gpg: New import and keyserver option "self-sigs-only"

* g10/options.h (IMPORT_SELF_SIGS_ONLY): New.
* g10/import.c (parse_import_options): Add option "self-sigs-only".
(read_block): Handle that option.
--

This option is intended to help against importing keys with many bogus
key-signatures.  It has obvious drawbacks and is not a bullet-proof
solution because a self-signature can also be faked and would be
detected only later.

GnuPG-bug-id: 4591
Signed-off-by: Werner Koch <wk@gnupg.org>

(cherry picked from commit 15a425a1dfe60bd976b17671aa8e3d9aed12e1c0)

doc/gpg.texi

@@ -2327,6 +2327,14 @@ opposite meaning. The options are:
   on the keyring. This option is the same as running the @option{--edit-key}
   command "clean" after import. Defaults to no.
 
+  @item self-sigs-only
+  Accept only self-signatures while importing a key.  All other
+  key-signatures are skipped at an early import stage.  This option
+  can be used with @code{keyserver-options} to mitigate attempts to
+  flood a key with bogus signatures from a keyserver.  The drawback is
+  that all other valid key-signatures, as required by the Web of Trust
+  are also not imported.
+
   @item repair-keys
   After import, fix various problems with the
   keys.  For example, this reorders signatures, and strips duplicate

g10/import.c

@@ -188,6 +188,9 @@ parse_import_options(char *str,unsigned int *options,int noisy)
       {"import-minimal",IMPORT_MINIMAL|IMPORT_CLEAN,NULL,
        N_("remove as much as possible from key after import")},
 
+      {"self-sigs-only", IMPORT_SELF_SIGS_ONLY, NULL,
+       N_("ignore key-signatures which are not self-signatures")},
+
       {"import-export", IMPORT_EXPORT, NULL,
        N_("run import filters and export key immediately")},
 
@@ -850,6 +853,8 @@ read_block( IOBUF a, unsigned int options,
   PACKET *pkt;
   kbnode_t root = NULL;
   int in_cert, in_v3key, skip_sigs;
+  u32 keyid[2];
+  unsigned int dropped_nonselfsigs = 0;
 
   *r_v3keys = 0;
 
@@ -974,6 +979,31 @@ read_block( IOBUF a, unsigned int options,
           init_packet(pkt);
           break;
 
+        case PKT_SIGNATURE:
+          if (!in_cert)
+            goto x_default;
+          if (!(options & IMPORT_SELF_SIGS_ONLY))
+            goto x_default;
+	  if (pkt->pkt.signature->keyid[0] == keyid[0]
+              && pkt->pkt.signature->keyid[1] == keyid[1])
+	    { /* This is likely a self-signature.  We import this one.
+               * Eventually we should use the ISSUER_FPR to compare
+               * self-signatures, but that will work only for v5 keys
+               * which are currently not even deployed.
+               * Note that we do not do any crypto verify here because
+               * that would defeat this very mitigation of DoS by
+               * importing a key with a huge amount of faked
+               * key-signatures.  A verification will be done later in
+               * the processing anyway.  Here we want a cheap an early
+               * way to drop non-self-signatures.  */
+              goto x_default;
+            }
+          /* Skip this signature.  */
+          dropped_nonselfsigs++;
+          free_packet (pkt, &parsectx);
+          init_packet(pkt);
+          break;
+
         case PKT_PUBLIC_KEY:
         case PKT_SECRET_KEY:
           if (in_cert) /* Store this packet.  */
@@ -983,7 +1013,9 @@ read_block( IOBUF a, unsigned int options,
               goto ready;
             }
           in_cert = 1;
-          /* fall through */
+          keyid_from_pk (pkt->pkt.public_key, keyid);
+          goto x_default;
+
         default:
         x_default:
           if (in_cert && valid_keyblock_packet (pkt->pkttype))
@@ -1012,6 +1044,10 @@ read_block( IOBUF a, unsigned int options,
   free_packet (pkt, &parsectx);
   deinit_parse_packet (&parsectx);
   xfree( pkt );
+  if (!rc && dropped_nonselfsigs && opt.verbose)
+    log_info ("key %s: number of dropped non-self-signatures: %u\n",
+              keystr (keyid), dropped_nonselfsigs);
+
   return rc;
 }
 

g10/options.h

@@ -356,6 +356,7 @@ EXTERN_UNLESS_MAIN_MODULE int memory_stat_debug_mode;
 #define IMPORT_RESTORE                   (1<<10)
 #define IMPORT_REPAIR_KEYS               (1<<11)
 #define IMPORT_DRY_RUN                   (1<<12)
+#define IMPORT_SELF_SIGS_ONLY            (1<<14)
 
 #define EXPORT_LOCAL_SIGS                (1<<0)
 #define EXPORT_ATTRIBUTES                (1<<1)