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moktor avatar Crypto

Is it safe to use AES-CCM only for authentication - i.e. for sending AAD without encrypted data?

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moktor

We are employing an AES-CCM 128-bit stream-cipher with 7-Byte Nonces and 12-Byte Authentication Tags in a communication protocol. Up until this point there was no need to use Additional Authenticated Data (AAD) in this protocol, as all transmitted data - apart from the Nonce - was encrypted.

In the meantime, some new networking-related requirements came up which may require one message-field to be transmitted as AAD instead of within the ciphertext. Some of the transmitted messages would then actually only consist of this one authenticated but plaintext field. There would be no additional ciphertext in these messages. The intendent message-recipient will always have to authenticate the message before actually interpreting it.

Would this be viable/safe to do or does this compromise the security of the system in any way?

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Maarten Bodewes avatar Crypto
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Maarten Bodewes

The NIST specification of CCM (in full: "Recommendation for Block Cipher Modes of Operation: The CCM Mode for Authentication and Confidentiality"): has an explicit remark about this in section 5.3:

The payload may also be empty, in which case the specification degenerates to an authentication mode on the associated data.

So yeah, it is safe and it doesn't compromise security. This is generally true for any authenticated cipher by the way.

As a sidenote, as with almost any cipher, any adversary can of course see that there is no ciphertext - and since CTR is used, it commonly knows the exact size of each plaintext message.

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