Case:
If you receive an encrypted mail (including subject encryption) from someone using Thunderbird, the email will have its subject “…” or “Re: …” in K9Mail and I see no way of deciphering it, it seems it gets lost at this point. The “…” is the placeholder subject in Thunderbird until you click on it and it gets decrypted, at which point the actual subject is shown, but not in K9.
Is this a known issue? Any workarounds?
For me the same but i dont know why
The way Thunderbird does this by means of a nonstandard GnuPG/PGP extension. At some point in time, it made it into the mainline code of Thunderbird.
K-9 does not handle encrypted mails itself. Instead, cryptography is delegated to another app (often, this is OpenKeychain). These other apps do neither know nor understand how Thunderbird encrypts the subject. If th y do subject encryption, the use other methods (which might be similar, but still different).
To be more precise: Thunderbird replaces the subject with “…”. Then, the cipher body is amended with a subject replacement link. Finally, the link is added by concatenation of the encrypted subject text to the MIME body. - This is the reason wh you cannot encrypt the subject for plaintext-encrypted mails; it only works in PGP-MIME mails.