[linux-elitists] Fwd: PGP signature attachments!
Fri Sep 7 10:51:00 PDT 2001
On Fri, Sep 07, 2001 at 10:44:55AM -0700, Wil Cooley wrote:
> How the hell do you respond to someone like this?
Try Karsten's canned response:
A (not so) Short Rant / FAQ on the Subject of
Signed E-Mail and Public Key Infrastructure
Karsten M. Self <firstname.lastname@example.org>
You're probably reading this because you either stumbled across it
at my website, or I sent it to you in response to an email you sent
me saying you can't read my mail. In the latter case, the short
answer is that:
- Your mailer is broken.
- This is your problem, not mine.
- File a bug report with your vendor.
- I'm going to continue signing my mail, and if you don't
change your end of things, you're going to continue
having problems reading it.
In some cases (you're cute, my mom, or you're offering
sufficient reasons per hour), I'll make exceptions, but
this is on a case-by-case basis, and I'm intentionally
leaving it as a PITA manual process so that each of us
is reminded it's a bad idea: me, when I do it, you,
when I forget and you're stuck with unreadable mail from
me. GET A REAL MAILER.
- No, this isn't a virus, a bomb, a bug, a worm, or any
other executable code. And if it is, that's your
problem, not mine.
- If your IT or MIS department is brain-dead enough to
actually strip off these attachments before you get your
mail, I'm going to laugh at you in public. Sorry, this
ain't the sympathy department. There's a nice rant
below about why this is such a pathetic action, though,
you might enjoy reading it.
The long answer is the rest of this document.
"Your Mail is Weird"
I use a combination of tools in my email to create messages which
are cryptographically signed in such a way that it is readily
possible for the recipient to gain a good level of assurance that
- Originates from me.
- Hasn't been modified in any way en route.
This is sometimes called a digital signature (a technical term, not
to be confused with the recently passed US legislation on
"electronic signatures", regarding legal contractual powers
associated with various, and largely very weak, methods of inserting
corporate hands into your wallet). The system under which it
operates is known as public key infrastructure, and is based on
public key encryption. You're probably going to start hearing a
whole lot about this over the next year or so.
That's the long description.
The short story is that there's a way for me to keep half a secret
and spread the other half to the world in such a way that you can
tell if a particular message came from my half of the secret. It's
The other part:
You're responsible for determining whether or not a communication
that purports to come from me is in fact from me. And if I didn't
sign it, it almost certainly didn't. If the message *is* signed,
it's still your obligation to verify the signature itself.
What is RFC 2015 and Why Can't I Read Your Mail?
There's an Internet standard, called a "request for
comments", or RFC, which covers MIME encoded encryption and
signatures. This is RFC 2015, "MIME Security with Pretty
Good Privacy", (more info below under "Resources"), and
defines how to handle public key infrastructure (PKI)
encryption and authentication via standard MIME protocols.
While it is still a draft standard, it is widely supported
on multiple platforms. There are some pieces of Internet
mail plumbing which break the protocol -- multiple mail
clients ("email applications" to you), as well as some
server applications. LISTSERV and beromail are two I'm
aware of -- but compatibility modes are frequently available
for such software, and in many cases, support is planned in
future upgrades. But that's another story.
If you're interested in the gory technical details, read on.
You should be able to save my email as a text file and open
it in a simple editor (e.g.: Notepad or Write under Legacy
MS Windows). You'll find that the message body content type
of my messages is expressed as:
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline Content-Transfer-Encoding:
...which should be handled properly as inline plain-text
content. If your mailer doesn't present the message body in
this format, you should report a bug to the program
maintainer or vendor.
The signature is presented as:
Again, this is plain text, non-executable, and in no way
represents a threat or possible exploit on your system. For
an intelligent mailer, this should be interpreted, rather
than presented, and used to validate the message content
itself. Otherwise, the content can be presented or
concealed, at the user's preference.
So, Why Do You Insist On Signing Your Mail Anyway?
Part of the reason is for your benefit, where you are the
reader of my mail. It is your responsibility to ensure that
what you are reading as attributed to me is in fact my own
writing. While digital (or sometimes "electronic"
signatures now carry some legal standing, I'm not vesting my
GPG hash with this power. However, you can be pretty
confident that words appearing over my signature, verified
against my public key, were written by me, or by someone who
has access to my computer, my private key, and the pass-key
necessary to utilize it.
Why is it your responsibility? Simple: you know you've
received mail from me. I may or may not know I've sent it.
As is well known, email is an insecure, unauthenticated
medium. It's quite possible that someone is sending
something claiming to be someone they aren't. In fact, this
happens as a matter of course with spam. Since you (the
recipient) have the evidence in front of your eyes, and I've
no idea it's been sent, if it's not from me, the burden of
authentication lies with the recipient.
If it's not signed by me, your assumption should be
that it isn't *from* me.
A large reason though is to encourage and advocate use and
adoption of tools that support public key infrastructure
(PKI) methods, both the ability to create and properly
process signed and encrypted mail. I've found myself at
several times needing to send authenticated or encrypted
mail to persons, only to find that the recipients did not
have a public key, PKI support within their mailer, or even,
at times, a mailer capable of supporting PKI.
