Mandate A256GCM for response encryption (compliant with 18013-7)

[peppelinux]

Let's use A256GCM by default

as it is supported by cryptojwt as well: https://github.com/search?q=repo%3AIdentityPython%2FJWTConnect-Python-CryptoJWT%20%20A256GCM&type=code

@Zicchio ^

[Zicchio]

Expanidng a little bit for documentation and other readers.

JWE with asymmetric content encryption use an hybrid encryption scheme (like TLS). The scheme has two component:
(1) a secret sharing algorithm (or key encryption algorithm), which is an asymmetric secret sharing scheme like diffie hellman, which uses a public/private key pair. This key is called Key Encryption Key (KEK). This algorithm will negotiate a (symmetric) Content Encryption Key (CEK).
Which secret sharing algorithm to use is contaned in the alg header https://datatracker.ietf.org/doc/html/rfc7516#section-4.1.1.
(2) a content encryption algorithm, which will use the CEK shared above to actually encrypt the content payload. Which algorithm is used to encrypt the payload is in the enc header https://datatracker.ietf.org/doc/html/rfc7516#section-4.1.2

We already do use AES-GCM with 256 bit size key as the default algorithm for content encryption (2).

Se default map for content encryption algorithm, which favours AES-GCM for EC KEK

eudi-wallet-it-python/pyeudiw/jwt/jws_helper.py

Lines 39 to 42 in ea3a917

	DEFAULT_ENC_ENC_MAP = {
	"RSA": "A256CBC-HS512",
	"EC": "A256GCM"
	}

and its usage

eudi-wallet-it-python/pyeudiw/jwt/jwe_helper.py

Lines 60 to 66 in ea3a917

	_keyobj = JWE_CLASS(
	_payload,
	alg=DEFAULT_ENC_ALG_MAP[key.kty],
	enc=DEFAULT_ENC_ENC_MAP[key.kty],
	kid=key.kid,
	**kwargs
	)

I'm not sure if there is a KEK (asymmetric) scheme (1) that involves AES-GCM.
Our default is ECDH-ES+A256KW, which I'm honestly clueless about the details. I was knowledgebale only about ECDH-ES.

eudi-wallet-it-python/pyeudiw/jwt/jws_helper.py

Lines 34 to 37 in ea3a917

	DEFAULT_ENC_ALG_MAP = {
	"RSA": "RSA-OAEP",
	"EC": "ECDH-ES+A256KW"
	}

Expanding the issue with current Potential LSP requirement, I'd personally prefer is Potential LSP mandated both a default content encryption algorithm (AES-GCM) AND key encryption algorithm (no proposal as of 2025-02-18), that is, clealy specified what to support in both alg and enc header parameters of a JWE.