What is the purpose of "prologue" field used by kernel module signing script "sign-file"

manisha.jnu08 · 12-12-2017, 03:23 AM

Hi,

I am going through the sign-file code. I need to understand the meaning of the “prologue” part in the script. The script actually prefixes the prologue to the digest of the module file and then signs it using the “openssl rsautl”.

Code snippet form "/usr/src/kernels/3.10.0-693.el7.x86_64/scripts/sign-file" script

$prologue = pack(“C*”,
0x30, 0x31, 0x30, 0x0d, 0x06, 0x09,
0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01,
0x05, 0x00, 0x04, 0x20);

Does anybody has any idea, what is the significance of this prologue?

Thanks

sundialsvcs · 12-13-2017, 09:48 AM

I would hazard a guess that this prologue serves the effect of being "salt." In this case, it is a fixed salt.

Quote:

Originally Posted by WikiPedia:

In cryptography, a salt is random data that is used as an additional input to a one-way function that "hashes" a password or passphrase. Salts are closely related to the concept of nonce. The primary function of salts is to defend against dictionary attacks or against its hashed equivalent, a pre-computed rainbow table attack.