --git a/doc/doxygen/doc_key_generation.h b/doc/doxygen/doc_key_generation.h
index 4bb9c708b5bcf64f1677766a4152f29bdd4e857f..cf04ba25513ed9000115eb29b22c2fc91bfa9f28 100644 (file)
--- a/doc/doxygen/doc_key_generation.h
+++ b/doc/doxygen/doc_key_generation.h
@@ -58,6 +58,12 @@
  *
  * @subsection key_generation_method_2 Key method 2
  *
+ * There are two methods for generating key data when using key method 2
+ * the first is OpenVPN's traditional approach that exchanges random
+ * data and uses a PRF and the other is using the RFC5705 keying material
+ * exporter to generate the key material. For both methods the random
+ * data is exchange but only used in the traditional method.
+ *
  * -# The client generates random material in the following amounts:
  *    - Pre-master secret: 48 bytes
  *    - Client's PRF seed for master secret: 32 bytes
@@ -73,8 +79,12 @@
  *    server's random material.
  *
  * %Key method 2 %key expansion is performed by the \c
- * generate_key_expansion() function.  Please refer to its source code for
- * details of the %key expansion process.
+ * generate_key_expansion_openvpn_prf() function.  Please refer to its source
+ * code for details of the %key expansion process.
+ *
+ * When the client sends the IV_PROTO_TLS_KEY_EXPORT flag and the server replies
+ * with `key-derivation tls-ekm` the RFC5705 key material exporter with the
+ * label EXPORTER-OpenVPN-datakeys is used for the key data.
  *
  * @subsection key_generation_random Source of random material
  *

diff --git a/src/openvpn/crypto.h b/src/openvpn/crypto.h
index 999f643edf76e78d469b8496a7e868a2b39f05f8..1ad669ce1a49a681092e8835d493efa3d80cceea 100644 (file)
--- a/src/openvpn/crypto.h
+++ b/src/openvpn/crypto.h
@@ -254,6 +254,10 @@ struct crypto_options
 #define CO_MUTE_REPLAY_WARNINGS (1<<2)
     /**< Bit-flag indicating not to display
      *   replay warnings. */
+#define CO_USE_TLS_KEY_MATERIAL_EXPORT  (1<<3)
+    /**< Bit-flag indicating that data channel key derivation
+     * is done using TLS keying material export [RFC5705]
+     */
     unsigned int flags;         /**< Bit-flags determining behavior of
                                  *   security operation functions. */
 };

diff --git a/src/openvpn/init.c b/src/openvpn/init.c
index f87c11e7464fd199da2f90115f898f2a19ea00f3..580a85500006d5d122848b566e5d94099bbc02a5 100644 (file)
--- a/src/openvpn/init.c
+++ b/src/openvpn/init.c
@@ -687,6 +687,7 @@ restore_ncp_options(struct context *c)
     c->options.ciphername = c->c1.ciphername;
     c->options.authname = c->c1.authname;
     c->options.keysize = c->c1.keysize;
+    c->options.data_channel_use_ekm = false;
 }
 
 void

diff --git a/src/openvpn/multi.c b/src/openvpn/multi.c
index 137381805f7d8cab2f2e12d53a6a5c120d868b5e..a5862020b81605416524b1beb925d2692c5c3eec 100644 (file)
--- a/src/openvpn/multi.c
+++ b/src/openvpn/multi.c
@@ -1817,6 +1817,10 @@ multi_client_set_protocol_options(struct context *c)
         c->c2.push_request_received = true;
     }
 
+#ifdef HAVE_EXPORT_KEYING_MATERIAL
+    o->data_channel_use_ekm = (proto & IV_PROTO_TLS_KEY_EXPORT);
+#endif
+
     /* Select cipher if client supports Negotiable Crypto Parameters */
     if (!o->ncp_enabled)
     {

