Screen keyserver responses.

* g10/main.h: Typedef import_filter for filter callbacks.
* g10/import.c (import): Add filter callbacks to param list.
(import_one): Ditto.
(import_secret_one): Ditto.
(import_keys_internal): Ditto.
(import_keys_stream): Ditto.
* g10/keyserver.c (keyserver_retrieval_filter): New.
(keyserver_spawn): Pass filter to import_keys_stream()

--
These changes introduces import functions that apply a constraining
filter to imported keys. These filters can verify the fingerprints of
the keys returned before importing them into the keyring, ensuring that
the keys fetched from the keyserver are in fact those selected by the
user beforehand.

Signed-off-by: Stefan Tomanek <tomanek@internet-sicherheit.de>

Re-indention and minor changes by wk.

g10/import.c

@@ -59,14 +59,17 @@ struct stats_s {
 
 
 static int import( IOBUF inp, const char* fname,struct stats_s *stats,
-		   unsigned char **fpr,size_t *fpr_len,unsigned int options );
+		   unsigned char **fpr,size_t *fpr_len,unsigned int options,
+		   import_filter filter, void *filter_arg );
 static int read_block( IOBUF a, PACKET **pending_pkt, KBNODE *ret_root );
 static void revocation_present(KBNODE keyblock);
 static int import_one(const char *fname, KBNODE keyblock,struct stats_s *stats,
 		      unsigned char **fpr,size_t *fpr_len,
-		      unsigned int options,int from_sk);
+		      unsigned int options,int from_sk,
+		      import_filter filter, void *filter_arg);
 static int import_secret_one( const char *fname, KBNODE keyblock,
-                              struct stats_s *stats, unsigned int options);
+                              struct stats_s *stats, unsigned int options,
+                              import_filter filter, void *filter_arg);
 static int import_revoke_cert( const char *fname, KBNODE node,
                                struct stats_s *stats);
 static int chk_self_sigs( const char *fname, KBNODE keyblock,
@@ -163,7 +166,8 @@ import_release_stats_handle (void *p)
 static int
 import_keys_internal( IOBUF inp, char **fnames, int nnames,
 		      void *stats_handle, unsigned char **fpr, size_t *fpr_len,
-		      unsigned int options )
+		      unsigned int options,
+		      import_filter filter, void *filter_arg)
 {
     int i, rc = 0;
     struct stats_s *stats = stats_handle;
@@ -172,7 +176,8 @@ import_keys_internal( IOBUF inp, char **fnames, int nnames,
         stats = import_new_stats_handle ();
 
     if (inp) {
-        rc = import( inp, "[stream]", stats, fpr, fpr_len, options);
+        rc = import (inp, "[stream]", stats, fpr, fpr_len, options,
+                     filter, filter_arg);
     }
     else {
         int once = (!fnames && !nnames);
@@ -192,7 +197,8 @@ import_keys_internal( IOBUF inp, char **fnames, int nnames,
 	        log_error(_("can't open `%s': %s\n"), fname, strerror(errno) );
 	    else
 	      {
-	        rc = import( inp2, fname, stats, fpr, fpr_len, options );
+	        rc = import (inp2, fname, stats, fpr, fpr_len, options,
+                             NULL, NULL);
 	        iobuf_close(inp2);
                 /* Must invalidate that ugly cache to actually close it. */
                 iobuf_ioctl (NULL, 2, 0, (char*)fname);
@@ -223,19 +229,23 @@ void
 import_keys( char **fnames, int nnames,
 	     void *stats_handle, unsigned int options )
 {
-  import_keys_internal(NULL,fnames,nnames,stats_handle,NULL,NULL,options);
+  import_keys_internal (NULL, fnames, nnames, stats_handle, NULL, NULL,
+                        options, NULL, NULL);
 }
 
 int
 import_keys_stream( IOBUF inp, void *stats_handle,
-		    unsigned char **fpr, size_t *fpr_len,unsigned int options )
+		    unsigned char **fpr, size_t *fpr_len,unsigned int options,
+	            import_filter filter, void *filter_arg )
 {
-  return import_keys_internal(inp,NULL,0,stats_handle,fpr,fpr_len,options);
+  return import_keys_internal (inp, NULL, 0, stats_handle, fpr, fpr_len,
+                               options, filter, filter_arg);
 }
 
