ssl: add SSLContext#tmp_dh=

Provide a wrapper of SSL_set0_tmp_dh_pkey()/SSL_CTX_set_tmp_dh(), which
sets the DH parameters used for ephemeral DH key exchange.

SSLContext#tmp_dh_callback= already exists for this purpose, as a
wrapper around SSL_CTX_set_tmp_dh_callback(), but it is considered
obsolete and the OpenSSL API is deprecated for future removal. There is
no practical use case where an application needs to use different DH
parameters nowadays. This was originally introduced to support export
grade ciphers.

RDoc for #tmp_dh_callback= is updated to recommend the new #tmp_dh=.

Note that current versions of OpenSSL support automatic ECDHE curve
selection which is enabled by default. SSLContext#tmp_dh= should only be
necessary if you must allow ancient clients which don't support ECDHE.

Author: rhenium

ext/openssl/extconf.rb

@@ -170,6 +170,9 @@ def find_openssl_library
 # added in 1.1.1
 have_func("EVP_PKEY_check")
 
+# added in 3.0.0
+have_func("SSL_set0_tmp_dh_pkey")
+
 Logging::message "=== Checking done. ===\n"
 
 create_header

ext/openssl/ossl_ssl.c

@@ -987,6 +987,52 @@ ossl_sslctx_set_ciphers(VALUE self, VALUE v)
     return v;
 }
 
+#ifndef OPENSSL_NO_DH
+/*
+ * call-seq:
+ *    ctx.tmp_dh = pkey
+ *
+ * Sets DH parameters used for ephemeral DH key exchange. This is relevant for
+ * servers only.
+ *
+ * +pkey+ is an instance of OpenSSL::PKey::DH. Note that key components
+ * contained in the key object, if any, are ignored. The server will always
+ * generate a new key pair for each handshake.
+ *
+ * Added in version 3.0. See also the man page SSL_set0_tmp_dh_pkey(3).
+ *
+ * Example:
+ *   ctx = OpenSSL::SSL::SSLContext.new
+ *   ctx.tmp_dh = OpenSSL::DH.generate(2048)
+ *   svr = OpenSSL::SSL::SSLServer.new(tcp_svr, ctx)
+ *   Thread.new { svr.accept }
+ */
+static VALUE
+ossl_sslctx_set_tmp_dh(VALUE self, VALUE arg)
+{
+    SSL_CTX *ctx;
+    EVP_PKEY *pkey;
+
+    rb_check_frozen(self);
+    GetSSLCTX(self, ctx);
+    pkey = GetPKeyPtr(arg);
+
+    if (EVP_PKEY_base_id(pkey) != EVP_PKEY_DH)
+        rb_raise(eSSLError, "invalid pkey type %s (expected DH)",
+                 OBJ_nid2sn(EVP_PKEY_base_id(pkey)));
+#ifdef HAVE_SSL_SET0_TMP_DH_PKEY
+    if (!SSL_CTX_set0_tmp_dh_pkey(ctx, pkey))
+        ossl_raise(eSSLError, "SSL_CTX_set0_tmp_dh_pkey");
+    EVP_PKEY_up_ref(pkey);
+#else
+    if (!SSL_CTX_set_tmp_dh(ctx, EVP_PKEY_get0_DH(pkey)))
+        ossl_raise(eSSLError, "SSL_CTX_set_tmp_dh");
+#endif
+
+    return arg;
+}
+#endif
+
 #if !defined(OPENSSL_NO_EC)
 /*
  * call-seq:
@@ -2670,6 +2716,9 @@ Init_ossl_ssl(void)
 			     ossl_sslctx_set_minmax_proto_version, 2);
     rb_define_method(cSSLContext, "ciphers",     ossl_sslctx_get_ciphers, 0);
     rb_define_method(cSSLContext, "ciphers=",    ossl_sslctx_set_ciphers, 1);
+#ifndef OPENSSL_NO_DH
+    rb_define_method(cSSLContext, "tmp_dh=", ossl_sslctx_set_tmp_dh, 1);
+#endif
     rb_define_method(cSSLContext, "ecdh_curves=", ossl_sslctx_set_ecdh_curves, 1);
     rb_define_method(cSSLContext, "security_level", ossl_sslctx_get_security_level, 0);
     rb_define_method(cSSLContext, "security_level=", ossl_sslctx_set_security_level, 1);

lib/openssl/ssl.rb

@@ -91,15 +91,17 @@ class SSLContext
       DEFAULT_CERT_STORE.set_default_paths
       DEFAULT_CERT_STORE.flags = OpenSSL::X509::V_FLAG_CRL_CHECK_ALL
 
-      # A callback invoked when DH parameters are required.
+      # A callback invoked when DH parameters are required for ephemeral DH key
+      # exchange.
       #
-      # The callback is invoked with the Session for the key exchange, an
+      # The callback is invoked with the SSLSocket, a
       # flag indicating the use of an export cipher and the keylength
       # required.
       #
       # The callback must return an OpenSSL::PKey::DH instance of the correct
       # key length.
-
+      #
+      # <b>Deprecated in version 3.0.</b> Use #tmp_dh= instead.
       attr_accessor :tmp_dh_callback
 
       # A callback invoked at connect time to distinguish between multiple

test/openssl/test_ssl.rb

@@ -1583,13 +1583,11 @@ def test_fallback_scsv
     end
   end
 
-  def test_dh_callback
-    pend "TLS 1.2 is not supported" unless tls12_supported?
-
+  def test_tmp_dh_callback
     dh = Fixtures.pkey("dh-1")
     called = false
     ctx_proc = -> ctx {
-      ctx.ssl_version = :TLSv1_2
+      ctx.max_version = :TLS1_2
       ctx.ciphers = "DH:!NULL"
       ctx.tmp_dh_callback = ->(*args) {
         called = true
@@ -1605,10 +1603,8 @@ def test_dh_callback
   end
 
   def test_connect_works_when_setting_dh_callback_to_nil
-    pend "TLS 1.2 is not supported" unless tls12_supported?
-
     ctx_proc = -> ctx {
-      ctx.ssl_version = :TLSv1_2
+      ctx.max_version = :TLS1_2
       ctx.ciphers = "DH:!NULL" # use DH
       ctx.tmp_dh_callback = nil
     }
@@ -1621,6 +1617,20 @@ def test_connect_works_when_setting_dh_callback_to_nil
     end
   end
 
+  def test_tmp_dh
+    dh = Fixtures.pkey("dh-1")
+    ctx_proc = -> ctx {
+      ctx.max_version = :TLS1_2
+      ctx.ciphers = "DH:!NULL" # use DH
+      ctx.tmp_dh = dh
+    }
+    start_server(ctx_proc: ctx_proc) do |port|
+      server_connect(port) { |ssl|
+        assert_equal dh.to_der, ssl.tmp_key.to_der
+      }
+    end
+  end
+
   def test_ecdh_curves_tls12
     pend "EC is disabled" unless defined?(OpenSSL::PKey::EC)