Ten Risks of PKI: What You're not Being Told about Public Key
Infrastructure By Carl Ellison and Bruce Schneier
Computer security has been victim of the "year of the..." syndrome.
First it was firewalls, then intrusion detection systems, then VPNs,
and now certification authorities (CAs) and public-key infrastructure
(PKI). "If you only buy X," the sales pitch goes, "then you will be
secure." But reality is never that simple, and that is especially
true with PKI.
Certificates provide an attractive business model. They cost almost
nothing to make, and if you can convince someone to buy a certificate
each year for $5, that times the population of the Internet is a big
yearly income. If you can convince someone to purchase a private CA
and pay you afee for every certificate he issues, you're also in good
shape. It's no wonder so many companies are trying to cash in on this
potential market.With that much money at stake, it is also no wonder
that almost all the literature and lobbying on the subject is produced
by PKI vendors. And this literature leaves some pretty basic
questions unanswered: What good are certificates anyway? Are they
secure? For what? In this essay, we hope to explore some of those
questions.
Security is a chain; it's only as strong as the weakest link. The
security of any CA-based system is based on many links and they're not
all cryptographic. People are involved.
Does the system aid those people, confuse them or just ignore them?
Does it rely inappropriately on the honesty or thoroughness of people?
Computer systems are involved. Are those systems secure? These all
work together in an overall process. Is the process designed to
maximize security or just profit?
Each of these questions can indicate security risks that need to be
addressed.
Before we start: "Do we even need a PKI for e-commerce?" Open any
article on PKI in the popular or technical press and you're likely to
find the statement that a PKI is desperately needed for e-commerce to
flourish. This statement is patently false. E-commerce is already
flourishing, and there is no such PKI. Web sites are happy to take
your order, whether or not you have a certificate. Still, as with
many other false statements, there is a related true statement:
commercial PKI desperately needs e-commerce in order to flourish. In
other words, PKI startups need the claim of being essential to e-
commerce in order to get investors.
There are risks in believing this popular falsehood. The immediate
risk is on the part of investors. The security risks are borne by
anyone who decides to actually use the product of a commercial PKI.
Risk #1: "Who do we trust, and for what?" There's a risk from an
imprecise use of the word "trust." 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?
A CA can do a superb job of writing a detailed Certificate Practice
Statement, or CPS ó all the ones we've read disclaim all liability and
any meaning to the certificate ó and then do a great job following
that CPS, but that doesn't mean you can trust a certificate for your
application. Many CAs sidestep the question of having no authority to
delegate authorizations by issuing ID certificates. Anyone can assign
names. We each do that all the time. This leaves the risk in the
hands of the verifier of the certificate, if he uses an ID certificate
as if it implied some kind of authorization.
There are those who even try to induce a PKI customer to do just that.
Their logic goes: (1) you have an ID certificate, (2) that gives you
the keyholder's name, (3) that means you know who the keyholder is,
(4) that's what you needed to know. Of course, that's not what you
needed to know. In addition, the logical links from 1 to 2, 2 to 3
and 3 to 4 are individually flawed. [We leave finding those as an
exercise for the reader.]
Risk #2: "Who is using my key?"
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 safe on your computer, is your computer 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? [Most are very weak.] If it is stored
in a truly attack-resistant device, can an infected driving computer
get the trustworthy device to sign something you didn't intend to
sign?
This matters mostly because of the term "non-repudiation." Like
"trusted," this term is taken from the literature of academic
cryptography. There it means something very specific: that the
digital-signature algorithm is not breakable, so a third party cannot
forge your signature. PKI vendors have latched onto the term and used
it in a legal sense, lobbying for laws to the effect that if someone
uses your private signing key, then you are not allowed to repudiate
the signature. In other words, under some digital signature laws
(e.g., Utah and Washington), if your signing key has been certified by
an approved CA, then you are responsible for whatever that private key
does. It does not matter who was at the computer keyboard or what
virus did the signing; you are legally responsible.
Contrast this with the practice regarding credit cards. Under mail-
order/telephone-order (MOTO) rules, if you object to a line item on
your credit card bill, you have the right to repudiate it ó to say you
didn't buy that ó and the merchant is required to prove that you did.
Risk #3: "How secure is the
verifying computer?"
The previous section showed that the computer holding or driving the
private key needs to be secure. Long keys don't make up for an
insecure system because total security is weaker than the weakest
component in the system.
The same applies to the verifying computer - the one that uses the
certificate.
