A Short Rant / FAQ on the Subject of
Signed E-Mail and Public Key Infrastructure
Karsten M. Self firstname.lastname@example.org
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 the message:
- 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
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 pretty cool.
The other part:
You're responsible for determining whether or not a
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.
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
end of things, you're going to continue having problems reading it.
- No, this isn't a virus, a bomb, a bug, a worm, or any
- If your IT or MIS department is brain-dead enough to
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.
There's an Internet standard, called a "request for comments",
which covers MIME encoded encryption and signatures. This is RFC 2015
(more info below under "Resources").
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
...which should be handled properly as inline plain-text
your mailer doens't present the message body in this format, you should
report a bug to the program maintainer or vendor.
The signature is presented as:
For an intelligent mailer, this should be interpreted, rather
presented, and used to validate the message content itself. Otherwise,
the content can be presented or concealed, at the user's preference.
You might ask why I insist on signing my mail. Fair question.
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 passkey necessary to utilize it. Consider it the
equivalent of an electronic signet ring.
I've been active on the Internet for over fifteen years. I've
dozens of email accounts, and have had my identity spoofed, publicly, in
front of those who know me well. By adopting the practice of signing
every piece of email I send, I'm establishing a practice of plausible
deniability in the event unauthorized communications are made in my
name. If it's not signed by me, your assumption should be that it isn't
*from* me. No, the system's not perfect, but it's a hell of a sight
better than nothing.
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
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
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
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.
Pallative measures to reduce tha apparent risk without
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 choice.
One particularly illuminating response I've receive runs more
or less as
> system so that if an email has suspected attachment, it will not be
> delivered. Instead, the recipient gets the following message:
> 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 message:
> <<< MIME ATTACHMENT STRIPPED >>>
> I keep getting empty emails as the result of this reconfiguration.
> <name deleted to protect the imbecillic>
This prompted a response:
So let me get this right.
You use a mail client which allows, among other things, attachments.
You use a mail client which, among other things, includes
content to be sent as attachments.
You use a mail client which, among other things,
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 but:
- Mail and/or OS features disguise the fact that the
is, in fact, an executable, by hiding such extensions as might
actually reveal such a fact.
- A file content coding scheme (file extentions) which is
by the fact that a program can be coded to open content which
should be safe (say, a text or RTF document) but then procedes
to allow execution of unsafe content within it (MS Word macro
- There is no ready tool for looking at the raw (text/binary)
of the attachment to detrmine 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
wreak havok 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
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
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 this.
And this is now a problem for third party sites to deal with
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,
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.
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 RFC2015.
Note also that munging the content of multipart/signed
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