i.MX Processorsのトピックcst different hashes from same input binary

cst different hashes from same input binary

paride_russo — Wed, 19 Aug 2020 08:10:22 GMT

Hi, can someone help me in understanding why cst is generating 2 different hashes for the same input binary?

I'm using this input csf:

[Header]
Version = 4.2
Engine = DCP
Engine Configuration = 0
Certificate Format = x509
Signature Format = CMS
Hash Algorithm = sha256
[Install SRK]
File = "keys/SRK_1_2_3_4_table.bin"
Source Index = 0
[Install CSFK]
File = "crts/CSF1_1_sha256_2048_65537_v3_usr_crt.pem"
Certificate Format = x509
[Authenticate CSF]
[Install Key]
File = "crts/IMG1_1_sha256_2048_65537_v3_usr_crt.pem"
Verification Index = 0
Target Index = 2
[Authenticate Data]
Verification Index = 2
Engine = DCP
Engine Configuration = 0
Blocks = 0x60001000 0x1000 0x40 "test.bin",\
0x60002000 0x2000 0x40090 "test.bin"
[Set Engine]
Hash Algorithm = sha256
Engine = DCP
Engine Configuration = 0
[Unlock]
Engine = SNVS
Features = ZMK WRITE

with this command line invocation:

cst.exe -o out_csf.bin -i input.csf

the result from cst seems ok:

CSF Processed successfully and signed data available in out_csf.bin

But if I execute this process twice, the content of out_csf.bin is different.

I tried to debug this thing by using the program hab_csf_parser part of the cst package and analyzing the 2 generated out_csf.bin but the only different part is what follows the HAB_TAG_SIG 0xD8 which from my understanding is the signature itself. Am I doing something wrong here?

Re: cst different hashes from same input binary

Yuri — Mon, 24 Aug 2020 05:44:45 GMT

paride.russo@orbotech.com

Hello,

Please use the following resources for i.MX RT:

"Security Application Note AN12079"

https://www.nxp.com/webapp/sps/download/mod_download.jsp?colCode=AN12079&appType=moderated

i.MX RT Secure Boot Lab Guide.pdf

MCUXpresso Secure Provisioning Tool | Software Development for NXP Microcontrollers (MCUs) | NXP | NXP

Regards,

Yuri.

Re: cst different hashes from same input binary

paride_russo — Mon, 24 Aug 2020 06:31:44 GMT

Thanks Yuri,

I've already read that documents but I need to drive "the low level" of the code signing tool because I'm bypassing elftosb and the flashloader; I'm loading the fw signed and encrypted via ethernet via a custom bootloader and some custom hardware that I did. It is working though, I generated multiple times the binary signed with cst and the i.mx is starting every time in closed mode so the signature makes sense and is valid. I just don't understand why the signature keeps changing value every time I generated even if the source binary is always the same. I was expecting the signature to be the same...are you including some random/time number/info in the signature?

Re: cst different hashes from same input binary

Yuri — Mon, 24 Aug 2020 11:09:47 GMT

paride.russo@orbotech.com

Hello,

It is expected behavior - that the signature is changed.

Details of signature data are not provided - sorry.

Regards,

Yuri.