gpg: Allow the use of an ADSK subkey as ADSK subkey.

* g10/packet.h (PKT_public_key): Increased size of req_usage to 16.
* g10/getkey.c (key_byname): Set allow_adsk in the context if ir was
requested via req_usage.
(finish_lookup): Allow RENC usage matching.
* g10/keyedit.c (append_adsk_to_key): Adjust the assert.
* g10/keygen.c (prepare_adsk): Also allow to find an RENC subkey.
--

If an ADSK is to be added it may happen that an ADSK subkey is found
first and this should then be used even that it does not have the E
usage.  However, it used to have that E usage when it was added.

While testing this I found another pecularity: If you do
  gpg -k ADSK_SUBKEY_FPR
without the '!' suffix and no corresponding encryption subkey is dound,
you will get an unusabe key error.  I hesitate to fix that due to
possible side-effects.

GnuPG-bug-id: 6882
Backported-from-master: d30e345692440b9c6677118c1d20b9d17d80f873

Note that we still use the NO_AKL and not the newer TRY_LDAP in 2.2.
We may want to backport that change as well.

4 files changed, 15 insertions, 9 deletions

diff --git a/g10/getkey.c b/g10/getkey.c
index d7e3909c0..105c95232 100644
--- a/g10/getkey.c
+++ b/g10/getkey.c
@@ -77,7 +77,8 @@ struct getkey_ctx_s
   /* Part of the search criteria: The type of the requested key.  A
      mask of PUBKEY_USAGE_SIG, PUBKEY_USAGE_ENC and PUBKEY_USAGE_CERT.
      If non-zero, then for a key to match, it must implement one of
-     the required uses.  */
+     the required uses.  FWIW: the req_usage field in PKT_public_key
+     used to be an u8 but meanwhile is an u16.  */
   int req_usage;
 
   /* The database handle.  */
@@ -978,7 +979,12 @@ key_byname (ctrl_t ctrl, GETKEY_CTX *retctx, strlist_t namelist,
 
   if (pk)
     {
+      /* It is a bit tricky to allow returning an ADSK key: lookup
+       * masks the req_usage flags using the standard usage maps and
+       * only if ctx->allow_adsk is set, sets the RENC flag again.  */
       ctx->req_usage = pk->req_usage;
+      if ((pk->req_usage & PUBKEY_USAGE_RENC))
+        ctx->allow_adsk = 1;
     }
 
   rc = lookup (ctrl, ctx, want_secret, ret_kb, &found_key);
@@ -3702,7 +3708,7 @@ 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.  */
+  /* In allow ADSK mode make sure both encryption bits are set.  */
   if (allow_adsk && (req_usage & PUBKEY_USAGE_XENC_MASK))
     req_usage |= PUBKEY_USAGE_XENC_MASK;
 
@@ -3807,7 +3813,8 @@ finish_lookup (kbnode_t keyblock, unsigned int req_usage, int want_exact,
 		log_debug ("\tsubkey not valid\n");
 	      continue;
 	    }
-	  if (!((pk->pubkey_usage & USAGE_MASK) & req_usage))
+	  if (!((pk->pubkey_usage & (USAGE_MASK | PUBKEY_USAGE_RENC))
+                & req_usage))
 	    {
 	      if (DBG_LOOKUP)
 		log_debug ("\tusage does not match: want=%x have=%x\n",
diff --git a/g10/keyedit.c b/g10/keyedit.c
index e31f13935..8c61e637e 100644
--- a/g10/keyedit.c
+++ b/g10/keyedit.c
@@ -4757,8 +4757,8 @@ append_adsk_to_key (ctrl_t ctrl, kbnode_t keyblock, PKT_public_key *adsk)
 
   /* Prepare and append the adsk.  */
   keyid_from_pk (main_pk, adsk->main_keyid); /* Fixup main keyid.  */
-  log_assert ((adsk->pubkey_usage & PUBKEY_USAGE_ENC));
-  adsk->pubkey_usage = PUBKEY_USAGE_RENC;    /* 'e' -> 'r'         */
+  log_assert ((adsk->pubkey_usage & PUBKEY_USAGE_XENC_MASK));
+  adsk->pubkey_usage = PUBKEY_USAGE_RENC;    /* 'e' or 'r' -> 'r'  */
   pkt = xtrycalloc (1, sizeof *pkt);
   if (!pkt)
     {
diff --git a/g10/keygen.c b/g10/keygen.c
index 79565b2ae..c9122f79e 100644
--- a/g10/keygen.c
+++ b/g10/keygen.c
@@ -4046,7 +4046,7 @@ prepare_adsk (ctrl_t ctrl, const char *name)
     }
 
   adsk_pk = xcalloc (1, sizeof *adsk_pk);
-  adsk_pk->req_usage = PUBKEY_USAGE_ENC;
+  adsk_pk->req_usage = PUBKEY_USAGE_ENC | PUBKEY_USAGE_RENC;
   err = get_pubkey_byname (ctrl, GET_PUBKEY_NO_AKL,
                            NULL, adsk_pk, name, NULL, NULL, 1);
   if (err)
diff --git a/g10/packet.h b/g10/packet.h
index 7621d85e6..4d9b68a62 100644
--- a/g10/packet.h
+++ b/g10/packet.h
@@ -400,10 +400,9 @@ typedef struct
      when serializing.  (Serialized.)  */
   byte    version;
   byte    selfsigversion; /* highest version of all of the self-sigs */
-  /* The public key algorithm.  (Serialized.)  */
-  byte    pubkey_algo;
+  byte    pubkey_algo;    /* The public key algorithm.  (PGP format)  */
   u16     pubkey_usage;   /* carries the usage info.            */
-  byte    req_usage;      /* hack to pass a request to getkey() */
+  u16     req_usage;      /* hack to pass a request to getkey() */
   u32     has_expired;    /* set to the expiration date if expired */
   /* keyid of the primary key.  Never access this value directly.
      Instead, use pk_main_keyid().  */