PKI: Who Do You Trust?


PKI (public key infrastructure) offers the advantage of rolling together
multiple security functions into a single technology In implementing PKI,
though, administrators must abide by rock solid security policies to
prevent abuses ranging from stolen cryptographic keys to forged digital
signatures.


Basically, PKI is meant to provide an end-to-end infrastructure for
security functions that include authentication; confidentiality; integrity;
and non-repudiation. The security protocol supports single sign-on, digital
signatures, and digital time stamping, as well.


PKI can also be used in conjunction with other security approaches. These
include S-MIME e-mail; SSL-secured Web sessions; LDAP-secured storage, and
IPSec. “In VPNs, IPSec can be secured using PKI components,” notes Uday O.
Ali Pabrai, chairman and CEO of Ecfirst.com, a security and application
solutions provider.


Big banks have been deploying PKI for at least six or seven years now, to
help secure online transactions. The US Department of Defense has gotten
into the act through CAC (Common Access Card), a initiative for issuing PKI
smart cards to military personnel. Other major PKI users include the
health care, insurance, and telecom industries.


Meanwhile, though, PKI has come under strong attack, due to the security
holes it can engender unless properly deployed. Criticisms revolve around
key distribution, management, and storage, as well as real world
implementation of PKI’s underlying “Web of trust.”


PKI uses two different kinds of keys: a public key, ideally stored in an
electronic vault; and a private key, generally stored on either the end
user’s PC or a separate smart card. Data encrypted using the public key can
only be decrypted with a complementary private key, for instance.


PKI’s key management system is optional, involving both a Certificate
Authority (required) and a Registration Authority (optional). The
Certificate Authority (CA) issues digital certificates by using a digital
signature algorithm which binds the identity of a user or a system to a
public key. CAs are also responsible for distributing digital certificates;
for scheduling expiration dates; and for revoking certificates when
necessary.


“The CA is the most critical component of PKI. A lot of time and effort is
expended in trying to understand the requirements of a CA. Should I host my
CA, or outsource my CA?” Pabrai observes. Organizations deciding to host
their own CAs can use a product such as RSA Security’s KeyOn.


Organizations that decide to outsource can either use the services of a
commercial CA such as VeriSign, Nortel Entrust, or GTE CyberTrust, or a
“trusted” third-party such as a government agency.


Some observers, though, take issue with the notion of “trust” as it applies
to CAs. “‘Who do we trust, and for what?’ There’s a risk from an imprecise
use of the word ‘trust,'” charge Bruce Schneier, CTO of Counterpane
Internet Security Inc., and Carl M. Ellison, senior security architect for
Intel Corp., in a paper posted on Counterpane’s Web site.


“A CA is often defined as ‘trusted.’ In the cryptographic literature, this
only means that it handles its own private keys well. This doesn’t mean you
can necessarily trust a certificate from that CA for a particular purpose:
making a micropayment or signing a million-dollar purchase order. Who gave
the CA the authority to grant such authorizations? Who made it trusted?”
they ask.


“The whole concept of a PKI is based on trust,” writes another observer, in
a message posted on the BugTraq forum. “You trust the issuing CA. If you
have no faith in the issuing CA then you cannot trust any of the
certificates that they have issued, or the organizations to which they were
issued. This is not the fault of the organizations, but of the CA itself,”
he adds.


“While risking the wrath of many, I’ll venture to say that unless public,
governmental organizations.act as Root CA’s and issue certificates to an
organization that specifically (prohibit) them from acting as a subordinate
CA to other organizations, or to individuals, we won’t see much trust in
CAs for the foreseeable future.”


The RA, where one exists, is supposed to authenticate users’ IDs – making
sure users “are who they say they are” – as well as to submit certificate
requests to the CA.


According to Pabrai, the State of Iowa is now considering a “smart card”
driver’s license implementation that might revolve around a combined CA/RA
model. “The smart cards will hold a digital certificate that will be issued
from the State of Iowa. So you can go to (a) Web site and access very
sensitive information about yourself stored on (the) state’s servers,” says
Pabrai, who delivered a talk on PKI at the Spring Internet World conference
in Los Angeles.


Some, though, oppose the combined CA/RA model. “The RA+CA model is
categorically less secure than a system with a CA at the authority’s desk.
The RA+CA model allows some entity (the CA) that is not an authority on the
contents to forge a certificate with the contents. Of course, the CA would
sign a contract promising not to do so, but that does not remove the
capability,” say Schneier and Ellsion.


Internal implementations can be tricky too.. Organizations need to weigh
issues such as “Where will you store keys, (and) what will you do if the
system crashes?” Pabrai points out.


“One of the biggest risks in any CA-based system is with your own private
signing key. How do you protect it? you almost certainly don’t own a
secure computing system with physical access controls, TEMPEST shielding,
“air wall” network security; and other protections; you store your private
key on a conventional computer. There, it’s subject to attack by viruses
and other malicious programs. Even if your private key is in a locked room,
with video surveillance, so that you know no one but you ever uses it? If
it’s protected by a password, how hard is it to guess that password? If
your key is stored on a smart card, how attack-resistant is the card?”
insist Schneier and Ellison.


Key distribution policies can be problematic, as well. Pabrai advocates
distributing multiple key pairs. An individual end user might receive one
private key for encryption, and another for digital signing. That way, when
an employee leaves, the organization might hold on to the encryption key,
for reading documents the employee previously wrote, while retiring the
signing key, so as to prevent future “signing abuses.”


PKI technology is complex enough that organizations definitely need to look
before they leap. “PKI is handy, but it’s not for everyone,” according to
Vic Wheatman, a Gartner Group analyst. In one report, Gartner spells out how
to tell whether an organization is ready for either full inhouse or
outsourced PKI deployment, or phased implementation.


Criteria cited by Gartner include level of inhouse expertise; willingness
to commit inhouse resources; track record of security budget; types of
transactions; sensitivity of transactions; type of industry; and existing
information security infrastructure.


As elements of an information security infrastructure, the analysts point
to security policies and business continuity plans, as well as to intrusion
detection (ID) and firewall and antivirus detection.


For organizations without extensive security infrastructures, “the first
priority is to put in place basic policies and infrastructure,” according
to the report. Over the short term, at least, organizations may be able to
get away with either digital certificates without PKI (such as SSL in Web
browsers) or symmetric encryption (such as MACs).


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Jacqueline Emigh

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