AzureSigner: import_ may return *unsupported* ecdsa-sha2-nistp521 scheme

[lukpueh]

AzureSigner.import_ supports importing a public key on curve nistp521, and returns it as SSlibKey with scheme "ecdsa-sha2-nistp521".

securesystemslib/securesystemslib/signer/_azure_signer.py

Lines 215 to 216 in 185aa9c

	elif key_vault_key.key.crv == KeyCurveName.p_521:
	crv = ec.SECP521R1()

SSlibKey.verify_signature, however, only supports nistp256 and nistp384 key:

securesystemslib/securesystemslib/signer/_key.py

Lines 382 to 385 in 185aa9c

	elif self.scheme in [
	"ecdsa-sha2-nistp256",
	"ecdsa-sha2-nistp384",
	]:

Possible fixes

• remove support for "ecdsa-sha2-nistp521" in AzureSigner.import_
• add support for "ecdsa-sha2-nistp521" in SSlibKey.verify_signature
• orthogonal: validate scheme in SSlibKey.__init__