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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