An introduction to Digital Signatures, by David Youd
Bob
(Bob's public key)
(Bob's private key)
Bob has been given two keys. One of Bob's keys is called a Public Key, the
other is called a Private Key.
Bob's Co-workers:
Anyone can get Bob's Public Key, but Bob keeps his Private Key to himself
Pat
Doug
Susan
Bob's Public key is available to anyone who needs it, but he keeps his Private Key
to himself. Keys are used to encrypt information. Encrypting information means
"scrambling it up", so that only a person with the appropriate key can make it
readable again. Either one of Bob's two keys can encrypt data, and the
other key can decrypt that data.
Susan (shown below) can encrypt a message using Bob's Public Key. Bob uses his
Private Key to decrypt the message. Any of Bob's coworkers might have access
to the message Susan encrypted, but without Bob's Private Key, the data is
worthless.
"Hey Bob, how about lunch at Taco Bell. I hear they have free refills!"
"Hey Bob, how about lunch at Taco Bell. I hear they have free refills!"
With his private key and the right software, Bob can put digital signatures on
documents and other data. A digital signature is a "stamp" Bob places on the
data which is unique to Bob, and is very difficult to forge. In addition, the
signature assures that any changes made to the data that has been signed can
not go undetected.
To sign a document, Bob's software will crunch down the data into just a few lines
by a process called "hashing". These few lines are called a message digest. (It
is not possible to change a message digest back into the original data from which
it was created.)
Bob's software then encrypts the message digest with his private key. The result
is the digital signature.
Finally, Bob's software appends the digital signature to document. All of the
data that was hashed has been signed.
Bob now passes the document on to Pat.
First, Pat's software decrypts the signature (using Bob's public key) changing it back
into a message digest. If this worked, then it proves that Bob signed the
document, because only Bob has his private key. Pat's software then hashes
the document data into a message digest. If the message digest is the same
as the message digest created when the signature was decrypted, then Pat
knows that the signed data has not been changed.
Plot complication...
Doug (our disgruntled employee) wishes to deceive Pat. Doug makes sure
that Pat receives a signed message and a public key that appears to belong
to Bob. Unbeknownst to Pat, Doug deceitfully sent a key pair he
created using Bob's name. Short of receiving Bob's public key from him
in person, how can Pat be sure that Bob's public key is authentic?
It just so happens that Susan works at the company's certificate
authority center. Susan can create a digital certificate for Bob simply
by signing Bob's public key as well as some information about Bob.
Bob Info:
Name
Department
Cubical Number
Certificate Info:
Expiration Date
Serial Number
Bob's Public Key:
Now Bob's co-workers can check Bob's trusted certificate to make sure
that his public key truly belongs to him. In fact, no one at Bob's
company accepts a signature for which there does not exist a certificate
generated by Susan. This gives Susan the power to revoke signatures if
private keys are compromised, or no longer needed. There are even more
widely accepted certificate authorities that certify Susan.
Let's say that Bob sends a signed document to Pat. To verify the signature
on the document, Pat's software first uses Susan's (the certificate
authority's) public key to check the signature on Bob's certificate.
Successful de-encryption of the certificate proves that Susan created it.
After the certificate is de-encrypted, Pat's software can check if
Bob is in good standing
with the certificate authority and that all of the certificate information
concerning Bob's identity has not been altered.
Pat's software then takes Bob's public key from the certificate and uses it
to check Bob's signature. If Bob's public key de-encrypts the signature
successfully, then Pat is assured that the signature was created using Bob's
private key, for Susan has certified the matching public key. And of course,
if the signature is valid, then we know that Doug didn't try to change the
signed content.
Although these steps may sound complicated, they are all handled behind
the scenes by Pat's user-friendly software. To verify a signature, Pat need
only click on it.
(c) 1996, David Youd
Permission to change or distribute is at the discretion of the author
One of the best ways I've seen to describe what digital signature really is. Thanks for a lovely article, it is very common this days, and people should know what it's all about.
2 Comments
One of the best ways I've seen to describe what digital signature really is. Thanks for a lovely article, it is very common this days, and people should know what it's all about.
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