Tell openssl to build its TLS contexts with security level 1

Fixes bug 27344, where we'd break compatibility with old tors by
rejecting RSA1024 and DH1024.

changes/bug27344

+  o Minor features (compatibility):
+    - Tell OpenSSL to maintain backward compatibility with previous
+      RSA1024/DH1024 users in Tor. With OpenSSL 1.1.1-pre6, these ciphers
+      are disabled by default. Closes ticket 27344.

configure.ac

@@ -678,6 +678,7 @@ AC_CHECK_FUNCS([ \
                 SSL_get_client_ciphers \
                 SSL_get_client_random \
 		SSL_CIPHER_find \
+                SSL_CTX_set_security_level \
 		TLS_method
 	       ])
 

src/common/tortls.c

@@ -1130,6 +1130,11 @@ tor_tls_context_new(crypto_pk_t *identity, unsigned int key_lifetime,
   if (!(result->ctx = SSL_CTX_new(SSLv23_method())))
     goto error;
 #endif
+#ifdef HAVE_SSL_CTX_SET_SECURITY_LEVEL
+  /* Level 1 re-enables RSA1024 and DH1024 for compatibility with old tors */
+  SSL_CTX_set_security_level(result->ctx, 1);
+#endif
+
   SSL_CTX_set_options(result->ctx, SSL_OP_NO_SSLv2);
   SSL_CTX_set_options(result->ctx, SSL_OP_NO_SSLv3);
 
@@ -2555,4 +2560,3 @@ evaluate_ecgroup_for_tls(const char *ecgroup)
 
   return ret;
 }
-