How a Digital Signature Works

R. A. Hettinga rah at shipwright.com
Tue Aug 10 07:34:48 EDT 2004


<http://www.businessweek.com/print/technology/content/aug2004/tc20040810_3053_tc024.htm?tc>

Business Week


 AUGUST 10, 2004

  NEWS ANALYSIS :TECH
 By Stephen H. Wildstrom





How a Digital Signature Works

Microsoft's new Service Pack makes life tough for programs lacking the
proper electronic credentials. Here's why
 A technology called public key cryptography makes it possible for you to
make sure that the publisher of any piece of software that claims to be
from Microsoft (MSFT ) or any other publisher really came from there. It
has the added benefit of insuring that the contents weren't maliciously
altered or damaged in transmission. Here's how it works:

 The publisher first has to obtain a digital certificate from a recognized
"certificate authority" or CA (VeriSign (VRSN ) is the largest and best
known CA in the U.S.). The publisher receives a private and a public key,
each of which is a long number of about 300 digits. These are used to
create a digital signature for each program (see BW Online, 8/10/04,
"Windows of Vulnerability No More?").

 When the software is ready to be posted for download, the publisher runs
it through a mathematical process called a one-way hash which reduces it to
a long number called the message digest. The message digest is then
encrypted using the publisher's private key, and the result, which looks
like a string of gibberish when displayed, is appended to the program when
it's downloaded.

HASH SLINGING.  The trick of public key encryption -- the best known
approach is called RSA for the initials of its inventors -- is that one key
can be used to scramble the data while a different, mathematically related,
key is used to unscramble it. When you download a digitally signed program,
the first thing your computer does is check the Web site's digital
certificate. It then queries the CA that issues the certificate to make
sure it's still valid and to obtain the public key.

 When the download is complete, your computer uses the public key to
decrypt the message digest. It also runs the same one-way hash procedure on
the downloaded software. If everything is as it should be, the decrypted
message digest and the one just created should be identical. If they differ
by a single bit, something is wrong and the downloaded software will be
rejected.

 For the curious, here's the message digest of the five paragraphs above
(as plain text), created using the MD5 algorithm from RSA Data Security
Inc: c21196eb8e026d47a67883d746c72c8d.



 Wildstrom is Technology & You columnist for BusinessWeek. Follow his Flash
Product Reviews, only at BusinessWeek Online


-- 
-----------------
R. A. Hettinga <mailto: rah at ibuc.com>
The Internet Bearer Underwriting Corporation <http://www.ibuc.com/>
44 Farquhar Street, Boston, MA 02131 USA
"... however it may deserve respect for its usefulness and antiquity,
[predicting the end of the world] has not been found agreeable to
experience." -- Edward Gibbon, 'Decline and Fall of the Roman Empire'

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