# Taken from:
# http://bugs.exim.org/show_bug.cgi?id=1397
# http://bugs.exim.org/attachment.cgi?id=661

Index: exim-4.83/src/globals.c
===================================================================
--- exim-4.83.orig/src/globals.c
+++ exim-4.83/src/globals.c
@@ -158,6 +158,7 @@ that's the interop problem which has bee
 bit-count as "NORMAL" (2432) and Thunderbird dropping connection. */
 int     tls_dh_max_bits        = 2236;
 uschar *tls_dhparam            = NULL;
+uschar *tls_eccurve            = NULL;
 #ifndef DISABLE_OCSP
 uschar *tls_ocsp_file          = NULL;
 #endif
Index: exim-4.83/src/globals.h
===================================================================
--- exim-4.83.orig/src/globals.h
+++ exim-4.83/src/globals.h
@@ -118,6 +118,7 @@ extern uschar *tls_channelbinding_b64; /
 extern uschar *tls_crl;                /* CRL File */
 extern int     tls_dh_max_bits;        /* don't accept higher lib suggestions */
 extern uschar *tls_dhparam;            /* DH param file */
+extern uschar *tls_eccurve;            /* EC curve */
 #ifndef DISABLE_OCSP
 extern uschar *tls_ocsp_file;          /* OCSP stapling proof file */
 #endif
Index: exim-4.83/src/readconf.c
===================================================================
--- exim-4.83.orig/src/readconf.c
+++ exim-4.83/src/readconf.c
@@ -443,6 +443,7 @@ static optionlist optionlist_config[] =
   { "tls_crl",                  opt_stringptr,   &tls_crl },
   { "tls_dh_max_bits",          opt_int,         &tls_dh_max_bits },
   { "tls_dhparam",              opt_stringptr,   &tls_dhparam },
+  { "tls_eccurve",              opt_stringptr,   &tls_eccurve },
 # ifndef DISABLE_OCSP
   { "tls_ocsp_file",            opt_stringptr,   &tls_ocsp_file },
 # endif
Index: exim-4.83/src/tls-openssl.c
===================================================================
--- exim-4.83.orig/src/tls-openssl.c
+++ exim-4.83/src/tls-openssl.c
@@ -612,6 +612,59 @@ return TRUE;
 
 
 
+#if !defined(OPENSSL_NO_ECDH)
+static BOOL
+init_ecdh(SSL_CTX *sctx, host_item *host)
+{
+EC_KEY *ecdh;
+int nid;
+
+# if !defined(OPENSSL_NO_ECDH) && OPENSSL_VERSION_NUMBER >= 0x10002000L
+/* check if OpenSSL >= 1.0.2 auto ECDH temp key parameter selection should be used */
+if (Ustrcmp(tls_eccurve, "auto") == 0)
+  {
+  DEBUG(D_tls) debug_printf("ECDH temp key parameter settings: OpenSSL 1.2+ autoselection\n");
+  SSL_CTX_set_ecdh_auto(sctx, 1);
+  return TRUE;
+  }
+# endif
+
+if (tls_eccurve == NULL)
+  {
+  DEBUG(D_tls)
+    debug_printf("ECDH curve (default): prime256v1\n", tls_eccurve);
+  nid = NID_X9_62_prime256v1;
+  }
+else
+  {
+  /* search curve name */
+  DEBUG(D_tls)
+    debug_printf("ECDH curve: %s\n", tls_eccurve);
+  nid = OBJ_sn2nid((uschar *)tls_eccurve);
+  if (nid == 0)
+    {
+    tls_error(string_sprintf("Unkown curve name tls_eccurve \"%s\"", tls_eccurve),
+      host, NULL);
+    return FALSE;
+    }
+  }
+
+ecdh = EC_KEY_new_by_curve_name(nid);
+if (ecdh == NULL)
+  {
+  tls_error("Unable to create ec curve",
+    host, NULL);
+  return FALSE;
+  }
+
+SSL_CTX_set_tmp_ecdh(sctx, ecdh);
+EC_KEY_free(ecdh);
+
+return TRUE;
+}
+#endif
+
+
 
 #ifndef DISABLE_OCSP
 /*************************************************
@@ -1254,6 +1307,11 @@ if (!init_dh(*ctxp, dhparam, host)) retu
 rc = tls_expand_session_files(*ctxp, cbinfo);
 if (rc != OK) return rc;
 
+#if !defined(OPENSSL_NO_ECDH)
+/* Initialize ECDH temp key parameter selection */
+if (!init_ecdh(*ctxp, host)) return DEFER;
+#endif
+
 /* If we need to handle SNI, do so */
 #ifdef EXIM_HAVE_OPENSSL_TLSEXT
 if (host == NULL)		/* server */