Setting up a gpg card with 2048bit, some notes

[Oliver Winker]

Hi,

Here some notes on the setup of my gpg card with 2048bit, and the 
different problems I meet with workarounds:
---
1) I could generate 1024 keys on card, but no 2048 keys. Setup is a 
ACR38U reader on a 32-bit linux system (Debian, Testing, 2.6.35.1) !? 

Generation stopped after 10-20sec with error message: "Key generation 
failed: general error"

=> Curiously! with the same hw running AMD64 I could generate 2048 
keys on card with the same ACR38U !? I don't know why ... maybe some 
USB problem with the ACR38U.

2) The encryption key, due to gpg issue #1230, is currently set to 1024 
bit.
3) A backup of the encryption key is saved (on choice) during 
generation, but not of the signing and auth key.
---

The following describes an experimental way and patch for gpg 1.4.10, 
which allowed me to work around this.

=> Only try if you feel experimental as well ;)! No guarantees !!

=> Maybe this is also (partly) none-sense: no guarantees either, but 
comments are highly welcome then ;) !!

So what the patch below does is the following: 
---
a) It uses the gen_card_key_with_backup() function for all three keys 
in keygen.c 
b) Sets the keys size to 2048 (hardcoded)
---

=> This solved my problems 2) and 3) ... and, since the keys are now 
generate in software by gpg (not on the card anymore), it also works 
around my problem 1). 

The final result is a gpg card with:

* All three keys (signing, encr, auth) are now 2048 RSA 
* Backups of all keys, which allows to create a full backup card

To create an initial card + backup card, the following are the steps:

I) Setup the initial card:
---
1) Patch gpg 1.4.10 and build 

2) Generate the keys (generate command), choose 2048 key size

	gpg.patched --card-edit
	admin
	generate

Info: The "passphrase" question is the passphrase, that gpg will use to 
protect the backup keys stored on disc!

=> During generation note the names of the backup files in sequence of 
the keys generated. The sequence is the following:

	1st) Signing key:	sk_rst....gpg
	2nd) Auth key:		sk_uvw....gpg
	3rd) Encr key:		sk_xyz....gpg

Info: If the card refuses the new keys, do first a 'normal' generate, 
on card, using a none patched gpg, with 1024 key sizes. This seems to 
reset things. 
---

II) Setup a backup card
---
1) Put the keys on the card from the created backup-files, using the 
'bkuptocard' command

	gpg --edit-key XYZ...
	toggle
	bkuptocard sk_rst....gpg
	bkuptocard sk_uvw....gpg
	bkuptocard sk_xyz....gpg
	
=> Choose the right key-type (1,2 or 3) using the sequence noted before!

Info: Also here, if the card refuses the new keys, do a 'normal' 
generate, on card, with a none patched gpg, with 1024 key sizes. This 
seems to reset things. 

2) Now ATTENTION: !!! Your secret key backups are still all on disc !!!

Depending on the 'passphrase' you gave them during the key generation, 
your secret keys are now more ???OR MAYBE LESS??? protected!

If e.g. your passphrase was just a 6 digit-pin code, then their 
protection is very low, because there is no chip around them anymore, 
that can lock them away after three retries!

=> Therefore: Either a) shred them, then no more backups possible: the 
key once on the card can't be recovered or b) store them away in a safe 
place! 
---

III) Using the backup card
---
In the gpg key-ring a 'stub' is pointing gpg to the card for the secret 
keys, and verifies the card serial number for this. The serial-number 
verification will fail with the backup card.

To use the backup card with an existing installation, you need to:

1) Delete the secret-key, which is in fact just the stub:

	gpg --delete-secret-key XYZ...
	
2) And let gpg setup a new stub to the backup card, by an --card-status

	gpg --card-status
---	

If something goes wrong: 

As long as you don't block the card by meddling around with wrong pins, 
you can always back out and start over again.

Cheers, Oliver
---

diff --git a/gnupg-1.4.10/g10/keygen.c b/gnupg-1.4.10/g10/keygen.c
--- a/gnupg-1.4.10/g10/keygen.c
+++ b/gnupg-1.4.10/g10/keygen.c
@@ -3138,6 +3138,19 @@
       }
     else
       {
+        if ((s = get_parameter_value (para, pBACKUPENCDIR)))
+            {
+              /* A backup of the key has been requested.
+                 Generate the key i software and import it then to
+                 the card.  Write a backup file. */
+          	rc = gen_card_key_with_backup (PUBKEY_ALGO_RSA, 1, 1, pub_root, sec_root,
+                      timestamp,
+                      get_parameter_u32 (para,
+                                         pKEYEXPIRE),
+                      para, s);
+
+            } else
+
         rc = gen_card_key (PUBKEY_ALGO_RSA, 1, 1, pub_root, sec_root, NULL,
                            &timestamp,
                            get_parameter_u32 (para, pKEYEXPIRE), para);
@@ -3176,6 +3189,20 @@
 
     if (!rc && card && get_parameter (para, pAUTHKEYTYPE))
       {
+
+        if ((s = get_parameter_value (para, pBACKUPENCDIR)))
+          {
+            /* A backup of the key has been requested.
+               Generate the key i software and import it then to
+               the card.  Write a backup file. */
+        	rc = gen_card_key_with_backup (PUBKEY_ALGO_RSA, 3, 0, pub_root, sec_root,
+                    timestamp,
+                    get_parameter_u32 (para,
+                                       pKEYEXPIRE),
+                    para, s);
+
+          } else
+
         rc = gen_card_key (PUBKEY_ALGO_RSA, 3, 0, pub_root, sec_root, NULL,
                            &timestamp,
                            get_parameter_u32 (para, pKEYEXPIRE), para);
@@ -3719,7 +3746,9 @@
 
   sk_unprotected = NULL;
   sk_protected = NULL;
-  rc = generate_raw_key (algo, 1024, timestamp,
+//  rc = generate_raw_key (algo, 1024, timestamp,
+//                         &sk_unprotected, &sk_protected);
+  rc = generate_raw_key (algo, 2048, timestamp,
                          &sk_unprotected, &sk_protected);
   if (rc)
     return rc;