[IPsec] Use of AEAD algorithms as pure encryption algorithms

[Valery Smyslov <[e-mail]>]

Hi,

I have a question to the crypto community regarding the use of AEAD algorithms as pure
encryption algorithms. The use case is as follows.

In G-IKEv2 (https://datatracker.ietf.org/doc/draft-ietf-ipsecme-g-ikev2/) we have a situation
where keys are transferred inside the G-IKEv2 message. The message itself is encrypted and
integrity protected. In addition, each of individual keys inside this message is encrypted too
with a different key(s) (it can be the same key for all encrypted keys or different key for each encrypted key,
but in any case these keys are different from the key protecting the message).
The reason for this construction is to prevent the G-IKEv2 engine which forms and parses 
messages from accessing any sensitive information inside the messages.

The algorithm for protecting the message itself and individual keys inside the message is the same - 
it is one of IKEv2 Encryption transforms 
https://www.iana.org/assignments/ikev2-parameters/ikev2-parameters.xhtml#ikev2-parameters-5
The reason for this is to simplify implementations - the algorithm for protecting the message will be 
supported anyway, so there seems to be no reason to negotiate another one.
In many cases this algorithm will be an AEAD algorithm (like AES-GCM).

The problem is that there may be quite a lot of encrypted keys inside a single message,
and since G-IKEv2 operates over UDP (and over multicast!), the size of the message matters - 
large messages will be fragmented by IP level and due to known issues with firewalls
might not get through, so we want to make the message small. And for each protected key 
the authentication tags would consume almost the same space, as the encrypted content.

So, the design is that even when using an AEAD algorithm, the individual
keys inside the protected message are only encrypted and their authentication tags produced by the AEAD algorithm,
are not transmitted. On a receiving side it must be possible to decrypt keys without performing an integrity check.
Note, that the message itself is encrypted and integrity protected, so we are sure that all message content, 
including all encrypted keys, is not altered.

My questions to the crypto community:
1. Is it generally OK to use AEAD algorithms as pure ciphers.
2. Do existing APIs to AEAD algorithms allow to decrypt an encrypted blob without checking its integrity.

Regards,
Valery.