diff --git a/src/openvpn/options.c b/src/openvpn/options.c
index 772323df4f37fa394addac46f56deea17d47619c..4e19d7cbcafc299fe1e12de2d2ded99dd36eeea4 100644 (file)
--- a/src/openvpn/options.c
+++ b/src/openvpn/options.c
@@ -7979,6 +7979,20 @@ add_option(struct options *options,
         }
         options->ncp_ciphers = p[1];
     }
+    else if (streq(p[0], "key-derivation") && p[1])
+    {
+        VERIFY_PERMISSION(OPT_P_NCP)
+#ifdef HAVE_EXPORT_KEYING_MATERIAL
+        if (streq(p[1], "tls-ekm"))
+        {
+            options->data_channel_use_ekm = true;
+        }
+        else
+#endif
+        {
+            msg(msglevel, "Unknown key-derivation method %s", p[1]);
+        }
+    }
     else if (streq(p[0], "ncp-disable") && !p[1])
     {
         VERIFY_PERMISSION(OPT_P_GENERAL|OPT_P_INSTANCE);
@@ -8707,6 +8721,11 @@ add_option(struct options *options,
                 "\"EXPORTER\"");
             goto err;
         }
+        if (streq(p[1], EXPORT_KEY_DATA_LABEL))
+        {
+            msg(msglevel, "Keying material exporter label must not be '"
+                EXPORT_KEY_DATA_LABEL "'.");
+        }
         if (ekm_length < 16 || ekm_length > 4095)
         {
             msg(msglevel, "Invalid keying material exporter length");

diff --git a/src/openvpn/options.h b/src/openvpn/options.h
index e527d70ee0a87d981348ea2a896116bd889b1d88..5d97779368bed86daf44c5f43bdea2d0e88bc1f3 100644 (file)
--- a/src/openvpn/options.h
+++ b/src/openvpn/options.h
@@ -648,6 +648,9 @@ struct options
     /* Useful when packets sent by openvpn itself are not subject
      * to the routing tables that would move packets into the tunnel. */
     bool allow_recursive_routing;
+
+    /* Use RFC5705 key export to generate data channel keys */
+    bool data_channel_use_ekm;
 };
 
 #define streq(x, y) (!strcmp((x), (y)))

diff --git a/src/openvpn/push.c b/src/openvpn/push.c
index e0d2eeaf28e6e7c04332f82d196ee0f05fa5e399..17bba9483768acff1cb068c5da73fe6aef1f339c 100644 (file)
--- a/src/openvpn/push.c
+++ b/src/openvpn/push.c
@@ -479,7 +479,10 @@ prepare_push_reply(struct context *c, struct gc_arena *gc,
     {
         push_option_fmt(gc, push_list, M_USAGE, "cipher %s", o->ciphername);
     }
-
+    if (o->data_channel_use_ekm)
+    {
+        push_option_fmt(gc, push_list, M_USAGE, "key-derivation tls-ekm");
+    }
     return true;
 }
 

diff --git a/src/openvpn/ssl.c b/src/openvpn/ssl.c
index f90c6a856773f06fc602e8e315d55926a4d43c13..b572b7b8c4533c1fe98bff936efa3fd5ba2af91c 100644 (file)
--- a/src/openvpn/ssl.c
+++ b/src/openvpn/ssl.c
@@ -1783,6 +1783,29 @@ init_key_contexts(struct key_ctx_bi *key,
 
 }
 
+static bool
+generate_key_expansion_tls_export(struct tls_session *session, struct key2 *key2)
+{
+    struct gc_arena gc = gc_new();
+    unsigned char *key2data;
+
+    key2data = key_state_export_keying_material(session,
+                                                EXPORT_KEY_DATA_LABEL,
+                                                strlen(EXPORT_KEY_DATA_LABEL),
+                                                sizeof(key2->keys),
+                                                &gc);
+    if (!key2data)
+    {
+        return false;
+    }
+    memcpy(key2->keys, key2data, sizeof(key2->keys));
+    secure_memzero(key2data, sizeof(key2->keys));
+    key2->n = 2;
+
+    gc_free(&gc);
+    return true;
+}
+
 static struct key2
 generate_key_expansion_openvpn_prf(const struct tls_session *session)
 {
@@ -1854,7 +1877,7 @@ generate_key_expansion_openvpn_prf(const struct tls_session *session)
  */
 static bool
 generate_key_expansion(struct key_ctx_bi *key,
-                       const struct tls_session *session)
+                       struct tls_session *session)
 {
     bool ret = false;
     struct key2 key2;
@@ -1865,10 +1888,20 @@ generate_key_expansion(struct key_ctx_bi *key,
         goto exit;
     }
 
-
     bool server = session->opt->server;
 
-    key2 = generate_key_expansion_openvpn_prf(session);
+    if (session->opt->crypto_flags & CO_USE_TLS_KEY_MATERIAL_EXPORT)
+    {
+        if (!generate_key_expansion_tls_export(session, &key2))
+        {
+            msg(D_TLS_ERRORS, "TLS Error: Keying material export failed");
+            goto exit;
+        }
+    }
+    else
+    {
+        key2 = generate_key_expansion_openvpn_prf(session);
+    }
 
     key2_print(&key2, &session->opt->key_type,
                "Master Encrypt", "Master Decrypt");
@@ -1967,6 +2000,11 @@ tls_session_update_crypto_params(struct tls_session *session,
         return false;
     }
 
