[messaging] Regarding the "AD" Value in X3DH Spec

[Nadim Kobeissi]

Hello,

In Section 3.3 of the X3DH specification [1], the following detail is
mentioned:

"Alice [...] calculates an "associated data" byte sequence AD that contains
identity information for both parties. Alice may optionally append
additional information to AD, such as Alice and Bob's usernames,
certificates, or other identifying information."

"Alice then sends [...] an initial ciphertext encrypted with some AEAD
encryption scheme [4] using AD as associated data and using an encryption
key which is either SK or the output from some cryptographic PRF keyed by
SK."

My question is the following: Is this "AD" value new? It's been mentioned
in the spec since its publication, but I'm not sure when it was introduced
in the implementations that pre-date the specification's publication.

Regards,
Nadim

[1] https://whispersystems.org/docs/specifications/x3dh/
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