gpg: Improve decryption diagnostic for an ADSK key.

* g10/keydb.h (GET_PUBKEYBLOCK_FLAG_ADSK): New constant.
* g10/packet.h (PUBKEY_USAGE_XENC_MASK): New constant.
* g10/pubkey-enc.c (get_it): Print a note if an ADSK key was used.
Use the new get_pubkeyblock flag.
* g10/getkey.c (struct getkey_ctx_s): Add field allow_adsk.
(get_pubkeyblock): Factor all code out to ...
(get_pubkeyblock_ext): new.
(finish_lookup): Add new arg allow_adsk and make use of it.
--

This patch solves two purposes:
- We write a note that the ADSK key was used for decryption
- We avoid running into a
  "oops: public key not found for preference check\n"
  due to ADSK keys.  The error is mostly harmless but lets gpg return
  with an exit code of 2.

Backported-from-master: 882ab7fef9bf4440900c32d7463469307224f11a

g10/getkey.c

@@ -66,6 +66,9 @@ struct getkey_ctx_s
      details.  */
   int exact;
 
+  /* Allow returning an ADSK key.  */
+  int allow_adsk;
+
   /* Part of the search criteria: Whether the caller only wants keys
      with an available secret key.  This is used by getkey_next to get
      the next result with the same initial criteria.  */
@@ -147,7 +150,8 @@ static int lookup (ctrl_t ctrl, getkey_ctx_t ctx, int want_secret,
 		   kbnode_t *ret_keyblock, kbnode_t *ret_found_key);
 static kbnode_t finish_lookup (kbnode_t keyblock,
                                unsigned int req_usage, int want_exact,
-                               int want_secret, unsigned int *r_flags);
+                               int want_secret, int allow_adsk,
+                               unsigned int *r_flags);
 static void print_status_key_considered (kbnode_t keyblock, unsigned int flags);
 
 
@@ -698,7 +702,7 @@ get_pubkeyblock_for_sig (ctrl_t ctrl, PKT_signature *sig)
  * The self-signed data has already been merged into the public key
  * using merge_selfsigs.  */
 kbnode_t
-get_pubkeyblock (ctrl_t ctrl, u32 * keyid)
+get_pubkeyblock_ext (ctrl_t ctrl, u32 * keyid, unsigned int flags)
 {
   struct getkey_ctx_s ctx;
   int rc = 0;
@@ -714,6 +718,7 @@ get_pubkeyblock (ctrl_t ctrl, u32 * keyid)
   ctx.items[0].mode = KEYDB_SEARCH_MODE_LONG_KID;
   ctx.items[0].u.kid[0] = keyid[0];
   ctx.items[0].u.kid[1] = keyid[1];
+  ctx.allow_adsk = !!(flags & GET_PUBKEYBLOCK_FLAG_ADSK);
   rc = lookup (ctrl, &ctx, 0, &keyblock, NULL);
   getkey_end (ctrl, &ctx);
 
@@ -721,6 +726,12 @@ get_pubkeyblock (ctrl_t ctrl, u32 * keyid)
 }
 
 
+kbnode_t
+get_pubkeyblock (ctrl_t ctrl, u32 * keyid)
+{
+  return get_pubkeyblock_ext (ctrl, keyid, 0);
+}
+
 /* Return the public key with the key id KEYID iff the secret key is
  * available and store it at PK.  The resources should be released
  * using release_public_key_parts().
@@ -1822,7 +1833,7 @@ get_pubkey_fromfile (ctrl_t ctrl, PKT_public_key *pk, const char *fname,
       /* Warning: node flag bits 0 and 1 should be preserved by
        * merge_selfsigs.  FIXME: Check whether this still holds. */
       merge_selfsigs (ctrl, keyblock);
-      found_key = finish_lookup (keyblock, pk->req_usage, 0, 0, &infoflags);
+      found_key = finish_lookup (keyblock, pk->req_usage, 0, 0, 0, &infoflags);
       print_status_key_considered (keyblock, infoflags);
       if (found_key)
         pk_from_block (pk, keyblock, found_key);
@@ -3670,7 +3681,7 @@ merge_selfsigs (ctrl_t ctrl, kbnode_t keyblock)
  */
 static kbnode_t
 finish_lookup (kbnode_t keyblock, unsigned int req_usage, int want_exact,
-               int want_secret, unsigned int *r_flags)
+               int want_secret, int allow_adsk, unsigned int *r_flags)
 {
   kbnode_t k;
 
@@ -3691,6 +3702,9 @@ finish_lookup (kbnode_t keyblock, unsigned int req_usage, int want_exact,
 
