Protect email experience not Subject:s (hypothesis, draft)

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Protect email experience not Subject:s (hypothesis, draft)

Bernhard Reiter-7
Hello,
for many months now, my feeling is growing that

  encrypted subject headers in emails
  shift the security balance in the wrong direction.

So I want to summarize and explore this hypothesis.
Here are some draft thoughts and notes.
Feedback welcome (either to me directly or on the list).

Not sure yet, where to place all this, but in the best tradition
I'll announce my intentions here and will follow up as I learn.

Best Regards,
Bernhard


== Idea of protected headers.
The subject of an email delives contents related to the body of the mail.
To raise the confidentiality (as one aspect of security),
it is proposed to do end-to-end encryption on the subject Header-Field.

== Hypothesis
The subject Header-Field is also meta data, needed to keep email usable.
Or in other words: to support the availability aspect of security.

Because email is the most popular decentral communication solution [5]
compatibility with existing installations and existing ways of working for
diverse user groups is more important than in other softwar contexts.

From an implementers point of view, protected headers seem to make
it more complicated and break some ways to implement good access
to emails.

Examples:
* IMAP servers can search emails by subject. A feature that cannot
  be kept functional with inaccessible contents.
* Fast access to filtering in clients by subject is broken, because
  it would mean indexes and indexes would need to come from a crypto
  secure store, one each per crypto context (private key).
* There are many old or non-header-decrypting clients around, otherwise fine
  and stable.

As observable metadata cannot be avoided totally, maybe the alternative to
automating the Header-Fields is to help people manage the different levels of
security they need per occasion.

Thought experiment:

If it is understood that the header section is like notes
on a paper envelope, needed for mail transport and to be able to be seen by
the transporting agents, this can be used to assess what can be learned
from it. And then common ways of distracting from the contents can be used.
(I write 'common ways', because this is a core of my concept about how to get
end-to-end encryption - especially email - more usable: People already know
social ways how to deal with different levels of confidentiality. Sofware
application need not to hide it the aspects too much.)

As a consequence encrypting the subject of an email can be seen as
contributing only very little to the confidentiality. The whole exchange has
to done so that it looks unsuspicous anyway.

Potential conclusion:
Encrypted subject headers contribute a little bit to confidentiality,
but they also lower availability for many cases (by the implementation
complications). They should not be send by default.


== Valid use cases?
Where the "Subject:" is a lot more than a writing on the envelope.

 * Example: a roundup-tracker fully run with OpenPGP/MIME mails,
   by default it changes the title of an issue and there can be
   commands to control the issue in the subject. (Also an example
   where backwards compatiblity failed.)

Implementation idea: per recipient (group) settings to explicitely
enable encrypted subjects for those groups and contexts where it is
known to be more useful.


== Material
[1]  https://github.com/autocrypt/memoryhole
  archived in favour of [2]
  lists some alternatives and links elder discussions  

[2] https://github.com/autocrypt/protected-headers
    Old source code repo of [3]?

[3] https://datatracker.ietf.org/doc/draft-ietf-lamps-header-protection/
   "Header Protection for S/MIME
   draft-ietf-lamps-header-protection-02 (Last updated 2020-11-02)"
   also aiming at OpenPGP/MIME?

[4] A widely spread version of Thunderbird v78 did not allow
    disabling the encryption of subjects.
   
https://support.mozilla.org/en-US/kb/openpgp-thunderbird-howto-and-faq#w_can-i-disable-the-encryption-of-the-email-subject

[5]  Mahn, Jan, 2021, c't 2021/03 pp50-53
     "Nichts zu ­verbergen?
      Sicher und vertraulich kommunizieren: Ein Grundrecht"
     https://www.heise.de/select/ct/2021/3/2030913061555053970

--
www.intevation.de/~bernhard   +49 541 33 508 3-3
Intevation GmbH, Osnabrück, DE; Amtsgericht Osnabrück, HRB 18998
Geschäftsführer Frank Koormann, Bernhard Reiter, Dr. Jan-Oliver Wagner

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Re: Protect email experience not Subject:s (hypothesis, draft)

GnuPG - User mailing list
Hi,

On Friday 29 January 2021 17:52:25 CET Bernhard Reiter wrote:
> for many months now, my feeling is growing that
>
>   encrypted subject headers in emails
>   shift the security balance in the wrong direction.

I share that feeling. My goal that encrypted mails do not feel much different
from unencrypted mails is made harder by subject encryption. So in a security
VS. usability standpoint that assumes that if usability is bad, users will not
encrypt mails or at least fewer mails I come to the same conclusion.

This discussion is very relevant for me because GpgOL is starting to include
protected-headers mime parts with the next version to transfer To and CC
information. Putting the subject into it would be easy but it's more of a
policy decision if we want to encourage or discourage this.

> If it is understood that the header section is like notes
> on a paper envelope, needed for mail transport and to be able to be seen by
> the transporting agents, this can be used to assess what can be learned
> from it. And then common ways of distracting from the contents can be used.
> (I write 'common ways', because this is a core of my concept about how to
> get  end-to-end encryption - especially email - more usable: People already
> know  social ways how to deal with different levels of confidentiality.
> Sofware application need not to hide it the aspects too much.)

I agree with the mental image of notes on an envelope, this is also how I try
to explain the Subject. We could probably try to explain this better. E.g. by
showning this as information once the first encrypted mail is sent.

> == Valid use cases?
> Where the "Subject:" is a lot more than a writing on the envelope.
>
>  * Example: a roundup-tracker fully run with OpenPGP/MIME mails,
>    by default it changes the title of an issue and there can be
>    commands to control the issue in the subject. (Also an example
>    where backwards compatiblity failed.)
>
> Implementation idea: per recipient (group) settings to explicitely
> enable encrypted subjects for those groups and contexts where it is
> known to be more useful.
I'm not sure, if the user configures such rules by themself they already have
an awareness that they don't really need automation for this. And if an Admin
preconfigures this for a whole instiution we have the bad user expierence that
the subject is "sometimes" encrypted. That would be even more confusion.

Currently for GpgOL I'm tending to a global option to encrypt the subject
which would be off by default and show a warning when it is activated that
recipients will only see "..." in their message list and threading etc. will
be broken. Just having the option and a warning related to the option could
raise awareness about the issue.


Best Regards,
Andre

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Re: Protect email experience not Subject:s (hypothesis, draft)

Bernhard Reiter-7
In reply to this post by Bernhard Reiter-7
Am Freitag, 29. Januar 2021, 17:52:25 CET schrieb Bernhard Reiter:
> From an implementers point of view, protected headers seem to make
> it more complicated and break some ways to implement good access
> to emails.

As Thunderbird as enabled "encrypted" subjects by default with 78
and additionally did not offer a way to disable this initially,
it created a number of real world examples.

Thunderbird forced this change on users, according to this
   https://support.mozilla.org/en-US/questions/1304451
there is only a hidden (expert) setting to disable it since 78.5.1.

Mentioned drawback:
 * Breaks filtering for the using company.
   " It's very important for mail filtering rules to be able to read the
      subject without opening the mail first."

Best Regards,
Bernhard
--
www.intevation.de/~bernhard   +49 541 33 508 3-3
Intevation GmbH, Osnabrück, DE; Amtsgericht Osnabrück, HRB 18998
Geschäftsführer Frank Koormann, Bernhard Reiter, Dr. Jan-Oliver Wagner
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