GnuPG card && using the backup secret key

[Thomas Jarosch]

Am 13.06.2017 um 12:20 schrieb Matthias Apitz:
>> AFAIK the "backup process" during key creation for the OpenPGP smartcard
>> is a bit different: There is no interface / function on the card to
>> export a key. Therefore, if you decide to create a backup, a key is
>> first created on the host and *then* transferred onto the card.
>> At least that's my understanding of it.
> 
> Thanks for your posting, but now I'm really confused. The howto about
> the card in https://gnupg.org/howtos/card-howto/en/smartcard-howto-single.html
> says:
> 
> ...
> 3.3.2. Generating keys
> 
> To generate a key on the card enter generate. You will be asked if you would like to make an off-card copy of the encryption key. It is useful to say yes here.
> Note
> 
> Without a backup you will not be able to access any data you encrypted
> with the card if it gets lost or damaged.
> ...
just checked the source code: If you want a backup of the key,
the "want_backup" variable is set. This later on translates
to the "card_backup_key" variable.

---keygen.c---
/*
 * Generate a keypair (fname is only used in batch mode) If
 * CARD_SERIALNO is not NULL the function will create the keys on an
 * OpenPGP Card.  If CARD_BACKUP_KEY has been set and CARD_SERIALNO is
 * NOT NULL, the encryption key for the card is generated on the host,
 * imported to the card and a backup file created by gpg-agent.  If
 * FULL is not set only the basic prompts are used (except for batch
 * mode).
 */
void
generate_keypair (ctrl_t ctrl, int full, const char *fname,
                  const char *card_serialno, int card_backup_key)
---keygen.c---


-> so yes, if you want a backup, the key is created on the host.
Security wise it would be bad if the card has a function to extract
a key from it and there's a bug that could somehow trigger this function.

Also it does not make a big difference if the key is created
on the host or on the card if it ends up on the host anyway :)

May be the documentation needs to clarify the situation a bit.

Cheers,
Thomas

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