DDSSEC12 — Parsing messages generated by encode_serialized_payload (auth only)

When "performing authentication only," the encode_serialized_payload operation wraps the SerializedPayload inside CryptoHeader and CryptoFooter. This resulting byte stream takes the place of the original SerializedPayload submessage element in a Data(Frag) submessage.

On the receiving side, this modified submessage element is passed to decode_serialized_payload. The problem comes in parsing this CryptoHeader-SeralizedPayload-CryptoFooter group.

The CryptoHeader is of fixed size and only contains octet-width data (therefore has no padding), so parsing it and determining where SerializedPaylod starts is trivial.

Then the implementation needs to determine where SerializedPayload ends in order to determine which bytes to authenticate. There is no in-stream indication of where the SerializePayload ends.

One possibility would be to look at the end of the byte sequence that decode_serialized_payload received and "step backwards" by the length of the CryptoFooter, however the CryptoFooter is variable length (with receiver_specific_macs). Even if the implementation has external knowledge that receiver_specific_macs are not in use, the alignment requirement of the plugin_sec_tag.receiver_specific_macs.length effectively makes this a variable-length element (also see issue #58).

To resolve this, an additional element for "length" could be added before the SerializedPayload, just like the CryptoContent submessage element does. This would make parsing the encoded payload similar for the encrypt and auth-only cases.