Drop support for AUTHTYPE_RSA_SHA256_RFC5705 authentication.

This was a stopgap method, designed on the theory that some routers
might support it before they could support Ed25519.  But it looks
like everybody who supports RFC5705 will also have an Ed25519 key,
so there's not a lot of reason to have this even supported.

src/or/connection_or.c

@@ -2278,9 +2278,9 @@ authchallenge_type_is_supported(uint16_t challenge_type)
 {
   switch (challenge_type) {
      case AUTHTYPE_RSA_SHA256_TLSSECRET:
-     case AUTHTYPE_RSA_SHA256_RFC5705:
      case AUTHTYPE_ED25519_SHA256_RFC5705:
        return 1;
+     case AUTHTYPE_RSA_SHA256_RFC5705:
      default:
        return 0;
   }
@@ -2321,7 +2321,9 @@ connection_or_send_auth_challenge_cell(or_connection_t *conn)
   crypto_rand((char*)ac->challenge, sizeof(ac->challenge));
 
   auth_challenge_cell_add_methods(ac, AUTHTYPE_RSA_SHA256_TLSSECRET);
-  auth_challenge_cell_add_methods(ac, AUTHTYPE_RSA_SHA256_RFC5705);
+  /* Disabled, because everything that supports this method also supports
+   * the much-superior ED25519_SHA256_RFC5705 */
+  /* auth_challenge_cell_add_methods(ac, AUTHTYPE_RSA_SHA256_RFC5705); */
   auth_challenge_cell_add_methods(ac, AUTHTYPE_ED25519_SHA256_RFC5705);
   auth_challenge_cell_set_n_methods(ac,
                                     auth_challenge_cell_getlen_methods(ac));

src/test/test_link_handshake.c

@@ -890,15 +890,15 @@ test_link_handshake_send_authchallenge(void *arg)
   cell1 = mock_got_var_cell;
   tt_int_op(0, ==, connection_or_send_auth_challenge_cell(c1));
   cell2 = mock_got_var_cell;
-  tt_int_op(40, ==, cell1->payload_len);
-  tt_int_op(40, ==, cell2->payload_len);
+  tt_int_op(38, ==, cell1->payload_len);
+  tt_int_op(38, ==, cell2->payload_len);
   tt_int_op(0, ==, cell1->circ_id);
   tt_int_op(0, ==, cell2->circ_id);
   tt_int_op(CELL_AUTH_CHALLENGE, ==, cell1->command);
   tt_int_op(CELL_AUTH_CHALLENGE, ==, cell2->command);
 
-  tt_mem_op("\x00\x03\x00\x01\x00\x02\x00\x03", ==, cell1->payload + 32, 8);
-  tt_mem_op("\x00\x03\x00\x01\x00\x02\x00\x03", ==, cell2->payload + 32, 8);
+  tt_mem_op("\x00\x02\x00\x01\x00\x03", ==, cell1->payload + 32, 6);
+  tt_mem_op("\x00\x02\x00\x01\x00\x03", ==, cell2->payload + 32, 6);
   tt_mem_op(cell1->payload, !=, cell2->payload, 32);
 
  done: