Re: [dm-crypt] How to get PBKDF settings of an existing key slot via libcryptsetup ?

[Ingo Franzki <ifranzki@linux.ibm.com>]

On 04.02.2019 14:44, Ingo Franzki wrote:
> On 04.02.2019 14:22, Milan Broz wrote:
>> Hi Ingo,
> Hi Milan,
>>
>> On 31/01/2019 11:14, Ingo Franzki wrote:
>>> Hi,
>>>
>>> is there a way to get the PBKDF settings (struct crypt_pbkdf_type) of an existing key slot in a LUKS2 volume via the libcryptsetup API? 
>>
>> Not yet, but see below.
>>
>>>
>>> This question is related to the default PBKDF algorithm Argon2i for LUKS2 and the out-of-memory errors that you might get when you unlock multiple LUKS2 volumes during system startup via /etc/crypttab.
>>>
>>> One of my application uses crypt_keyslot_add_by_key() to add a new unbound key slot. Unfortunately this new key slot gets the default PBKDF settings, thus it gets Argon2i. I guess if I would use crypt_set_pbkdf_type() before to set PBKDF2, then the new key slot would get PBKDF2 instead of Argon2i. However, I don't want to hard code PBKDF2 here, but I would like to use the PBKDF settings of the key slot that was unlocked before. So I would need a way to get the PBKDF settings of a key slot and then use crypt_set_pbkdf_type() with those settings before calling crypt_keyslot_add_by_key(). That way the new key slot would get the same PBKDF settings as the current one.
>>>
>>> Using crypt_get_pbkdf_type() seems to return the default PBKDF algorithm, thus Argon2i for LUKS2. 
>>
>> Yes, you describe exactly how I intended to use it. (I guess your use key is s390 crypto, so PBKDF2 ok, because it is wrapped key for crypto accelerator,
>> without the hw attacker cannot run offline attacks here.)
>>
>> But since we can now get per-keyslot encryption in LUKS2 through API, there should be also way how to get specific keyslot PBKDF setting (and not only the default).
>>
>> It should be relatively simple, so I tried to add such a call - it something like this what you need?
>>   https://gitlab.com/cryptsetup/cryptsetup/commit/8c3be56418248ef5b96265f901122effa88e446b
>>
> Yes, that's exactly what I need ! 
> Thanks a lot !

It would be nice if I could check via #ifdef if that new function is available or not. 
For example have a symbol in libcryptsetup.h that contains the current version:
#define LIBCRYPTSETUP_VERSION     0x00020006
That way one could use 
#ifdef LIBCRYPTSETUP_VERSION >= 0x00020006
to check for a feature.

That would allow me to write code that does not hardly depend on a certain cryptsetup version. If the new function is not there I could use PBKDF2 unconditionally, and when it is there I use the new function to find out what PBKDF is used by the existing keyslot. 


>> Thanks,
>> Milan
>>
>>
> 
> 


-- 
Ingo Franzki
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