iMX6 - 256-bit Zeroizable Master Key

[viniciusmaciel]

Hi,

I am using kernel 4.9.11 running on Wandboard. How do I read the 256-bit Zeroizable Master Key from iMX6 CPU fuse? 

Can I read from userspace or only from kernel space? Can I get this key from the CAAM driver?

Vini

[Bio_TICFSL · Accepted solution]

Hi Vinicius,

ZMK value (or the ZMK XORed with the OTPMK) can be selected as the  master key input to 
the DCP. Please refer to Security Reference Manual for the i.MX 6ULL Applications Processor for more details.
  The unique OTP Master Key (OTPMK) is a random value stored in the non-volatile memory 

(typically fuses) outside the SNVS module.  OTPMK is factory preprogrammed. 

  HAB incorporates signature verification using RSA public keys and the SHA-256 hash algorithm, but i.MX6 CAAM does not support asymmetric cryptographic acceleration..

Regards

[viniciusmaciel]

Hi,

I am using sm_test.c(crypto/caam/sm_test.c) driver to try retrieve the OTP Master Key, but this driver actually has 4 keys(64 bits, 128 bits, 192 bits and 256 bits) with 2 groups: clear keys and black keys. The clear keys does not matter because is just a progression (0, 1, 2, 3, 4, ...). The "true" random keys are the black keys, so that is what we want. 

The problem is that everytime I reboot the Linux, i am getting different values for the 4 keys (64 bits, 128 bits, 192 bits and 256 bits).

Example:

[ 3.350716] platform caam_sm: 64-bit black key:
[ 3.353955] platform caam_sm: [0000] 12 e5 59 04 33 25 46 f9
[ 3.358318] platform caam_sm: [0008] c8 13 e4 d3 c1 45 27 bd

And in another boot:

[ 3.420722] platform caam_sm: 64-bit black key:
[ 3.423961] platform caam_sm: [0000] 67 f5 f6 f7 5a 3b 19 14
[ 3.428322] platform caam_sm: [0008] 4e f7 a7 85 66 14 44 f2

Are these keys supposed to be invariable?