 #define USAGE_MASK  (PUBKEY_USAGE_SIG|PUBKEY_USAGE_ENC|PUBKEY_USAGE_CERT)
   req_usage &= USAGE_MASK;
+  /* In allow ADSK mode make sure both encryption bis are set.  */
+  if (allow_adsk && (req_usage & PUBKEY_USAGE_XENC_MASK))
+    req_usage |= PUBKEY_USAGE_XENC_MASK;
 
   /* Request the primary if we're certifying another key, and also if
    * signing data while --pgp6 or --pgp7 is on since pgp 6 and 7 do
@@ -3718,7 +3732,8 @@ finish_lookup (kbnode_t keyblock, unsigned int req_usage, int want_exact,
               pk->flags.exact = 1;
               break;
             }
-          else if ((k->pkt->pkt.public_key->pubkey_usage == PUBKEY_USAGE_RENC))
+          else if (!allow_adsk && (k->pkt->pkt.public_key->pubkey_usage
+                                   == PUBKEY_USAGE_RENC))
             {
               if (DBG_LOOKUP)
                 log_debug ("finish_lookup: found via ADSK - not selected\n");
@@ -4018,7 +4033,8 @@ lookup (ctrl_t ctrl, getkey_ctx_t ctx, int want_secret,
        * merge_selfsigs.  */
       merge_selfsigs (ctrl, keyblock);
       found_key = finish_lookup (keyblock, ctx->req_usage, ctx->exact,
-                                 want_secret, &infoflags);
+                                 want_secret, ctx->allow_adsk,
+                                 &infoflags);
       print_status_key_considered (keyblock, infoflags);
       if (found_key)
 	{

g10/keydb.h

@@ -337,6 +337,8 @@ int get_pubkey_fast (PKT_public_key *pk, u32 *keyid);
 kbnode_t get_pubkeyblock_for_sig (ctrl_t ctrl, PKT_signature *sig);
 
 /* Return the key block for the key with KEYID.  */
+#define GET_PUBKEYBLOCK_FLAG_ADSK  1 /* Allow returning ADSK key.  */
+kbnode_t get_pubkeyblock_ext (ctrl_t ctrl, u32 *keyid, unsigned int flags);
 kbnode_t get_pubkeyblock (ctrl_t ctrl, u32 *keyid);
 
 /* A list used by get_pubkeys to gather all of the matches.  */

g10/packet.h

@@ -60,10 +60,14 @@
 #define PUBKEY_USAGE_RENC    1024                /* Restricted encryption.  */
 #define PUBKEY_USAGE_TIME    2048                /* Timestamp use.  */
 
+
 /* The usage bits which can be derived from the algo.  */
 #define PUBKEY_USAGE_BASIC_MASK  (PUBKEY_USAGE_SIG|PUBKEY_USAGE_ENC\
                                   |PUBKEY_USAGE_CERT|PUBKEY_USAGE_AUTH)
 
+/* The usage bits which define encryption.  */
+#define PUBKEY_USAGE_XENC_MASK  (PUBKEY_USAGE_ENC | PUBKEY_USAGE_RENC)
+
 
 /* Helper macros.  */
 #define is_RSA(a)     ((a)==PUBKEY_ALGO_RSA || (a)==PUBKEY_ALGO_RSA_E \

g10/pubkey-enc.c

@@ -369,7 +369,7 @@ get_it (ctrl_t ctrl,
   {
     PKT_public_key *pk = NULL;
     PKT_public_key *mainpk = NULL;
-    KBNODE pkb = get_pubkeyblock (ctrl, keyid);
+    KBNODE pkb = get_pubkeyblock_ext (ctrl, keyid, GET_PUBKEYBLOCK_FLAG_ADSK);
 
     if (!pkb)
       {
@@ -418,6 +418,13 @@ get_it (ctrl_t ctrl,
           }
       }
 
+    if (pk && !(pk->pubkey_usage & PUBKEY_USAGE_ENC)
+        && (pk->pubkey_usage & PUBKEY_USAGE_RENC))
+      {
+        log_info (_("Note: ADSK key has been used for decryption"));
+        log_printf ("\n");
+      }
+
     if (pk && pk->flags.revoked)
       {
         log_info (_("Note: key has been revoked"));
@@ -435,7 +442,7 @@ get_it (ctrl_t ctrl,
 
         /* Note that we do not want to create a trustdb just for
          * getting the ownertrust: If there is no trustdb there can't
-         * be ulitmately trusted key anyway and thus the ownertrust
+         * be ultimately trusted key anyway and thus the ownertrust
          * value is irrelevant.  */
         write_status_printf (STATUS_DECRYPTION_KEY, "%s %s %c",
                              pkhex, mainpkhex,