scd: Convey the correct length for Le

* scd/app-openpgp.c (determine_rsa_response): Round bits up.
--

Co-authored-by: Arnaud Fontaine <arnaud.fontaine@ssi.gouv.fr>

Arnaud wrote:

  Actually, when the incorrect expected response length (i.e. Le
  field) is transmitted to the card, the card's answer is missing a
  byte (i.e. ...  6101) so an additional command has to be sent to the
  card to retrieve the last byte. Using the correct length avoids to
  send the additional command to retrieve the missing byte, when the
  computed length is wrong.

Note that an value of 65537 for E is pretty standard and thus we can
avoid the 6101 return code inmost cases.

Signed-off-by: Werner Koch <wk@gnupg.org>

scd/app-openpgp.c

@@ -943,8 +943,8 @@ determine_rsa_response (app_t app, int keyno)
   int size;
 
   size = 2 + 3 /* header */
-    + 4 /* tag+len */ + app->app_local->keyattr[keyno].rsa.n_bits/8
-    + 2 /* tag+len */ + app->app_local->keyattr[keyno].rsa.e_bits/8;
+    + 4 /* tag+len */ + (app->app_local->keyattr[keyno].rsa.n_bits+7)/8
+    + 2 /* tag+len */ + (app->app_local->keyattr[keyno].rsa.e_bits+7)/8;
 
   return size;
 }