Add support for elliptic curve diffie-hellmann key exchange (ECDH)

This patch is based on Jan Just Keijser's patch from Feb 7, 2012.

When OpenSSL 1.0.2+ or PolarSSL is used, lets the crypto library do the
heavy lifting. For OpenSSL builds, if a user specifies a curve using
--ecdh-curve, it first tries to override automatic selection using that
curve.

For older OpenSSL, tries the following things (in order of preference):
 * When supplied, use the ecdh curve specified by the user.
 * Try to extract the curve from the private key, use the same curve.
 * Fall back on secp384r1 curve.

Note that although a curve lookup might succeed, OpenSSL 1.0.0 and older do
*not* support TLSv1.1 or TLSv1.2, which means no that no EC-crypto can be
used.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <53597BEA.6080408@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8625
Signed-off-by: Gert Doering <gert@greenie.muc.de>

README.ec

@@ -0,0 +1,35 @@
+Since 2.4.0, OpenVPN has official support for elliptic curve crypto. Elliptic
+curves are an alternative to RSA for asymmetric encryption.
+
+Elliptic curve crypto ('ECC') can be used for the ('TLS') control channel only
+in OpenVPN; the data channel (encrypting the actual network traffic) uses
+symmetric encryption. ECC can be used in TLS for authentication (ECDSA) and key
+exchange (ECDH).
+
+Key exchange (ECDH)
+-------------------
+OpenVPN 2.4.0 and newer automatically initialize ECDH parameters. When ECDSA is
+used for authentication, the curve used for the server certificate will be used
+for ECDH too. When autodetection fails (e.g. when using RSA certificates)
+OpenVPN lets the crypto library decide if possible, or falls back to the
+secp384r1 curve.
+
+An administrator can force an OpenVPN/OpenSSL server to use a specific curve
+using the --ecdh-curve <curvename> option with one of the curves listed as
+available by the --show-curves option. Clients will use the same curve as
+selected by the server.
+
+Note that not all curves listed by --show-curves are available for use with TLS;
+in that case connecting will fail with a 'no shared cipher' TLS error.
+
+Authentication (ECDSA)
+----------------------
+Since OpenVPN 2.4.0, using ECDSA certificates works 'out of the box'. Which
+specific curves and cipher suites are available depends on your version and
+configuration of the crypto library. The crypto library will automatically
+select a cipher suite for the TLS control channel.
+
+Support for generating an ECDSA certificate chain is available in EasyRSA (in
+spite of it's name) since EasyRSA 3.0. The parameters you're looking for are
+'--use-algo=ec' and '--curve=<curve_name>'. See the EasyRSA documentation for
+more details on generating ECDSA certificates.

doc/openvpn.8

@@ -4246,6 +4246,13 @@ included with the OpenVPN distribution.  Diffie Hellman parameters
 may be considered public.
 .\"*********************************************************
 .TP
+.B \-\-ecdh-curve name
+Specify the curve to use for elliptic curve Diffie Hellman. Available
+curves can be listed with
+.B \-\-show-curves
+. The specified curve will only be used for ECDH TLS-ciphers.
+.\"*********************************************************
+.TP
 .B \-\-cert file
 Local peer's signed certificate in .pem format \-\- must be signed
 by a certificate authority whose certificate is in
@@ -5027,6 +5034,13 @@ lowest.
 Show currently available hardware-based crypto acceleration
 engines supported by the OpenSSL library.
 .\"*********************************************************
+.TP
+.B \-\-show-curves
+(Standalone)
+Show all available elliptic curves to use with the
+.B \-\-ecdh-curve
+option.
+.\"*********************************************************
 .SS Generate a random key:
 Used only for non-TLS static key encryption mode.
 .\"*********************************************************

src/openvpn/init.c

@@ -871,7 +871,7 @@ print_openssl_info (const struct options *options)
 #ifdef ENABLE_CRYPTO
   if (options->show_ciphers || options->show_digests || options->show_engines
 #ifdef ENABLE_SSL
-      || options->show_tls_ciphers
+      || options->show_tls_ciphers || options->show_curves
 #endif
     )
     {
@@ -884,6 +884,8 @@ print_openssl_info (const struct options *options)
 #ifdef ENABLE_SSL
       if (options->show_tls_ciphers)
 	show_available_tls_ciphers (options->cipher_list);
+      if (options->show_curves)
+	show_available_curves();
 #endif
       return true;
     }