Certificate verification does not use a secret key, only public keys.
Therefore, there are no secrets to protect. However, it does use one
or more "root" public keys. If the attacker can add his own public
key to that list, then he can issue his own certificates, which will
be treated exactly like the legitimate certificates. They can even
match legitimate certificates in every other field except that they
would contain a public key of the attacker instead of the correct one.
It doesn't help to hold these root keys in "root certificates." Such
a certificate is self-signed and offers no increased security. The
only answer is to do all certificate verification on a computer system
that is invulnerable to penetration by hostile code or to physical
tampering.
Risk #4: "Which John Robinson is he?"
Certificates generally associate a public key with a name, but few
people talk about how useful that association is. Imagine that you
receive the certificate of John Robinson. You may know only one John
Robinson personally, but how many does the CA know? How do you find
out if the particular John Robinson certificate you received is your
friend's certificate? You could have received his public key in
person or verified it in person (PGP allows this), but more likely you
received a certificate in e-mail and are simply trusting that it is
the correct John Robinson. The certificate's Common Name will
probably be extended with some other information, in order to make it
unique among names issued by that one CA.
Do you know that other information about your friend? Do you know
what CA his certificate should come from?
When Diffie and Hellman introduced public-key cryptography, they
proposed a modified telephone directory in which you could find public
keys. Instead of name, address, and phone number, it would have name,
address, and public key. If you wanted to find John Robinson's public
key you would look him up in the directory, get his public key and
send him a message for his eyes only using that public key. This
might have worked with the Stanford Computer Science Department phone
directory in 1976, but how many John Robinsons are in the New York
City phone book, much less in a hypothetical phone book for the global
Internet?
We grow up in small families where names work as identifiers. By the
time we're 5 years old, we know that lesson. Names work. That is
false in the bigger world, but things we learn as toddlers we never
forget. In this case, we need to think carefully about names and not
blindly accept their value by the 5-year-old's lessons locked into our
memories.
Risk #5: "Is the CA an authority?"
The CA may be an authority on making certificates, but is it an
authority on what the certificate contains? For example, an SSL server
certificate contains two pieces of data of potential security
interest: the name of the keyholder (usually a corporate name) and the
DNS name for the server. There are authorities on DNS name
assignments, but none of the SSL CAs listed in the popular browsers is
such an authority. That means that the DNS name in the certificate is
not an authoritative statement. There are authorities on corporate
names. These names need to be registered when one gets a business
license. However, none of the SSL CAs listed in the browsers is such
an authority. In addition, when some server holds an SSL server
certificate, it has permission to do SSL. Who granted the authority
to an SSL CA to control that permission? Is the control of that
permission even necessary? It serves an economic purpose (generating
an income stream for CAs) but does it serve a security purpose? What
harm is done if an uncertified server were allowed to use encryption?
None.
Risk #6: "Is the
user part of the security design?"
Does the application using certificates take the user into account or
does it concern itself only with cryptography?
For example, a normal user makes a decision of whether to shop with a
given SSL-protected Web page based on what is displayed on that page.
The certificate is not displayed and does not necessarily have a
relation to what is displayed. SSL security does not have the ability
to control or even react to the content of the Web page, only its DNS
address. The corporate name is not compared to anything the user sees
and there are some Web pages whose certificate is for a company that
does Web hosting, not for the company whose logo appears on the
displayed page. Users can't, and can't be expected to, sort this all
out.
Risk #7: "Was it one CA or a CA plus a Registration Authority?"
Some CAs, in response to the fact that they are not authorities on the
certificate contents, have created a two-part certification structure:
a Registration Authority (RA), run by the authority on the contents,
in secure communication with the CA that just issues certificates.
Other vendors sell CA machinery directly to the content authority.
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
that contents. Of course, the CA would sign a contract promising not
to do so, but that does not remove the capability. Meanwhile, since
security of a chain is weaker than the weakest link, the RA+CA is less
secure than either the RA or the CA, no matter how strong the CA or
how good the contract with the CA. Of course, the model with a CA at
the authority's desk (not at the vendor's site) violates some PKI
vendors' business models. It's harder to charge for certificates when
you sell someone the CA code (or they get it for free, as Open
Source).
Risk #8: "How did the CA identify the
certificate holder?"
Whether a certificate holds just an identifier or some specific
authorization, the CA needs to identify the applicant before issuing
the certificate.
There was a credit bureau that thought they would get into the CA
business.