 static int
 import( IOBUF inp, const char* fname,struct stats_s *stats,
-	unsigned char **fpr,size_t *fpr_len,unsigned int options )
+	unsigned char **fpr,size_t *fpr_len,unsigned int options,
+	import_filter filter, void *filter_arg)
 {
     PACKET *pending_pkt = NULL;
     KBNODE keyblock = NULL;
@@ -252,9 +262,11 @@ import( IOBUF inp, const char* fname,struct stats_s *stats,
 
     while( !(rc = read_block( inp, &pending_pkt, &keyblock) )) {
 	if( keyblock->pkt->pkttype == PKT_PUBLIC_KEY )
-	    rc = import_one( fname, keyblock, stats, fpr, fpr_len, options, 0);
+	    rc = import_one (fname, keyblock, stats, fpr, fpr_len, options, 0,
-	else if( keyblock->pkt->pkttype == PKT_SECRET_KEY )
+                             filter, filter_arg);
-                rc = import_secret_one( fname, keyblock, stats, options );
+        else if( keyblock->pkt->pkttype == PKT_SECRET_KEY )
+            rc = import_secret_one (fname, keyblock, stats, options,
+                                    filter, filter_arg);
 	else if( keyblock->pkt->pkttype == PKT_SIGNATURE
 		 && keyblock->pkt->pkt.signature->sig_class == 0x20 )
 	    rc = import_revoke_cert( fname, keyblock, stats );
@@ -738,7 +750,7 @@ check_prefs(KBNODE keyblock)
 static int
 import_one( const char *fname, KBNODE keyblock, struct stats_s *stats,
 	    unsigned char **fpr,size_t *fpr_len,unsigned int options,
-	    int from_sk )
+	    int from_sk, import_filter filter, void *filter_arg)
 {
     PKT_public_key *pk;
     PKT_public_key *pk_orig;
@@ -778,6 +790,13 @@ import_one( const char *fname, KBNODE keyblock, struct stats_s *stats,
 	return 0;
       }
 
+    if (filter && filter (pk, NULL, filter_arg))
+      {
+        log_error (_("key %s: %s\n"), keystr_from_pk(pk),
+                   _("rejected by import filter"));
+        return 0;
+      }
+
     if (opt.interactive) {
         if(is_status_enabled())
 	  print_import_check (pk, uidnode->pkt->pkt.user_id);
@@ -1146,7 +1165,8 @@ sec_to_pub_keyblock(KBNODE sec_keyblock)
  */
 static int
 import_secret_one( const char *fname, KBNODE keyblock,
-                   struct stats_s *stats, unsigned int options)
+                   struct stats_s *stats, unsigned int options,
+                   import_filter filter, void *filter_arg)
 {
     PKT_secret_key *sk;
     KBNODE node, uidnode;
@@ -1162,6 +1182,12 @@ import_secret_one( const char *fname, KBNODE keyblock,
     keyid_from_sk( sk, keyid );
     uidnode = find_next_kbnode( keyblock, PKT_USER_ID );
 
+    if (filter && filter (NULL, sk, filter_arg)) {
+        log_error (_("secret key %s: %s\n"), keystr_from_sk(sk),
+                   _("rejected by import filter"));
+        return 0;
+    }
+
     if( opt.verbose )
       {
 	log_info( "sec  %4u%c/%s %s   ",
@@ -1240,8 +1266,9 @@ import_secret_one( const char *fname, KBNODE keyblock,
 	    KBNODE pub_keyblock=sec_to_pub_keyblock(keyblock);
 	    if(pub_keyblock)
 	      {
-		import_one(fname,pub_keyblock,stats,
+		import_one (fname, pub_keyblock, stats,
-			   NULL,NULL,opt.import_options,1);
+                            NULL, NULL, opt.import_options, 1,
+                            NULL, NULL);
 		release_kbnode(pub_keyblock);
 	      }
 	  }

g10/keyserver.c

@@ -959,6 +959,51 @@ direct_uri_map(const char *scheme,unsigned int is_direct)
 #define KEYSERVER_ARGS_KEEP " -o \"%O\" \"%I\""
 #define KEYSERVER_ARGS_NOKEEP " -o \"%o\" \"%i\""
 
