Presumably you are reading this because you received an e-mail from me with this signature:
This message was digitally signed as a security measure. If you're curious, or worried about an "unknown attachment", see http://cromwell-intl.com/digsig/ -===---=-=-=-=---=-----------=-===-===---=-===---=-===-=-=---=-===-=-=---=-=-=- All unencrypted communication by Internet, telephone, and fax is subject to interception and archiving. Belief otherwise is folly. Belief that this is somehow changed by stern corporate announcements of wishes for deletion by unintended recipients is arrogant folly. -------=-=-=-=---=-===---=-=-=-===---=---------=---=-===---=-===-=---=-=-=-----
A digital signature is a security mechanism based on cryptography. It allows someone to verify two crucial aspects of information security:
If the digital signature can be verified, then you can have very high confidence in data integrity and sender identity. If it cannot, then either the data has been somehow modified or it is an attempt to spoof the identity of another sender. You cannot tell precisely how it was modified — what was changed, added, or deleted — just that something was done to it.
The simple answer, what you would need to do, is import my public key into your PGP keyring and make sure that your mail tool uses it.
Unfortunately, despite the fact that my messages are signed using the OpenPGP standard, a standard dating from November 1998, many people seem to use mail tools that do not understand it. If you're reading this because you were puzzled or worried by mysterious "unknown attachments" reported by your mail tool, then you are one of those people.
If you're curious about the cryptography, about the mechanics
(or really the mathematics) of how a digital signature is
created and verified,
see my "Just Enough Cryptography" page
for an overview.
You will need to look at these sections:
— Terms
— Encryption and Decryption
— Symmetric versus Asymmetric Cryptography
— Hashes
— Digital Signatures
You must understand that digital signatures do not provide confidentiality.
While they are based on cryptography, digital signatures do not encrypt the message. Anyone can read a digitally signed message. The signature itself is just a distraction or is ignored if you or your mail software do not use it.
All communication on public telecommunication networks — Internet, telephone, facsimile, etc — is subject to interception and archiving. It is easy for governments to do this because Internet and telephone traffic must pass through a limited number of backbone interconnection points. The governments simply obligate the telecommunications companies to provide access, or even to do the data collection on behalf of the government.
Yes, this process has been greatly expanded in the U.S. during the Cheney/Bush administration, but it had already been underway for many years. See, for example:
The only defense, potentially very powerful if done very carefully, is to encrypt the message. The encrypted message, the ciphertext, can still be intercepted and archived. However, the intercepting agency would have to decrypt the message to make any sense of it beyond the fact that at some time person A sent a message to person B.
Again, see my "Just Enough Crypto" page to see what would be involved to protect your communication, and what would be required to attack the encryption.
Below is an actual message. First, notice the header field specifying the boundary between message components. The randomly generated delimiter nextPart1304702.zeePTlEpVM is highlighted where it appears in the message body to separate the message itself from the signature. The sender's mail application randomly generated this distinctive string that does not appear within the message body.
Then the message body itself has a green background, and the PGP digital signature has an orange background.
From bob.cromwe11@comcast.net Sun Dec 16 11:55:04 2007
From: Bob Cromwell <bob.cromwe11@comcast.net>
Reply-To: bob.cromwe11@comcast.net
Organization: Cromwell Intl
To: cromwe11@ecn.purdue.edu
Subject: Here is an example of an OpenPGP message
Date: Sun, 16 Dec 2007 11:55:04 -0500
User-Agent: KMail/1.9.7
MIME-Version: 1.0
Content-Type: multipart/signed;
boundary="nextPart1304702.zeePTlEpVM";
protocol="application/pgp-signature";
micalg=pgp-sha1
Content-Transfer-Encoding: 7bit
Message-Id: <200712161155.11830.bob.cromwe11@comcast.net>
Status: RO
X-Status: RSC
X-KMail-EncryptionState:
X-KMail-SignatureState:
X-KMail-MDN-Sent:
--nextPart1304702.zeePTlEpVM
Content-Type: text/plain;
charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
Content-Disposition: inline
This is the message body. My signature will appear below.
All of the message has been protected with cryptographic
"tamper detection".
Bob
--
This message was digitally signed as a security measure. If you're curious, or
worried about an "unknown attachment", see http://cromwell-intl.com/digsig/
-===---=-=-=-=---=-----------=-===-===---=-===---=-===-=-=---=-===-=-=---=-=-=-
All unencrypted communication by Internet, telephone, and fax is subject to
interception and archiving. Belief otherwise is folly. Belief that this is
somehow changed by stern corporate announcements of wishes for deletion by
unintended recipients is arrogant folly.
-------=-=-=-=---=-===---=-=-=-===---=---------=---=-===---=-===-=---=-=-=-----
--nextPart1304702.zeePTlEpVM
Content-Type: application/pgp-signature; name=signature.asc
Content-Description: This is a digitally signed message part.
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.7 (OpenBSD)
iD8DBQBHZVhrE0PBXmL+TdERAv2sAJ47xNrSKEL5zx1PqvnRS6m5l7NAVQCfXhp3
WmEEaCLe4LgSS+wvDApJ5g0=
=CnGa
-----END PGP SIGNATURE-----
--nextPart1304702.zeePTlEpVM--
As you can see from the message header and the PGP signature block, this message was digitally signed with GNU Privacy Guard (GnuPG), through the KMail mail tool, part of the KDE desktop environment, running on the OpenBSD operating system.
However, since the message uses the OpenPGP standard format (from Nov 1998), specifically OpenPGP/MIME (from Aug 2001), modern mail tools should be able to handle it regardless of application, graphical environment, or operating system.
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