Re: [dm-crypt] IV generation for dm-crypt

[Milan Broz <gmazyland@gmail.com>]

Dear Sharmila,

when I sent you to the list, because the issue tracker is not a support forum
and our time is extremely limited, I expected you will AT LEAST read the link
to the documentation I provided.

Please do not expect from the community any support for questions you just
copy&paste without even trying to study documentation yourself.

Anyway.

First, really think about using integritysetup/cryptsetup. Using dmsetup
is super fragile (it is the lowest tool, basically just wrapper through IOCTL).

On 29/01/2020 09:18, EXTERNAL D Sharmila (Iwave, RBEI/PAC-PF) wrote:
> Query:
>         Whether Random number generator used to generate IV, generates
> same IV repeatedly or unique IV will be generated ? How the IV
> generation handled in dm-crypt ?

https://gitlab.com/cryptsetup/cryptsetup/-/wikis/DMCrypt#iv-generators

So, if you use "random" IV it is regenerated on every write (can be used onlt for
authenticated encryption).
For other IVs it is either fixed (null) or derived from sector offset.

I would strongly suggest to use aes-xts-plain64 these days for length preserving
encryption in dm-crypt.

> The commands I used to create dm-integrity and dm-crypt is as below
> 
> dmsetup create test-integrity --table '0 14240 integrity /dev/mmcblk0p5
> 0 32 J 1
> internal_hash:hmac(sha256):30313233343536373839414243444546303132333435
> 36373839414243444546' 
> 
> dmsetup create test-integrity-crypt --table '0 14240 crypt aes-cbc-
> essiv:sha256 :32:user:test-key 0 /dev/mapper/test-integrity 0'

So here you use integrity protected device with keyded MAC, and over
it length preserving encryption using aes-cbc-essiv.

So IV in dmcrypt is fixed, but unpredictable (derived from encryption key).

Again, read the documentation.
https://gitlab.com/cryptsetup/cryptsetup/-/wikis/DMIntegrity

Namely that key specification for internal_hash is in hexa.
Your "random" HMAC key above is very suspicious in this regard, for example.

m.