src/openvpn/options.c

@@ -854,6 +854,7 @@ init_options (struct options *o, const bool init_gc)
   o->renegotiate_seconds = 3600;
   o->handshake_window = 60;
   o->transition_window = 3600;
+  o->ecdh_curve = NULL;
 #ifdef ENABLE_X509ALTUSERNAME
   o->x509_username_field = X509_USERNAME_FIELD_DEFAULT;
 #endif
@@ -6516,6 +6517,16 @@ add_option (struct options *options,
       VERIFY_PERMISSION (OPT_P_GENERAL);
       options->show_tls_ciphers = true;
     }
+  else if (streq (p[0], "show-curves"))
+    {
+      VERIFY_PERMISSION (OPT_P_GENERAL);
+      options->show_curves = true;
+    }
+  else if (streq (p[0], "ecdh-curve") && p[1])
+    {
+      VERIFY_PERMISSION (OPT_P_CRYPTO);
+      options->ecdh_curve= p[1];
+    }
   else if (streq (p[0], "tls-server"))
     {
       VERIFY_PERMISSION (OPT_P_GENERAL);

src/openvpn/options.h

@@ -201,6 +201,7 @@ struct options
   bool show_engines;
 #ifdef ENABLE_SSL
   bool show_tls_ciphers;
+  bool show_curves;
 #endif
   bool genkey;
 #endif
@@ -515,6 +516,7 @@ struct options
   const char *priv_key_file;
   const char *pkcs12_file;
   const char *cipher_list;
+  const char *ecdh_curve;
   const char *tls_verify;
   int verify_x509_type;
   const char *verify_x509_name;

src/openvpn/ssl.c

@@ -555,6 +555,10 @@ init_ssl (const struct options *options, struct tls_root_ctx *new_ctx)
       tls_ctx_load_extra_certs(new_ctx, options->extra_certs_file, options->extra_certs_file_inline);
     }
 
+  /* Once keys and cert are loaded, load ECDH parameters */
+  if (options->tls_server)
+    tls_ctx_load_ecdh_params(new_ctx, options->ecdh_curve);
+
   /* Allowable ciphers */
   tls_ctx_restrict_ciphers(new_ctx, options->cipher_list);
 

src/openvpn/ssl_backend.h

@@ -185,6 +185,16 @@ void tls_ctx_restrict_ciphers(struct tls_root_ctx *ctx, const char *ciphers);
 void tls_ctx_load_dh_params(struct tls_root_ctx *ctx, const char *dh_file,
     const char *dh_file_inline);
 
+/**
+ * Load Elliptic Curve Parameters, and load them into the library-specific
+ * TLS context.
+ *
+ * @param ctx          TLS context to use
+ * @param curve_name   The name of the elliptic curve to load.
+ */
+void tls_ctx_load_ecdh_params(struct tls_root_ctx *ctx, const char *curve_name
+    );
+
 /**
  * Load PKCS #12 file for key, cert and (optionally) CA certs, and add to
  * library-specific TLS context.
@@ -460,6 +470,11 @@ void print_details (struct key_state_ssl * ks_ssl, const char *prefix);
  */
 void show_available_tls_ciphers (const char *tls_ciphers);
 
+/*
+ * Show the available elliptic curves in the crypto library
+ */
+void show_available_curves (void);
+
 /*
  * The OpenSSL library has a notion of preference in TLS ciphers.  Higher
  * preference == more secure. Return the highest preference cipher.

src/openvpn/ssl_openssl.c

@@ -56,6 +56,9 @@
 #include <openssl/pkcs12.h>
 #include <openssl/x509.h>
 #include <openssl/crypto.h>
+#ifndef OPENSSL_NO_EC
+#include <openssl/ec.h>
+#endif
 
 /*
  * Allocate space in SSL objects in which to store a struct tls_session
@@ -329,6 +332,78 @@ tls_ctx_load_dh_params (struct tls_root_ctx *ctx, const char *dh_file,
   DH_free (dh);
 }
 
