Pass through nil as digest when signing certificates

In order to sign certificates with Ed25519 keys, NULL must be passed
as md to X509_sign.  This NULL is then passed
(via ASN1_item_sign_ex) as type to EVP_DigestSignInit.  The
documentation[1] of EVP_DigestSignInit states that type must be NULL
for various key types, including Ed25519.

[1]: https://www.openssl.org/docs/manmaster/man3/EVP_DigestSignInit.html

ext/openssl/ossl_x509cert.c

@@ -539,7 +539,11 @@ ossl_x509_sign(VALUE self, VALUE key, VALUE digest)
     const EVP_MD *md;
 
     pkey = GetPrivPKeyPtr(key); /* NO NEED TO DUP */
-    md = ossl_evp_get_digestbyname(digest);
+    if (NIL_P(digest)) {
+      md = NULL; /* needed for some key types, e.g. Ed25519 */
+    } else {
+      md = ossl_evp_get_digestbyname(digest);
+    }
     GetX509(self, x509);
     if (!X509_sign(x509, pkey, md)) {
 	ossl_raise(eX509CertError, NULL);

test/openssl/fixtures/pkey/ed25519.pem

@@ -0,0 +1,3 @@
+-----BEGIN PRIVATE KEY-----
+MC4CAQAwBQYDK2VwBCIEIFfR9+7eHflbSCNLLWz50cKZG9J/dzNTyYCggFG1MKG9
+-----END PRIVATE KEY-----

test/openssl/test_x509cert.rb

@@ -10,6 +10,7 @@ def setup
     @rsa2048 = Fixtures.pkey("rsa2048")
     @dsa256  = Fixtures.pkey("dsa256")
     @dsa512  = Fixtures.pkey("dsa512")
+    @ed25519  = Fixtures.pkey("ed25519")
     @ca = OpenSSL::X509::Name.parse("/DC=org/DC=ruby-lang/CN=CA")
     @ee1 = OpenSSL::X509::Name.parse("/DC=org/DC=ruby-lang/CN=EE1")
   end
@@ -222,6 +223,11 @@ def test_sign_and_verify_dsa_md5
     }
   end
 
+  def test_sign_and_verify_ed25519
+    cert = issue_cert(@ca, @ed25519, 1, [], nil, nil, digest: nil)
+    assert_equal(true, cert.verify(@ed25519))
+  end
+
   def test_dsa_with_sha2
     cert = issue_cert(@ca, @dsa256, 1, [], nil, nil, digest: "sha256")
     assert_equal("dsa_with_SHA256", cert.signature_algorithm)