SigstoreSigner: public key should match signer

[lukpueh]

SigstoreSigner takes a signing token and a public key that contains the corresponding OIDC identity and issuer. They should be checked if they indeed correspond:

securesystemslib/securesystemslib/signer/_sigstore_signer.py

Lines 137 to 142 in 2acf318

	def __init__(self, token: str, public_key: Key):
	# TODO: Vet public key
	# - signer eligible for keytype/scheme?
	# - token matches identity/issuer?
	self.public_key = public_key
	self._token = token

Moreover, this relation could be facilitated by constraining oauth + oidc flow to use the identity and issuer set by the user when the token is created:

securesystemslib/securesystemslib/signer/_sigstore_signer.py

Lines 168 to 171 in 2acf318

	# TODO: Restrict oauth flow to use identity/issuer from public_key
	# TODO: Use secrets_handler for identity_token() secret arg
	issuer = Issuer.production()
	token = issuer.identity_token()

(currently, the sigstore proxy lets you choose from a list of providers and use arbitrary identities in the flow)

[ckilcoin]

Hi Lukas, I am interested in working on this issue as a first task to get more involved in the Secure Systems Lab. Do you think it would be good introductory work for the project?

[jku]

it's unfortunately not 100% clear what the right path is:

• the info we want to verify is in the token...
• but the token is also sort of a black box that we as users of sigstore-python just should not need to parse

I'm still hoping that sisgtore-python would add some functionality to make this easier (such as being able to call issuer.identity_token() with arguments issuer / identity), sigstore/sigstore-python#567 but it is not clear if that will happen or not

[jku]

if you're just looking for something to start with, #498 or #512 might be good

[jku]

I believe this is now doable by comparing the public key to sigstore-pythons
IdentityToken.expected_certificate_subject and IdentityToken.expected_certificate_subject.identity

[jku]

this is now doable by comparing the public key to sigstore-pythons
IdentityToken.expected_certificate_subject and IdentityToken.expected_certificate_subject.identity

Actually this bit is now implemented (for interactive auth at least). I'll close this one.