+void
+tls_ctx_load_ecdh_params (struct tls_root_ctx *ctx, const char *curve_name
+    )
+{
+#ifndef OPENSSL_NO_EC
+  int nid = NID_undef;
+  EC_KEY *ecdh = NULL;
+  const char *sname = NULL;
+
+  /* Generate a new ECDH key for each SSL session (for non-ephemeral ECDH) */
+  SSL_CTX_set_options(ctx->ctx, SSL_OP_SINGLE_ECDH_USE);
+#if OPENSSL_VERSION_NUMBER >= 0x10002000L
+  /* OpenSSL 1.0.2 and newer can automatically handle ECDH parameter loading */
+  if (NULL == curve_name) {
+    SSL_CTX_set_ecdh_auto(ctx->ctx, 1);
+    return;
+  }
+#endif
+  /* For older OpenSSL, we'll have to do the parameter loading on our own */
+  if (curve_name != NULL)
+    {
+      /* Use user supplied curve if given */
+      msg (D_TLS_DEBUG, "Using user specified ECDH curve (%s)", curve_name);
+      nid = OBJ_sn2nid(curve_name);
+    }
+  else
+    {
+      /* Extract curve from key */
+      EC_KEY *eckey = NULL;
+      const EC_GROUP *ecgrp = NULL;
+      EVP_PKEY *pkey = NULL;
+
+      /* Little hack to get private key ref from SSL_CTX, yay OpenSSL... */
+      SSL ssl;
+      ssl.cert = ctx->ctx->cert;
+      pkey = SSL_get_privatekey(&ssl);
+
+      msg (D_TLS_DEBUG, "Extracting ECDH curve from private key");
+
+      if (pkey != NULL && (eckey = EVP_PKEY_get1_EC_KEY(pkey)) != NULL &&
+          (ecgrp = EC_KEY_get0_group(eckey)) != NULL)
+        nid = EC_GROUP_get_curve_name(ecgrp);
+    }
+
+  /* Translate NID back to name , just for kicks */
+  sname = OBJ_nid2sn(nid);
+  if (sname == NULL) sname = "(Unknown)";
+
+  /* Create new EC key and set as ECDH key */
+  if (NID_undef == nid || NULL == (ecdh = EC_KEY_new_by_curve_name(nid)))
+    {
+      /* Creating key failed, fall back on sane default */
+      ecdh = EC_KEY_new_by_curve_name(NID_secp384r1);
+      const char *source = (NULL == curve_name) ?
+          "extract curve from certificate" : "use supplied curve";
+      msg (D_TLS_DEBUG_LOW,
+          "Failed to %s (%s), using secp384r1 instead.", source, sname);
+      sname = OBJ_nid2sn(NID_secp384r1);
+    }
+
+  if (!SSL_CTX_set_tmp_ecdh(ctx->ctx, ecdh))
+    msg (M_SSLERR, "SSL_CTX_set_tmp_ecdh: cannot add curve");
+
+  msg (D_TLS_DEBUG_LOW, "ECDH curve %s added", sname);
+
+  EC_KEY_free(ecdh);
+#else
+  msg (M_DEBUG, "Your OpenSSL library was built without elliptic curve support."
+		" Skipping ECDH parameter loading.");
+#endif /* OPENSSL_NO_EC */
+}
+
 int
 tls_ctx_load_pkcs12(struct tls_root_ctx *ctx, const char *pkcs12_file,
     const char *pkcs12_file_inline,
@@ -1299,6 +1374,50 @@ show_available_tls_ciphers (const char *cipher_list)
   SSL_CTX_free (tls_ctx.ctx);
 }
 
+/*
+ * Show the Elliptic curves that are available for us to use
+ * in the OpenSSL library.
+ */
+void
+show_available_curves()
+{
+#ifndef OPENSSL_NO_EC
+  EC_builtin_curve *curves = NULL;
+  size_t crv_len = 0;
+  size_t n = 0;
+
+  crv_len = EC_get_builtin_curves(NULL, 0);
+
+  curves = OPENSSL_malloc((int)(sizeof(EC_builtin_curve) * crv_len));
+
+  if (curves == NULL)
+    msg (M_SSLERR, "Cannot create EC_builtin_curve object");
+  else
+  {
+    if (EC_get_builtin_curves(curves, crv_len))
+    {
+      printf ("Available Elliptic curves:\n");
+      for (n = 0; n < crv_len; n++)
+      {
+        const char *sname;
+        sname   = OBJ_nid2sn(curves[n].nid);
+        if (sname == NULL) sname = "";
+
+        printf("%s\n", sname);
+      }
+    }
+    else
+    {
+      msg (M_SSLERR, "Cannot get list of builtin curves");
+    }
+    OPENSSL_free(curves);
+  }
+#else
+  msg (M_WARN, "Your OpenSSL library was built without elliptic curve support. "
+	       "No curves available.");
+#endif
+}
+
 void
 get_highest_preference_tls_cipher (char *buf, int size)
 {

src/openvpn/ssl_polarssl.c

@@ -228,6 +228,15 @@ else
       (counter_type) 8 * mpi_size(&ctx->dhm_ctx->P));
 }
 
+void
+tls_ctx_load_ecdh_params (struct tls_root_ctx *ctx, const char *curve_name
+    )
+{
+    if (NULL != curve_name)
+      msg(M_WARN, "WARNING: PolarSSL builds do not support specifying an ECDH "
+                  "curve, using default curves.");
+}
+
 int
 tls_ctx_load_pkcs12(struct tls_root_ctx *ctx, const char *pkcs12_file,
     const char *pkcs12_file_inline,
@@ -1083,6 +1092,23 @@ show_available_tls_ciphers (const char *cipher_list)
   tls_ctx_free(&tls_ctx);
 }
 
+void
+show_available_curves (void)
+{
+  const ecp_curve_info *pcurve = ecp_curve_list();
+
+  if (NULL == pcurve)
+    msg (M_FATAL, "Cannot retrieve curve list from PolarSSL");
+
+  /* Print curve list */
+  printf ("Available Elliptic curves, listed in order of preference:\n\n");
+  while (POLARSSL_ECP_DP_NONE != pcurve->grp_id)
+    {
+      printf("%s\n", pcurve->name);
+      pcurve++;
+    }
+}
+
 void
 get_highest_preference_tls_cipher (char *buf, int size)
 {