Hi all, To my understanding, DTLS 1.3 defines AEAD additional data for record protection as the record header as seen on the wire. Quoting Draft 37, Section 4:
``` The entire header value shown in Figure 4 (but prior to record number encryption) is used as as the additional data value for the AEAD function. For instance, if the minimal variant is used, the AAD is 2 octets long. Note that this design is different from the additional data calculation for DTLS 1.2 and for DTLS 1.2 with Connection ID. ``` I would like to suggest that DTLS 1.3 uses a structured representation of the record header instead, as do all other versions of [D]TLS as far as I understand. The reasons for this are as follows, in decreasing order of my perception of importance: - Omission of Connection ID Regarding the presence of Connection IDs in multiple records within a single datagram, Draft 37 says: ``` Implementations which send multiple records in the same datagram SHOULD omit the connection id from all but the first record; receiving implementations MUST assume that any subsequent records without connection IDs belong to the same assocatiation. ``` This means that the Connection ID for non-initial records in a datagram containing multiple records is _not_ part of the AEAD additional data for those records, which seems wrong. Concretely, one could inject such non-initial records into other datagrams using different CIDs, and the record protection wouldn't notice it. One might argue that CID shouldn't be part of the AEAD in the first place, but in any case, I believe the treatment should be uniform and not distinguish between initial and non-initial records in a datagram. - Modularity Decoupling the wire-presentation of the record header from record protection allows to implement record protection and the choice of record header independently: One piece of the implementation can take care of record protection - using the structured presentation of the record header - while another takes care of the wire-encoding. It is even possible to change the record header format in transit. One might of course turn this argument around and say that such modifications are unwanted and using the header on-the-wire as the AEAD allows to detect them, but I don't yet see the concrete problem with them so far. - Simplicity At first it seems that using the record header as an unstructured binary blob for AEAD makes things simpler, but I don't think this is the case: Prior to record decryption, the record sequence number needs to be decrypted, and for that purpose, the record header already has to be parsed. Hence, at the time of record decryption, the record header is already be present a modified, structured form, and retaining the corresponding modified binary form appears to create additional complexity which would be avoided if record protection would use the structured header presentation. - Uniformity with other [D]TLS versions Let me know what you think, Best, Hanno IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.
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