It's been suggested variously that I sign messages inline,
or in some instances, that mailing lists drop all
MIME-encoded attachments. I believe this is the wrong
solution for two reasons:
- It breaks useful behavior. MIME attachments *can*
provide useful information, including support of
non-ASCII charactersets, required for basic
communications in much of the world. In the case of
signed messages, a recent SANS alert of the BIND exploit
of the day was copied to a mailing list I'm subscribed
to as cleartext-signed message. The body of the message
was modified in two generations of distribution and the
signature rendered invalid. This is not immediately
apparent as messages which are cleartext- signed must be
verified as a separate, manual, step. In the case of
security exploits and announcements, such verification
and authentication is of some importance.
- It's not the root problem. The root problem is mail
clients which handle untrusted content in an insecure
fashion. This is like dousing 75% of the population
with gasoline, then placing match-confiscating personnel
at the doors of all public arenas. The problem isn't
the matches. It's the gasoline.
Palliative measures to reduce the apparent risk without
addressing the actual cause mask the problem without
fixing it. If sufficient people feel the pain, we'll
eventually see changes either to client behavior or
So Where Can I Find RFC 2015 Compliant Mailers
MUA implementations of RFC 2015:
GNU/Linux / UNIX (most also ported to other platforms via
compatibility kits such as Cygwin, UWIN, MKS, MS Unix
Services for NT, etc.)
emacs (through various mail extensions), exmh,
ishmail, mew (an emacs mail reader), mutt, premail
(Netscape plugin), Gnus 5.6.45 with TM and
Mailcrypt, KMail, Mixmaster 2.9 (internal mail
reader), Sylpheed 0.4.63, TkRat, XCmail, XFMail 1.3.
Legacy MS Windows, Macintosh, and other platforms.
Claris Em@iler with PGP 5 (?), Datula (plugin), Edmax
(plugin), MS Outlook Express (plugin), MS Outlook
(plugin), Mulberry (plugin), PMMail (native), Qualcomm
Eudora (full and light version for Windows and Mac)
since version 3.02 with PGP 5.5.3i and 6.0.2i Plugin,
Turnpike (native), Voodo on Amiga.
Got Any Funny Clueless Luser Stories For Us?
Funny you should ask.
One particularly illuminating response I've receive runs
more or less as follows:
My company's MIS department has recently configured the
email system so that if an email has suspected attachment,
it will not be delivered. Instead, the recipient gets the
This message uses a character set that is not
supported by the Internet Service. To view the
original message content, open the attached message.
If the text doesn't display correctly, save the
attachment to disk, and then open it using a viewer
that can display the original character set.
If you try to save it as instructed, you will see another
<<< MIME ATTACHMENT STRIPPED >>>
I keep getting empty emails as the result of this
<name deleted to protect the lusers>
This prompted the following response from me:
So let me get this right.
You use a mail client which allows, among other things,
You use a mail client which, among other things, includes
executable content to be sent as attachments.
You use a mail client which, among other things,
*automatically executes* this content, without verifying its
source or asking for user intervention. As an added bonus,
there is content which *does* require the user to launch it
- Mail and/or OS features disguise the fact that the
attachment is, in fact, an executable, by hiding such
extensions as might actually reveal such a fact.
- A file content coding scheme (file extensions) which is
bypassed by the fact that a program can be coded to open
content which should be safe (say, a text or RTF
document) but then proceeds to allow execution of unsafe
content within it (MS Word macro viruses).
- There is no ready tool for looking at the raw
(text/binary) form of the attachment to determine its
true Buddha nature. I've got raw text and binary
viewers at my fingertips, and use them.
- OS features and security are such that unprivileged
users can wreak havoc on their own systems, networked
storage, and other users systems, without protections
afforded by sane filesystem security, user permissions,
and file organization.
- OS and application features are such that users
routinely send, and are expected to utilized, arbitrary
content, much of which may be executable. Which might
be translated as "the user is an idiot", but is
conditioned by the fact that the user has been trained
that acting like an idiot: running arbitrary software,
or engaging arbitrary methods, which may or may not
include executing code, on arbitrary content, is not
only a perfectly acceptable standard of operation, but
_is required to perform basic job functions_.
- In order to compensate for all these "features", your
system administrators have seen fit, in their divine
wisdom, to extricate all attachments from email.
Including such attachments as might actually serve to
provide some level of authentication as to whether or
not the source of a particular email is who it claims to
be, and possibly even a trust level associated with
And this is now a problem for third party sites to deal with
on your behalf?
I'm sorry. I don't follow the logic.
This is your problem, not mine.
If you're going to strip all such features from your email, why
don't you just go back to a plain text mailer and stop asking the
rest of the world to please stop passing bombs your way with fuses
you insist on lighting.
Some additional informational resources for your benefit:
- RFC 2015, "MIME Security with Pretty Good Privacy", M. Elkins,
http://www.cis.ohio-state.edu/cgi-bin/rfc/rfc2015.html , spells
out the standards for encrypted and cryptographically signed
email. Note that signature-as-attachment is required by
Note also that munging the content of multipart/signed messages
violates RFC2015. This addresses issues with several broken
mailing list management software packages.
- For a list of mailers supporting RFC2015, see:
- For information on GnuPG, the GNU Privacy Guard, see
Copyright (c) 2001, Karsten M. Self <email@example.com>
This document may be freely distributed with attribution.
More information about the linux-elitists