[TECH DEBT] Migrating away from `M2Crypto`

Description of the tech debt to be addressed, include links and screenshots

Following up on https://github.com/saltstack/salt/issues/63066: Salt currently uses M2Crypto for some of its cryptographic operations. M2Crypto is a wrapper around OpenSSL's APIs that has some drawbacks:

• It does not provide memory safety
• It's currently in maintenance mode (the recommended alternative is pyca/cryptography)
• It does not provide wheels on PyPI, which means users installing it also have to build it from source
• The build/installation process requires installing swig, something that adds complexity and results in user installation issues.

I'm opening this issue to list the places where M2Crypto is still being used inside Salt, and to assess the interest in migrating away from it towards more well-maintained alternatives, such as pyca/cryptography. To note: Salt already depends on pyca/cryptography, so this migration would not add new dependencies.

Going through the codebase, these are the usages that I could find, along with the feasibility of migrating to alternatives:

• runners/digicertapi.py: Used to generate a RSA private key
  • Fallbacks to using PyCryptodome or else PyCrypto if M2Crypto is not found
  • Can be migrated to cryptography using RSA.generate_private_key()
  • PR now open here: https://github.com/saltstack/salt/pull/66166
• x509 module: Already deprecated
  • Its replacement module x509v2 was re-written to use pyca/cryptography instead
• salt/crypt.py: Used to generate an RSA key pair, load/encrypt/sign using a RSA private key, load/encrypt/decrypt/verify using a RSA public key, load an RSA private key, load an RSA public key, signing and verifying using RSA, encrypts and decrypts using RSA with X931 padding, decrypts using RSA with PKCS1.5 padding, and others.
  • Fallbacks to using PyCryptodome or else PyCrypto if M2Crypto is not found
  • Most uses can be migrated to pyca/cryptography. An exception is encryption/decryption with X931 padding, but Salt already has its own fallback implementation for it.
• channel/server.py: Used to encrypt/decrypt with RSA with PKCS1 OAEP padding
  • Fallbacks to using PyCryptodome or else PyCrypto if M2Crypto is not found
  • Same as above, can be migrated to pyca/cryptography
• auth/pki.py: Used to load and verify a X.509 certificate
  • Fallbacks to using pyOpenSSL and PyCryptodome/PyCrypto if M2Crypto is not available
  • Can be migrated to pyca/cryptography using https://cryptography.io/en/latest/x509/verification/
• clouds/joyent.py: Used to calculate a SHA256 digest and sign using RSA
  • Fallbacks to using PyCryptodome or else PyCrypto if M2Crypto is not found
  • Same as above, can be migrated to pyca/cryptography

I'm opening the discussion to see if there is interest in migrating away from M2Crypto, and to discuss related issues such as blockers, deprecation paths, fallback alternatives, etc.

Review the listed M2Crypto call sites in salt/crypt.py, salt/channel/server.py, salt/auth/pki.py, and salt/cloud/clouds/joyent.py, noting that runners/digicertapi.py already has a pull request and the x509 module is deprecated. Start by comparing each fallback path with the suggested pyca/cryptography APIs. Done means the remaining unchecked usages have a decided migration or deprecation path without relying on M2Crypto.