Re: How to get PBKDF settings of an existing key slot via libcryptsetup ?

Hello Ingo,

On 14/02/2019 10:28, Ingo Franzki wrote:
> I have now added support for calling crypt_keyslot_get_pbkdf(). 
> Works great, with one little exception:
> 
> When getting a PBKDF from a key slot, the field time_ms is zero. 

Yes, it is zero here, because it is the time intended for a benchmark.
(IOW it says: benchmark the PBKDF and set parameters such a way, that unlocking should take approximately this time.)

This requested time is not stored in keyslot (only the final calculated values are stored here).

> When setting this PBKDF using crypt_set_pbkdf_type() I get an error that time can not be zero:
> Message "Requested PBKDF target time cannot be zero." from function verify_pbkdf_params().
> 
> Looking at function LUKS2_keyslot_pbkdf() it seems that there is a little copy & paste error:
> 
> 	if (json_object_object_get_ex(jobj_kdf, "iterations", &jobj))
> 		pbkdf->iterations = json_object_get_int(jobj);
> 	if (json_object_object_get_ex(jobj_kdf, "time", &jobj))
> 		pbkdf->iterations = json_object_get_int(jobj); <--- should set field time_ms, not iterations

The code is correct ;-)

It is tricky, for PBKDF2 it is named "iterations" , for Argon it is "time" (means time cost),
but it is in the end stored in the same pbkdf->iterations value.

Only one of the value is in JSON, so it will not overwrite..

I admit this is not the best solution but we had already stable data structure in API.

> Not a big deal, I can set time_ms manually before calling crypt_set_pbkdf_type() when it is zero.

If you want to run benchmark, then you need to set time_ms to requested value (default is 2000 ms).

The other option is to force reuse the same parameters and skip benchmark, then you can keep
time_ms value to 0 but you have to set pbkdf->flags bit CRYPT_PBKDF_NO_BENCHMARK.

Milan
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