Add PKCS#8 identifier. (ESPTOOL-584)

[StevenMacias]

Description of change

Add extra condition to allow the serialization of PKCS#8 format.

This change fixes the following bug(s):

The documentation suggests to use the following command:

openssl genrsa -out my_secure_boot_signing_key.pem 3072

This generates a PKCS#8 pem file starting with -----BEGIN PRIVATE KEY----- which is different to the expected ---BEGIN RSA PRIVATE KEY---.

Since cryptography uses OpenSSL in the background this change should not be dangerous. I have tested espsecure.py locally and it works as expected.

cryptography.exceptions.UnsupportedAlgorithm – If the serialized key type is not supported by the OpenSSL version cryptography is using.

Before:

espsecure.py verify_signature --version 2 --keyfile development_private_key_rsa_3072.pem fbebbae17cf48279e7dcbc3782f8581542b281ca.bin 
espsecure.py v4.5-dev

A fatal error occurred: Verification key does not appear to be an RSA Private or Public key in PEM format. Unsupported

After:

espsecure.py verify_signature --version 2 --keyfile development_private_key_rsa_3072.pem fbebbae17cf48279e7dcbc3782f8581542b281ca.bin 
espsecure.py v4.5-dev
Signature block 0 is valid (RSA). 
Signature block 0 verification successful with /home/steven/Workspace/development_private_key_rsa_3072.pem (RSA).
Signature block 1 invalid. Skipping.
Signature block 2 invalid. Skipping.

I have tested this change with the following hardware & software combinations:

Operating system: Arch Linux x86_64
Kernel: 6.1.1-arch1-1
Chip name: ESP32-S3
IDF version: 4.4.3
OpenSSL version: OpenSSL 3.0.7 1 Nov 2022 (Library: OpenSSL 3.0.7 1 Nov 2022)

[radimkarnis]

Hi @StevenMacias,
Thank you for contributing! Your PR will be reviewed and hopefully merged soon.

In the meantime, there some stylistic issues. This repo follows black formatting and style is enforced with flake8. Details are described in the Contributing guide. Please resolve this small issue and squash the commits afterwards.

[StevenMacias]

Format and style should be fixed now!

[radimkarnis]

This was merged in 1d34837. Thank you!