GPG: Are attachments or headers also encrypted on C-c C-m C-c?

GPG: Are attachments or headers also encrypted on C-c C-m C-c?

[Marius Hofert]

Hi,

Is a message encrypted with GnuPG (PGP/MIME) via C-c C-m C-c (or signed and encrypted with C-c C-m
C-e) fully encrypted in the sense that attachments are also encrypted? 

I would have guessed that only the main part/body of the email (without subject
headers or attachments) are encrypted, but I couldn't find sufficient information on this. 

Cheers,

Marius

Re: GPG: Are attachments or headers also encrypted on C-c C-m C-c?

[Kevin Brubeck Unhammer]

Marius Hofert <marius.hofert@math.ethz.ch> writes:

> Hi,
>
> Is a message encrypted with GnuPG (PGP/MIME) via C-c C-m C-c (or
> signed and encrypted with C-c C-m
> C-e) fully encrypted in the sense that attachments are also encrypted? 
>
> I would have guessed that only the main part/body of the email (without subject
> headers or attachments) are encrypted, but I couldn't find sufficient
> information on this.

I just tested by doing `C-c C-m C-c' and then `C-c C-m f', sending to a
webmail. The webmail showed the unencrypted subject, so I assume all
headers are unencrypted. The body was empty, and there were two
attachments:


$ file Attachment*
Attachment1: ASCII text
Attachment2: PGP message
$ cat Attachment1
Version: 1
$ gpg -d Attachment2 > Attachment2.decrypted
# [enter passphrase]
$ file Attachment2.decrypted
Attachment2.decrypted: multipart/mixed; boundary="=-=-=", 


From what I can tell, the (contents of the) file I attached does not
appears unencrypted anywhere in the email sent by Gnus; everything is
encrypted[1].

But of course, if you want to make really sure that it's encrypted
before sending, you could simply

$ gpg -r alice@example.com -e myattachment.txt

and attach the resulting myattachment.txt.gpg


[1] Apropos, I'm pretty sure K-9 mail on Android does _not_ encrypt
    attachments, yet.

-- 
Kevin Brubeck Unhammer

GPG: 0x766AC60C

Re: GPG: Are attachments or headers also encrypted on C-c C-m C-c?

[Marius Hofert]

> > Hi,
> 
> >
> 
> > Is a message encrypted with GnuPG (PGP/MIME) via C-c C-m C-c (or
> 
> > signed and encrypted with C-c C-m
> 
> > C-e) fully encrypted in the sense that attachments are also encrypted? 
> 
> >
> 
> > I would have guessed that only the main part/body of the email (without subject
> 
> > headers or attachments) are encrypted, but I couldn't find sufficient
> 
> > information on this.
> 
> 
> 
> I just tested by doing `C-c C-m C-c' and then `C-c C-m f', sending to a
> 
> webmail. 

Hi Kevin,

Many thanks for your quick reply. 

What exactly did you send? 

I assume your sent message was encrypted and appeared as the encrypted attachment 2 on the webmail. But what did you send such that it appeared (unencrypted) as attachment 1? Was it a .txt attachment containing "Version: 1"? If so, it wasn't encrypted ... 

I was wondering if, for example, a standard .pdf document attached will appear at the recipient encrypted as .pdf.gpg? If so, this would not require oneself to encrypt it manually (although I agree that this is safer)

