[openpgp] Session-Key-Reuse and Intended Recipient

[Kai Engert <[e-mail]>]

At the recent OpenPGP for email summit [1], we had a session in which we 
discussed the practicality of the Intended Recipient Subpackage (IRS) 
for PGP/MIME email. It was mentioned that it's recommended to use 
protected headers in such messages. With that, the intended recipients 
are already known. Having equivalent information twice, e.g. both 
protected headers with the TO/CC fields, plus IRS, might be 
unnecessarily complex, and MUA implementations might prefer to avoid the 
IRS.

Based on that, would it make sense to make the following two changes to 
the crypto-refresh document, in the places that mention IRS ?

Change 1:
=========

Section 5.2.3.7 currently says:

"To avoid surreptitious forwarding (see Section 13.12), implementations 
SHOULD also implement the "Intended Recipients" subpacket."

This could be changed to:

"To avoid surreptitious forwarding (see Section 13.12), implementations 
SHOULD also implement the "Intended Recipients" subpacket, unless the 
signed data uses an application data format that already lists the 
intended recipients."

Change 2:
=========

Section 13.8 currently says:

"Implementations are encouraged to use the Intended Recipient 
Fingerprint (Section 5.2.3.36) subpacket when composing messages and to 
use it to check the consistency of the set of recipients of a message 
before replying to it with session-key reuse."

This could be changed to:

"When composing messages, implementations are encouraged to check the 
consistency of the set of recipients of a message before replying to it 
with session-key reuse, either by using the Intended Recipient 
Fingerprint (Section 5.2.3.36) subpacket, or by using equivalent 
information contained in the signed data."

Kai

[1] https://wiki.gnupg.org/OpenPGPEmailSummit202305