gpg: New option --override-compliance-check

* g10/gpg.c (oOverrideComplianceCheck): New.
(opts): Add new option.
(main): Set option and add check for batch mode.
* g10/options.h (opt): Add flags.override_compliance_check.

* g10/sig-check.c (check_signature2): Factor complaince checking out
to ...
(check_key_verify_compliance): this.  Turn error into a warning in
override mode.
--

There is one important use case for this: For systems configured
globally to use de-vs mode, Ed25519 and other key types are not
allowed because they are not listred in the BSI algorithm catalog.
Now, our release signing keys happen to be Ed25519 and thus we need to
offer a way for users to check new versions even if the system is in
de-vs mode.  This does on purpose not work in --batch mode so that
scripted solutions won't accidently pass a signature check.

GnuPG-bug-id: 5655

doc/gpg.texi

@@ -3471,6 +3471,15 @@ To avoid a minor risk of collision attacks on third-party key
 signatures made using SHA-1, those key signatures are considered
 invalid.  This options allows to override this restriction.
 
+@item --override-compliance-check
+@opindex --override-compliance-check
+The signature verification only allows the use of keys suitable in the
+current compliance mode.  If the compliance mode has been forced by a
+global option, there might be no way to check certain signature.  This
+option allows to override this and prints an extra warning in such a
+case.  This option is ignored in --batch mode so that no accidental
+unattended verification may happen.
+
 @item --no-default-keyring
 @opindex no-default-keyring
 Do not add the default keyring to the list of keyrings. Note that

g10/gpg.c

@@ -357,6 +357,7 @@ enum cmd_and_opt_values
     oShowSessionKey,
     oOverrideSessionKey,
     oOverrideSessionKeyFD,
+    oOverrideComplianceCheck,
     oNoRandomSeedFile,
     oAutoKeyRetrieve,
     oNoAutoKeyRetrieve,
@@ -840,7 +841,6 @@ static gpgrt_opt_t opts[] = {
   ARGPARSE_s_s (oOverrideSessionKey, "override-session-key", "@"),
   ARGPARSE_s_i (oOverrideSessionKeyFD, "override-session-key-fd", "@"),
 
-
   ARGPARSE_header ("Security", N_("Options controlling the security")),
 
   ARGPARSE_s_i (oS2KMode, "s2k-mode", "@"),
@@ -871,6 +871,7 @@ static gpgrt_opt_t opts[] = {
   ARGPARSE_s_s (oAEADAlgo,   "aead-algo", "@"),
   ARGPARSE_s_s (oDigestAlgo, "digest-algo", "@"),
   ARGPARSE_s_s (oCertDigestAlgo, "cert-digest-algo", "@"),
+  ARGPARSE_s_n (oOverrideComplianceCheck, "override-compliance-check", "@"),
 
 
   ARGPARSE_header (NULL, N_("Options for unattended use")),
@@ -3660,6 +3661,10 @@ main (int argc, char **argv)
             opt.flags.allow_old_cipher_algos = 1;
             break;
 
+          case oOverrideComplianceCheck:
+            opt.flags.override_compliance_check = 1;
+            break;
+
           case oFakedSystemTime:
             {
               size_t len = strlen (pargs.r.ret_str);
@@ -3869,6 +3874,15 @@ main (int argc, char **argv)
 	g10_exit(2);
       }
 
+    /* We allow overriding the compliance check only in non-batch mode
+     * so that the user has a chance to see the message.  */
+    if (opt.flags.override_compliance_check && opt.batch)
+      {
+        opt.flags.override_compliance_check = 0;
+        log_info ("Note: '%s' ignored due to batch mode\n",
+                  "--override-compliance-check");
+      }
+
     set_debug (debug_level);
     if (DBG_CLOCK)
       log_clock ("start");

g10/options.h

@@ -245,6 +245,7 @@ struct
     unsigned int allow_old_cipher_algos:1;
     unsigned int allow_weak_digest_algos:1;
     unsigned int allow_weak_key_signatures:1;
+    unsigned int override_compliance_check:1;
     unsigned int large_rsa:1;
     unsigned int disable_signer_uid:1;
     unsigned int include_key_block:1;

g10/sig-check.c

@@ -67,6 +67,35 @@ sig_check_dump_stats (void)
 }
 
 
+static gpg_error_t
+check_key_verify_compliance (PKT_public_key *pk)
+{
+  gpg_error_t err = 0;
+
+  if (!gnupg_pk_is_allowed (opt.compliance, PK_USE_VERIFICATION,
+                            pk->pubkey_algo, 0, pk->pkey,
+                            nbits_from_pk (pk),
+                            NULL))
+    {
+      /* Compliance failure.  */
+      log_info (_("key %s may not be used for signing in %s mode\n"),
+                 keystr_from_pk (pk),
+                 gnupg_compliance_option_string (opt.compliance));
+      if (opt.flags.override_compliance_check)
+        log_info (_("continuing verification anyway due to option %s\n"),
+                  "--override-compliance-failure");
+      else
+        {
+          log_inc_errorcount (); /* We used log info above.  */
+          err = gpg_error (GPG_ERR_PUBKEY_ALGO);
+        }
+    }
+
+  return err;
+}
+
+
+
 /* Check a signature.  This is shorthand for check_signature2 with
    the unnamed arguments passed as NULL.  */
 int
@@ -172,17 +201,8 @@ check_signature2 (ctrl_t ctrl,
     }
   else if (get_pubkey_for_sig (ctrl, pk, sig, forced_pk))
     rc = gpg_error (GPG_ERR_NO_PUBKEY);
-  else if (!gnupg_pk_is_allowed (opt.compliance, PK_USE_VERIFICATION,
-                                 pk->pubkey_algo, 0, pk->pkey,
-                                 nbits_from_pk (pk),
-                                 NULL))
-    {
-      /* Compliance failure.  */
-      log_error (_("key %s may not be used for signing in %s mode\n"),
-                 keystr_from_pk (pk),
-                 gnupg_compliance_option_string (opt.compliance));
-      rc = gpg_error (GPG_ERR_PUBKEY_ALGO);
-    }
+  else if ((rc = check_key_verify_compliance (pk)))
+    ;/* Compliance failure.  */
   else if (!pk->flags.valid)
     {
       /* You cannot have a good sig from an invalid key.  */