Cheers,

Marius

> The webmail showed the unencrypted subject, so I assume all
> 
> headers are unencrypted. The body was empty, and there were two
> 
> attachments:
> 
> 
> 
> 
> 
> $ file Attachment*
> 
> Attachment1: ASCII text
> 
> Attachment2: PGP message
> 
> $ cat Attachment1
> 
> Version: 1
> 
> $ gpg -d Attachment2 > Attachment2.decrypted
> 
> # [enter passphrase]
> 
> $ file Attachment2.decrypted
> 
> Attachment2.decrypted: multipart/mixed; boundary="=-=-=", 
> 
> 
> 
> 
> 
> >From what I can tell, the (contents of the) file I attached does not
> 
> appears unencrypted anywhere in the email sent by Gnus; everything is
> 
> encrypted[1].
> 
> 
> 
> But of course, if you want to make really sure that it's encrypted
> 
> before sending, you could simply
> 
> 
> 
> $ gpg -r alice@example.com -e myattachment.txt
> 
> 
> 
> and attach the resulting myattachment.txt.gpg
> 
> 
> 
> 
> 
> [1] Apropos, I'm pretty sure K-9 mail on Android does _not_ encrypt
> 
>     attachments, yet.
> 
> 
> 
> -- 
> 
> Kevin Brubeck Unhammer
> 
> 
> 
> GPG: 0x766AC60C

Re: GPG: Are attachments or headers also encrypted on C-c C-m C-c?

[Kevin Brubeck Unhammer]

Marius Hofert <marius.hofert@math.ethz.ch> writes:

>> > Hi,
>> 
>> >
>> 
>> > Is a message encrypted with GnuPG (PGP/MIME) via C-c C-m C-c (or
>> 
>> > signed and encrypted with C-c C-m
>> 
>> > C-e) fully encrypted in the sense that attachments are also encrypted? 
>> 
>> >
>> 
>> > I would have guessed that only the main part/body of the email (without subject
>> 
>> > headers or attachments) are encrypted, but I couldn't find sufficient
>> 
>> > information on this.
>> 
>> 
>> 
>> I just tested by doing `C-c C-m C-c' and then `C-c C-m f', sending to a
>> 
>> webmail. 
>
> Hi Kevin,
>
> Many thanks for your quick reply. 
>
> What exactly did you send? 
>
> I assume your sent message was encrypted and appeared as the encrypted
> attachment 2 on the webmail. But what did you send such that it
> appeared (unencrypted) as attachment 1? Was it a .txt attachment
> containing "Version: 1"? If so, it wasn't encrypted ...

The first attachment, named Attachment1, and containing the text
"Version 1", must be something Gnus or message-mode creates (presumably
to note which OpenPGP version is used?). It seems that every time I
encrypt an email, what really happens is that Gnus sends an email with
two attachments, Attachment1 just containing "Version 1", and
Attachment2 being a PGP encrypted file containing the email body +
anything I attached.

So the attachment that _I_ attached with `C-c C-m f' was part of the
file Attachment2, of which everything was encrypted. 

But don't take my word for it, just try it yourself, e.g. with some
webmail system. If you want to send to your throwaway webmail at
foo@bar.com, and you don't have an encryption key, you can do (setq
mm-encrypt-option 'guided) first, then when you `C-c C-c', you'll be
given the option to encrypt using your own public key.

> I was wondering if, for example, a standard .pdf document attached
> will appear at the recipient encrypted as .pdf.gpg? If so, this would
> not require oneself to encrypt it manually (although I agree that this
> is safer)
>
> Cheers,
>
> Marius

If you attach a pdf, it'll be embedded in the file Attachment2. Note
that when you open that email in _Gnus_, not some dumb webmail, Gnus
automatically figures out that Attachment1 and Attachment2 represent an
encrypted email, and shows it like


[[PGP Encrypted Part:OK]]
[1. application/octet-stream; somefile.pdf]...

Hi Alice,
See attached pdf file.
-- 
Bob
[[End of PGP Encrypted Part]]


That is, at least it will if you've got

#+begin_src emacs-lisp
(setq mm-verify-option 'always
      mm-decrypt-option 'always
      gnus-buttonized-mime-types '("multipart/encrypted"
				   "multipart/signed"))
#+end_src

in your ~/.gnus.

I assume K9 and Thunderbird with Enigmail use the same conventions,
though I haven't tried.


-- 
Kevin Brubeck Unhammer

GPG: 0x766AC60C

Re: GPG: Are attachments or headers also encrypted on C-c C-m C-c?

[Marius Hofert]

Very good, many thanks for the detailed explanations, Kevin.

Cheers,

Marius