GPG Mail: Can't decrypt email sent via Windows Outlook gpg4o PGP/Inline (Lacking PGP/MIME support for sending messages)

N's Avatar


25 Mar, 2021 12:04 PM

Me and another Mac users I know can no longer decrypt email sent via the outlook extension. The applies to manual command line decryption and the Mail extension. Is doesn't seem to be recognised as a valid PGP format.

This is causing a lot of frustration on both sides as Windows users need to perform the encryption manually and Mac users have to ask them to resend everything.

  1. Support Staff 1 Posted by Steve on 25 Mar, 2021 12:32 PM

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    Hi N,

    welcome to the GPGTools support platform. Sorry to hear you are having problems using GPG Suite.

    Do you know which OpenPGP implementation for Outlook is used for encryption on the side creating the encrypted messages? And do you know which format they are using? Could you ask them to make sure to use PGP/MIME and see if you are then able to decrypt as expected.


  2. Support Staff 2 Posted by Steve on 25 Mar, 2021 12:35 PM

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    I see we already had a conversation about this in december 2020:

    Please provide screenshots and eml files of problematic messages so we can take a closer look.

  3. 3 Posted by N on 25 Mar, 2021 03:39 PM

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    I don’t have an outlook account to send a test email from, and I can’t send screenshots of other people’s emails. I have asked 4 other MacBook users and we are all experiencing the same problem.

    The implementation seems to be gpg4o. I can decrypt the text via command line but not attachments such as word documents. Apple mail doesn’t even see the email as PGP.. I will suggested making sure the type is PGP/MINE.

  4. 4 Posted by N on 25 Mar, 2021 03:46 PM

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    The email body just shows:

    -----BEGIN PGP MESSAGE----- hQI {rest of encypted email} ln -----END PGP MESSAGE-----

    and the comment is:

    gpg4o v6.0.9.9529

    I know it is harder without screenshots, but I would rather not send any work-related information just in case.

  5. Support Staff 5 Posted by Steve on 25 Mar, 2021 07:34 PM

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    Hi N,

    gpg4o is the Outlook OpenPGP plugin for Outlook on Windows and developed by Tesis SYSware.

    At this moment gpg4o supports the PGP/MIME format only when receiving messages. For unknown reasons it does not support PGP/MIME when sending emails. This results in messages that may not be decrypted by GPG Mail and make manual decryption neccessary.

    Excursus about PGP/MIME

    GPG Mail defaults to the only standard there is for sending OpenPGP Mails, which is PGP/MIME. The format gpg4o is sending encrypted messages in is called PGP/Inline and is an undocumented non-standard format, which leads to several problems.

    If you want to read more about the deficites of PGP/Inline you may want to read this note of Daniel Kahn Gillmore called Inline PGP signatures considered harmful. This is the GnuPG FAQ entry covering this question.

    To deal with this situation there are a few options:

    1. ask your contact to switch to a modern email client. Thunderbird for example is able to handle PGP/MIME messages, even if run on an old windows machine
    2. ask TESIS SYSware to implement support for sending PGP/MIME messages

    Let me know if that was helpful,

  6. 6 Posted by N on 26 Mar, 2021 01:37 PM

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    Thanks, that is very useful.

    I can manually decrypt the message body, but can’t see any attachments.

    Unfortunately, the majority of Windows users seem to use that extension. Would you consider supporting reading PGP/Inline? Whilst I can see why you don’t want them sending in this format, I think it is reasonable to be able to receive them.

    Windows users vastly outnumber Mac users so it isn’t reasonable to ask them all to switch to Thunderbird. I wasn’t having issues before the latest big GPG Suite update.

  7. Support Staff 7 Posted by Steve on 26 Mar, 2021 01:49 PM

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    We have a ticket for this problem. I connected this discussion with the existing ticket. That means, should this discussion get closed, it will be re-opened as soon as the ticket is closed. That way you stay in the loop and will receive info as soon as we have news. Feel free to open a new discussion should you run into further problems or need assistance.

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