PIV APT parsing compatibility

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PIV APT parsing compatibility

Valtteri Vuorikoski

Re-sending this (with some edits) since I think the first attempt a few
days ago disappeared due to a mail misconfig on my side:


Hi, I was giving the PIV support in master a try last night, but
the current code for parsing APT objects 0x4F and 0x79 stopped me
short.

It appears that SP-800-73-x is not too clear about the format of these
objects. Many current cards (such as the Yubikey 5 series) apparently
have only the PIX in DO 0x4F and only the RID in object 0x79/0x4F.

However, other cards as well as the PivApplet Javacard applet have the
full AID in 0x4F (which actually seems closer to what the standard
says). PivApplet also has the full AID in 0x79/0x4F, but this is
probably incorrect. (Here is a long discussion of the matter from an
OpenSC author: https://github.com/arekinath/PivApplet/issues/43#issuecomment-772649709)

Anyway, I think it would be useful for GnuPG 2.3 to support both
formats, since both appear in the wild currently. I have attached the
diff I used to make a PivApplet card pass the initial probe. It also
still works for a Yubikey 5.

(The retry counter in gpg-card also shows an error for PivApplet
because app-piv.c queries the Global PIN even though the Discovery Object
doesn't indicate support for it. I couldn't try actual sign/encrypt
operations yet since card-keys.c can't read certs from gpg/gpgsm.)

 -Valtteri

diff --git a/scd/app-piv.c b/scd/app-piv.c
index 36324e630..d36c5c63f 100644
--- a/scd/app-piv.c
+++ b/scd/app-piv.c
@@ -3641,20 +3641,23 @@ app_select_piv (app_t app)
     }
 
   s = find_tlv (apt, aptlen, 0x4F, &n);
-  if (!s || n != 6 || memcmp (s, piv_aid+5, 4))
+  /* Some cards (new Yubikey) return only the PIX, while others
+   * (old Yubikey, PivApplet) return the RID+PIX. */
+  if (!s || !((n == 6 && memcmp (s, piv_aid+5, 4) == 0) ||
+      (n == 11 && memcmp (s, piv_aid, 9) == 0)))
     {
       /* The PIX does not match.  */
       log_error ("piv: missing or invalid DO 0x4F in APT\n");
       err = gpg_error (GPG_ERR_CARD);
       goto leave;
     }
-  if (s[4] != 1 || s[5] != 0)
+  if (s[n-2] != 1 || s[n-1] != 0)
     {
       log_error ("piv: unknown PIV version %u.%u\n", s[4], s[5]);
       err = gpg_error (GPG_ERR_CARD);
       goto leave;
     }
-  app->appversion = ((s[4] << 8) | s[5]);
+  app->appversion = ((s[n-2] << 8) | s[n-1]);
 
   s = find_tlv (apt, aptlen, 0x79, &n);
   if (!s || n < 7)
@@ -3664,7 +3667,9 @@ app_select_piv (app_t app)
       goto leave;
     }
   s = find_tlv (s, n, 0x4F, &n);
-  if (!s || n != 5 || memcmp (s, piv_aid, 5))
+  /* Some cards may also return the full AID instead of just
+   * the 5-byte RID here. */
+  if (!s || !(n == 5 || n == 11) || memcmp (s, piv_aid, 5))
     {
       /* The RID does not match.  */
       log_error ("piv: missing or invalid DO 0x79.4F in APT\n");

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