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Security alert: Do you want to proceed?
BY WAI CHOI AND RICH GUSKI
How would you answer this question when
you go to a Web site and a Security Alert
window pops up telling you that there is
a problem with the site’s certificate – Yes,
No or View Certificate? Or you might hope
there’s a fourth option – I don’t know.
This article helps explain Digital
Certificates in a simple way, starting from
a daily experience—online shopping.
When you click Check Out after you pick
the product you want, do you know what
happens before it displays a page for you to
enter your personal information and your
credit card number?
Actually, quite a bit happens. Your
browser and the host site negotiate what
is called a Secure Socket Layer (SSL)
handshake, a process that goes
like this: the Web site sends its
certificate to your browser
to prove its identity to
ensure you that you are
really dealing with the
Web site that you think
you are. Your browser
then determines
whether it can trust that
certificate. If it can’t
decide, it will ask you to
make a decision. That’s
why the Security Alert
window pops up.
But how does the
browser decide whether
to trust the certificate?
The first thing it checks
is whether it knows the
issuer of the certificate
by checking whether a
certificate for the issuer
is in the browser’s trusted certificate store.
Certificates are issued by a Certificate
Authority (CA). The CA must have its
own certificate, either issued by another
CA or issued by itself, before it can issue
certificates for other parties. When you
install your browser, there are a number of
well-known CA certificates that are placed
in your browser’s trusted certificate store.
Usually, a Web site applies for a certificate
from a well-known CA, so when a user
visits the site, the certificate it sends is
known by the browser.
Let’s dive a little deeper. How does the
browser know which certificate in the
trusted certificate store is the site’s issuer
certificate? If you look at the certificate,
you find the following:
• Version
• Serial number
• Signature algorithm ID
• Issuer’s name
• Validity period
• Subject’s name
• Subject’s public key
• Extension(s)
• Issuer’s signature.
The browser finds the issuer’s name from
the host site’s certificate. It then tries
to find a certificate in the store, whose
subject name is the same as that issuer’s
name. After it finds the issuer’s certificate,
it can verify the signature on that site’s
certificate by using the public key from the
issuer’s certificate.
Public Key cryptography involves
a function in which encryption and
decryption are using associated keys from
a mathematically related key pair. The
content encrypted with one key can only
be decrypted by the other. One key of the
pair, which is called the public key, is put
in a digital certificate for public use, while
the other key, which is called the private
key, is kept securely by the owner. When
a CA issues a certificate, it vouches for
the binding between the owner’s public
key and the owner’s name by
cryptographically signing the
certificate with its own private
key. Public Key cryptography
is the soul of a digital certificate when
contrasted with Secret Key cryptography
in which only one key is used to encrypt
and decrypt.
Now you can understand why the
verification on the certificate’s signature
is done using the issuer’s public key
– because that signature was generated
using the issuer’s private key. Let’s review
the verification steps involved so far.
The browser identifies the issuer of
the incoming certificate from the
issuer’s name in the certificate.
The browser then verifies the
certificate by checking
the signature with the
issuer’s public key. The
public key is located in
the issuer’s certificate,
which is stored
in the trusted
certificate store.
But wait
there’s more!
Validity period
and revocation
status also play a part
in the validation process. If a certificate
has expired or is revoked, it should not
be trusted. While validity period can be
checked directly from the information
within the certificate itself, revocation
status is checked from a Certificate
Revocation List (CRL) which is located
elsewhere, as indicated by the Certificate
Revocation List Distribution Points
extension.
After a certificate is issued, its owner
can use it until it expires. However, if,
during that period, the certificate needs
to be revoked, the CA of that certificate
puts that certificate on the Certificate
February 2006 z/OS HOT TOPICS Newsletter, Issue 14 25