Add "OfflineMasterKey" option

When this is set, and Tor is running as a relay, it will not
generate or load its secret identity key.  You can manage the secret
identity key with --keygen.  Implements ticket 16944.

changes/feature16944

@@ -0,0 +1,5 @@
+  o Minor features (relay, ed25519):
+    - Add a new OfflineMasterKey option to tell Tor never to try
+      loading or generating a secret ed25519 identity key. You can use
+      this in combination with tor --keygen to manage offline and/or
+      encrypted ed25519 keys. Implements ticket 16944.

doc/tor.1.txt

@@ -1877,6 +1877,11 @@ is non-zero):
     configures their lifetime.
     (Default: 30 days)
 
+[[OfflineMasterKey]] **OfflineMasterKey** **0**|**1**::
+    If non-zero, the Tor relay will never generate or load its master secret
+    key.  Instead, you'll have to use "tor --keygen" to manage the master
+    secret key. (Default: 0)
+
 DIRECTORY SERVER OPTIONS
 ------------------------
 

src/or/config.c

@@ -333,6 +333,7 @@ static config_var_t option_vars_[] = {
   V(NumCPUs,                     UINT,     "0"),
   V(NumDirectoryGuards,          UINT,     "0"),
   V(NumEntryGuards,              UINT,     "0"),
+  V(OfflineMasterKey,            BOOL,     "0"),
   V(ORListenAddress,             LINELIST, NULL),
   VPORT(ORPort,                      LINELIST, NULL),
   V(OutboundBindAddress,         LINELIST,   NULL),

src/or/or.h

@@ -4295,6 +4295,10 @@ typedef struct {
   /** How long before auth keys expire will we try to make a new one? */
   int TestingAuthKeySlop;
 
+  /** Force use of offline master key features: never generate a master
+   * ed25519 identity key except from tor --keygen */
+  int OfflineMasterKey;
+
   enum {
     FORCE_PASSPHRASE_AUTO=0,
     FORCE_PASSPHRASE_ON,
@@ -4302,6 +4306,7 @@ typedef struct {
   } keygen_force_passphrase;
   int use_keygen_passphrase_fd;
   int keygen_passphrase_fd;
+
 } or_options_t;
 
 /** Persistent state for an onion router, as saved to disk. */

src/or/routerkeys.c

@@ -668,6 +668,9 @@ load_ed_keys(const or_options_t *options, time_t now)
     use_signing = sign;
   }
 
+  const int offline_master =
+    options->OfflineMasterKey && options->command != CMD_KEYGEN;
+
   const int need_new_signing_key =
     NULL == use_signing ||
     EXPIRES_SOON(check_signing_cert, 0) ||
@@ -676,29 +679,43 @@ load_ed_keys(const or_options_t *options, time_t now)
     need_new_signing_key ||
     EXPIRES_SOON(check_signing_cert, options->TestingSigningKeySlop);
 
+  /* We can only create a master key if we haven't been told that the
+   * master key will always be offline.  Also, if we have a signing key,
+   * then we shouldn't make a new master ID key. */
+  const int can_make_master_id_key = !offline_master &&
+    NULL == use_signing;
+
   if (need_new_signing_key) {
     log_notice(LD_OR, "It looks like I need to generate and sign a new "
                "medium-term signing key, because %s. To do that, I need to "
-               "load (or create) the permanent master identity key.",
+               "load%s the permanent master identity key.",
             (NULL == use_signing) ? "I don't have one" :
             EXPIRES_SOON(check_signing_cert, 0) ? "the one I have is expired" :
-            "you asked me to make one with --keygen");
-  } else if (want_new_signing_key) {
+               "you asked me to make one with --keygen",
+            can_make_master_id_key ? " (or create)" : "");
+  } else if (want_new_signing_key && !offline_master) {
     log_notice(LD_OR, "It looks like I should try to generate and sign a "
                "new medium-term signing key, because the one I have is "
                "going to expire soon. To do that, I'm going to have to try to "
                "load the permanent master identity key.");
+  } else if (want_new_signing_key) {
+    log_notice(LD_OR, "It looks like I should try to generate and sign a "
+               "new medium-term signing key, because the one I have is "
+               "going to expire soon. But OfflineMasterKey is set, so I "
+               "won't try to load a permanent master identity key is set. "
+               "You will need to use 'tor --keygen' make a new signing key "
+               "and certificate.");
   }
 
   {
     uint32_t flags =
       (INIT_ED_KEY_SPLIT|
        INIT_ED_KEY_EXTRA_STRONG|INIT_ED_KEY_NO_REPAIR);
-    if (! use_signing)
+    if (can_make_master_id_key)
       flags |= INIT_ED_KEY_CREATE;
     if (! need_new_signing_key)
       flags |= INIT_ED_KEY_MISSING_SECRET_OK;
-    if (! want_new_signing_key)
+    if (! want_new_signing_key || offline_master)
       flags |= INIT_ED_KEY_OMIT_SECRET;
     if (options->command == CMD_KEYGEN)
       flags |= INIT_ED_KEY_TRY_ENCRYPTED;
@@ -724,6 +741,9 @@ load_ed_keys(const or_options_t *options, time_t now)
     tor_free(fname);
     if (!id) {
       if (need_new_signing_key) {
+        if (offline_master)
+          FAIL("Can't load master identity key; OfflineMasterKey is set.");
+        else
           FAIL("Missing identity key");
       } else {
         log_warn(LD_OR, "Master public key was absent; inferring from "