+    if (options->data_channel_use_ekm)
+    {
+        session->opt->crypto_flags |= CO_USE_TLS_KEY_MATERIAL_EXPORT;
+    }
+
     if (strcmp(options->ciphername, session->opt->config_ciphername))
     {
         msg(D_HANDSHAKE, "Data Channel: using negotiated cipher '%s'",
@@ -2251,13 +2289,11 @@ push_peer_info(struct buffer *buf, struct tls_session *session)
          * push request, also signal that the client wants
          * to get push-reply messages without without requiring a round
          * trip for a push request message*/
-        if(session->opt->pull)
+        if (session->opt->pull)
         {
             iv_proto |= IV_PROTO_REQUEST_PUSH;
         }
 
-        buf_printf(&out, "IV_PROTO=%d\n", iv_proto);
-
         /* support for Negotiable Crypto Parameters */
         if (session->opt->ncp_enabled
             && (session->opt->mode == MODE_SERVER || session->opt->pull))
@@ -2269,8 +2305,14 @@ push_peer_info(struct buffer *buf, struct tls_session *session)
                 buf_printf(&out, "IV_NCP=2\n");
             }
             buf_printf(&out, "IV_CIPHERS=%s\n", session->opt->config_ncp_ciphers);
+
+#ifdef HAVE_EXPORT_KEYING_MATERIAL
+            iv_proto |= IV_PROTO_TLS_KEY_EXPORT;
+#endif
         }
 
+        buf_printf(&out, "IV_PROTO=%d\n", iv_proto);
+
         /* push compression status */
 #ifdef USE_COMP
         comp_generate_peer_info_string(&session->opt->comp_options, &out);

diff --git a/src/openvpn/ssl.h b/src/openvpn/ssl.h
index 005628f664687b2cec5062636742e26d3936619e..f00f8abd53a3bb1bab946168288cd5e7ae3ec0a5 100644 (file)
--- a/src/openvpn/ssl.h
+++ b/src/openvpn/ssl.h
@@ -116,6 +116,8 @@
  * to wait for a push-request to send a push-reply */
 #define IV_PROTO_REQUEST_PUSH   (1<<2)
 
+/** Supports key derivation via TLS key material exporter [RFC5705] */
+#define IV_PROTO_TLS_KEY_EXPORT (1<<3)
 
 /* Default field in X509 to be username */
 #define X509_USERNAME_FIELD_DEFAULT "CN"

diff --git a/src/openvpn/ssl_backend.h b/src/openvpn/ssl_backend.h
index cf9fba254a89ecf03a5ae8ff5cad10deeb3d21ac..4bcb3181c6224ff5ba315bd34dc3a0c2efe67e57 100644 (file)
--- a/src/openvpn/ssl_backend.h
+++ b/src/openvpn/ssl_backend.h
@@ -389,6 +389,7 @@ void key_state_ssl_free(struct key_state_ssl *ks_ssl);
 void backend_tls_ctx_reload_crl(struct tls_root_ctx *ssl_ctx,
                                 const char *crl_file, bool crl_inline);
 
+#define EXPORT_KEY_DATA_LABEL       "EXPORTER-OpenVPN-datakeys"
 /**
  * Keying Material Exporters [RFC 5705] allows additional keying material to be
  * derived from existing TLS channel. This exported keying material can then be

diff --git a/src/openvpn/ssl_mbedtls.c b/src/openvpn/ssl_mbedtls.c
index 4ec355a9034a2d996266b3fc3d120d90d2d7a5c5..f375e957b2c35d9a13587d4e3740649608aaf441 100644 (file)
--- a/src/openvpn/ssl_mbedtls.c
+++ b/src/openvpn/ssl_mbedtls.c
@@ -1168,11 +1168,8 @@ key_state_ssl_init(struct key_state_ssl *ks_ssl,
 
 #ifdef HAVE_EXPORT_KEYING_MATERIAL
     /* Initialize keying material exporter */
-    if (session->opt->ekm_size)
-    {
-        mbedtls_ssl_conf_export_keys_ext_cb(ks_ssl->ssl_config,
-                                            mbedtls_ssl_export_keys_cb, session);
-    }
+    mbedtls_ssl_conf_export_keys_ext_cb(ks_ssl->ssl_config,
+                                        mbedtls_ssl_export_keys_cb, session);
 #endif
 
     /* Initialise SSL context */