One of the more difficult issues for OpenPGP-aware E-Mail clients is the support and treatment of different e-mail formats, i.e. PGP/INLINE, PGP/MIME, and the variants thereof. While developers are usually aware of the quirks of their own implementations, it is difficult to find reliable documentation on the matter without doing extensive research on the clients.

I would like to collect on this page the behavior of different implementations, for both sending and receiving e-mail. It is more important to have a general overview here than a deep understanding of each implementation, so information should be kept to the point. I will make a start, feel free to use the same fields or suggest a better way to structure this information when you add your own.

Format overview (did I forget anything?):

• PGP/MIME (RFC1847)
• PGP/MIME (Combined)
• PGP/INLINE as PGP MESSAGE
• PGP/INLINE clearsigned
• PGP/PARTITIONED (partitioned)
• PGP/PEP (definition will follow soon in p≡p development wiki)

• Memoryhole (be the first!)

notmuch-emacs

Supported receiving formats: PGP/MIME
Preferred receiving format: PGP/MIME

Supported sending formats: PGP/MIME; PGP/INLINE Preferred sending format: PGP/MIME

Contacts:

• <notmuch@notmuchmail.org>
• https://notmuchmail.org
• irc:irc.freenode.net:#notmuch

Thunderbird + Enigmail

Supported receiving formats: PGP/MIME, PGP/INLINE
Preferred receiving format: PGP/MIME

Supported sending formats: PGP/MIME; PGP/INLINE Preferred sending format: PGP/MIME

PGP/MIME specifics:

• Sending: only combined method.
• Receiving: combined method preferred; encapsulated format supported.

Inline PGP specifics:

• Encryption: Attachments are Armor-encrypted with file names ending as *.asc
• Signing: an additional attachment with a signature file is added for each attachment.

Enigmail-specific user-agent is off by default
Standard Thunderbird User-Agent: Mozilla/5.0 (<OS/version>) Gecko/20100101 Thunderbird/<version>

Contacts:

• https://enigmail.net
• <enigmail-users@enigmail.net>

K-9 Mail + OpenKeychain

Supported receiving formats: PGP/INLINE, encrypted and clearsigned
Preferred receiving format: PGP/INLINE

Supported sending formats: PGP/INLINE (no attachment support)
Preferred sending format: PGP/INLINE (no attachment support)

Platform limitations and quirks:

The current stable release is based on the original Android Open Source Project E-Mail client, which squashed all mail at receive time into a single html blob to display for performance reasons, irrecoverably losing necessary information for PGP/MIME processing.

Planned development:

Full PGP/MIME support is in the works. The development master has a revised database architecture but is missing a migration mechanism. PGP supported is patched directly into the K-9 code, so there are no technical limitations.

Contact for questions: Vincent Breitmoser <look@my.amazin.horse>

User-Agent Header: "K-9 Mail for Android"

GpgOL

Outlook >= 2010 (Version 2.3)

Supported receiving formats: PGP/MIME, PGP/INLINE, encrypted, clearsigned and opaque
Preferred receiving format: PGP/MIME

Supported sending formats: PGP/MIME (PGP/INLINE optional for mails without attachments)
Preferred sending format: PGP/MIME

Attachment filenames

Attachment file names are always a bit of a problem with outlook as it uses RFC2047 encoding and not RFC 2231

Sending:

GpgOL tries to send filenames encoded like outlook does. This is currently RFC2047. Quoted Printable if less then 20% of the characters need to be encoded otherwise base64. Charset is always UTF-8.

Recieving: GpgOL reads encoding like outlook does. It prefers the "Content-Type name" value.

Ideally you should send someting like thunderbird / enigmail does e.g for "Däß üst äin Töst.txt":

Content-Type: text/plain; charset=UTF-8;
 name="=?UTF-8?B?RMOkw58gw7xzdCDDpGluIFTDtnN0LnR4dA==?="
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
 filename*0*=utf-8''%44%C3%A4%C3%9F%20%C3%BC%73%74%20%C3%A4%69%6E%20%54%C3;
 filename*1*=%B6%73%74%2E%74%78%74

No user agent header. But sends:

X-Mailer: Microsoft Outlook <version>

Contact for questions: Andre Heinecke <aheinecke@intevation.de>

gpg4o

Supported receiving formats: PGP/MIME (only combined yet), PGP/Partitioned, PGP/INLINE
Preferred receiving format: we like all of them :)

Supported sending formats: PGP/Partitioned, PGP/INLINE (clearsigned)
Preferred sending format: PGP/Partitioned for HTML-Mails, PGP/INLINE for Plaintext

No header yet, we will evaluate if we can add one.

Planned development

We are currently researching full PGP/MIME sending support and want to add this asap.

Contact for questions: Alexander Strobel <alexander.strobel@giepa.de>, Daniel Ranft <daniel.ranft@giepa.de>

KMail

Supported receiving formats: PGP/MIME, PGP/INLINE
Preferred receiving format: PGP/MIME

Supported sending formats: PGP/MIME, PGP/INLINE (clearsigned)
Preferred sending format: PGP/MIME

Can send GpgOL compatibile attachment names in PGP/MIME mails when the option "Outlook compatibile attachment names" is configured.

User-Agent Header: KMail/<version>

Contact for questions: Andre Heinecke <aheinecke@intevation.de>

p≡p

Supported receiving formats (OpenPGP compatible only): PGP/MIME, PGP/PARTITIONED, PGP/PEP
Supported sending formats (OpenPGP compatible only): PGP/MIME, PGP/PARTITIONED, PGP/PEP

Preferred sending format for OpenPGP communication partners: PGP/MIME
Preferred sending format for p≡p communication partners: PGP/PEP

x-pep-version: 1.0

Contact for questions: p≡p communication team <pep@pep-project.org>

Memoryhole will be fully supported in case of support of other major contributors

GPGMail

Supported receiving formats: PGP/MIME, PGP/Partitioned, PGP/INLINE
Preferred receiving format: PGP/MIME

Supported sending formats: PGP/MIME, PGP/INLINE (text only, no attachments)
Preferred sending format: PGP/MIME

X-Pgp-Agent: GPGMail

Contact for questions: team@gpgtools.org <team@gpgtools.org>