+
+/* Check whether a key matches the search description.  The filter
+   returns 0 if the key shall be imported.  Note that this kind of
+   filter is not related to the iobuf filters. */
+static int
+keyserver_retrieval_filter (PKT_public_key *pk, PKT_secret_key *sk, void *arg)
+{
+  KEYDB_SEARCH_DESC *desc = arg;
+  u32 keyid[2];
+  byte fpr[MAX_FINGERPRINT_LEN];
+  size_t fpr_len = 0;
+
+  /* Secret keys are not expected from a keyserver.  Do not import.  */
+  if (sk)
+    return G10ERR_GENERAL;
+
+  fingerprint_from_pk (pk, fpr, &fpr_len);
+  keyid_from_pk (pk, keyid);
+
+  /* Compare requested and returned fingerprints if available. */
+  if (desc->mode == KEYDB_SEARCH_MODE_FPR20)
+    {
+      if (fpr_len != 20 || memcmp (fpr, desc->u.fpr, 20))
+        return G10ERR_GENERAL;
+    }
+  else if (desc->mode == KEYDB_SEARCH_MODE_FPR16)
+    {
+      if (fpr_len != 16 || memcmp (fpr, desc->u.fpr, 16))
+        return G10ERR_GENERAL;
+    }
+  else if (desc->mode == KEYDB_SEARCH_MODE_LONG_KID)
+    {
+      if (keyid[0] != desc->u.kid[0] || keyid[1] != desc->u.kid[1])
+        return G10ERR_GENERAL;
+    }
+  else if (desc->mode == KEYDB_SEARCH_MODE_SHORT_KID)
+    {
+      if (keyid[1] != desc->u.kid[1])
+        return G10ERR_GENERAL;
+    }
+
+  return 0;
+}
+
+
 static int
 keyserver_spawn(enum ks_action action,STRLIST list,KEYDB_SEARCH_DESC *desc,
 		int count,int *prog,unsigned char **fpr,size_t *fpr_len,
@@ -1509,8 +1554,9 @@ keyserver_spawn(enum ks_action action,STRLIST list,KEYDB_SEARCH_DESC *desc,
 	     but we better protect against rogue keyservers. */
 
 	  import_keys_stream (spawn->fromchild, stats_handle, fpr, fpr_len,
-                              (opt.keyserver_options.import_options
+                             (opt.keyserver_options.import_options
-                               | IMPORT_NO_SECKEY));
+                              | IMPORT_NO_SECKEY),
+                              keyserver_retrieval_filter, desc);
 
 	  import_print_stats(stats_handle);
 	  import_release_stats_handle(stats_handle);
@@ -2043,7 +2089,7 @@ keyserver_import_cert(const char *name,unsigned char **fpr,size_t *fpr_len)
 
       rc=import_keys_stream (key, NULL, fpr, fpr_len,
                              (opt.keyserver_options.import_options
-                              | IMPORT_NO_SECKEY));
+                              | IMPORT_NO_SECKEY), NULL, NULL);
 
       opt.no_armor=armor_status;
 

g10/main.h

@@ -207,11 +207,15 @@ MPI encode_md_value( PKT_public_key *pk, PKT_secret_key *sk,
 		     MD_HANDLE md, int hash_algo );
 
 /*-- import.c --*/
+
+typedef int (*import_filter)(PKT_public_key *pk, PKT_secret_key *sk, void *arg);
+
 int parse_import_options(char *str,unsigned int *options,int noisy);
 void import_keys( char **fnames, int nnames,
 		  void *stats_hd, unsigned int options );
-int import_keys_stream( IOBUF inp,void *stats_hd,unsigned char **fpr,
+int import_keys_stream (IOBUF inp,void *stats_hd,unsigned char **fpr,
-			size_t *fpr_len,unsigned int options );
+                        size_t *fpr_len,unsigned int options,
+                        import_filter filter, void *filter_arg);
 void *import_new_stats_handle (void);
 void import_release_stats_handle (void *p);
 void import_print_stats (void *hd);