How should I load signers for immediate signing, e.g. in CLI? about securesystemslib

[lukpueh]

likely all Signers could have more ergonomic signer-specific (or use-case-specific) constructors

Yes, a usable CryptoSigner.__init__ would already go a long way. Currently, CryptoSigner is abstract and all its subclasses are protected, which makes it unusable outside of from_priv_key_uri. If I could create a CryptoSigner from a pyca/cryptography private key object, then it would be easy for an application to create their own from_pem, e.g.:

from cryptography.hazmat.primitives.serialization import load_pem_private_key

private_key = load_pem_private_key(data, password)
signer = CryptoSigner(private_key)  # generate default public Key, if not passed explicitly

[jku]

I see no problems with this.

• from_priv_key_uri() is generic but clumsy
• likely all Signers could have more ergonomic signer-specific (or use-case-specific) constructors

I've never had a use-case where I wanted to avoid generic signer loading to have the latter but I can see others might.
Let's not add code "just because" but if an application use case gets easier by adding a constructor, let's do that.