After all, they had a vast database on people, so, the thinking ran,
they should be able to establish someone's identity online with ease.
If you want to establish identity online, you can do that provided you
have a shared secret with the subject and a secure channel over which
to reveal that secret. SSL provides the secure channel.
The trouble with a credit bureau serving this role is that in their
vast database there is not one secret shared with the subject. This
is because credit bureaus are in the business of selling their
information to people other than the subject. Worse, because credit
bureaus do such a good job at collecting and selling facts about
people, others who might have information about a subject are probably
hard pressed to find any datum shared with the subject that is not
already available through some credit bureau. This puts at risk
commercial CAs that use credit bureau information to verify identity
on-line; the model just doesn't work.
Meanwhile, having identified the applicant somehow, how did the CA
verify that the applicant really controlled the private key
corresponding to the public key being certified? Some CAs don't even
consider that to be part of the application process. Others might
demand that the applicant sign some challenge right there on the spot,
while the CA watches.
Risk #9: "How secure are the
certificate practices?"
Certificates aren't like some magic security elixir, where you can
just add a drop to your system and it will become secure.
Certificates must be used properly if you want security. Are these
practices designed with solid security reasons, or are they just
rituals or imitations of the behavior of someone else? Many such
practices and even parts of some standards are just imitations which,
when carefully traced back, started out as arbitrary choices by people
who didn't try to get a real answer.
How is key lifetime computed? Does the vendor use 1 year, just
because that's common? A key has a cryptographic lifetime. It also
has a theft lifetime, as a function of the vulnerability of the
subsystem storing it, the rate of physical and network exposure,
attractiveness of the key to an attacker, etc. From these, one can
compute the probability of loss of key as a function of time and
usage. Does the vendor do that computation? What probability
threshold is used to consider a key invalid?
Does the vendor support certificate or key revocation? Certificate
Revocation Lists (CRLs) are built into some certificate standards, but
many implementations avoid them because they seem to be archaic
remnants of the newsprint booklets of bad checking account numbers one
used to find at the supermarket checkout stand. Like those booklets,
CRLs are seen as too big and too outdated to be relevant. However, if
CRLs are not used, how is revocation handled?
If revocation is handled, how is compromise of a key detected in order
to trigger that revocation? Can revocation be retroactive? That is,
can a certificate holder deny having made some signature in the past?
If so, are signatures dated so that one knows good signatures from
suspect ones? Is that dating done by a secure timestamp service?
How long are the generated public keys and why was that length chosen?
Does the vendor support 512-bit RSA keys just because they're fast or
2048-bit keys because someone over there in the corner said he thought
it was secure?
Does the proper use of these certificates require user actions? Do
users perform those actions? For example, when you establish an SSL
connection with your browser, there's a visual indication that the SSL
protocol worked and the link is encrypted. But who are you talking
securely with? Unless you take the time to read the certificate that
you received, you don't know.
Even then, you may not know (cf., Risk #4, above) but if you don't
even look, it's much like going into a private room with the lights
off: you might know that someone else is there and your conversation
is private, but until you know who that other person is, you shouldn't
reveal any secret information.
Risk #10: "Why are we using the CA process, anyway?"
One PKI vendor employee confided in us a few years ago that they had
great success selling their PKI solution, but that customers were
still unhappy.
After the CA was installed and all employees had been issued
certificates, the customer turned to the PKI vendor and asked, "OK,
how do we do single sign-on?" The answer was, "You don't. That
requires a massive change in the underlying system software."
Single Sign-On (SSO) might be the killer app of PKI. Under SSO, you
come into work in the morning, plug in your smart-card, enter the PIN
that activates it, and for the rest of the day, you don't have to do
any more logins. All of that is handled for you by the SSO mechanism.
Attractive isn't it? Of course, it's attractive. Authentication is a
pain.
Anything we can do to avoid it, we'll jump at.
Unfortunately, the security value of authentication is all but
completely defeated by SSO. Authentication is supposed to prove that
the user is present at the controlling computer, at the time of the
test. Under SSO, when the user has to rush to the washroom, any
passing person can walk up to that user's computer and sign on
someplace via the SSO mechanism.
So, why are so many jumping at the CA process with such fervor? Do
they use certificates out of empty ritual, just because the other guy
does and it's the thing to do this year? Do they do it in order to
pass the liability buck: to be able to blame the PKI experts if any
insecurity sneaks through?
We are not that cynical. Our assessment is that security is very
difficult, both to understand and to implement. Busy system
administrators and IT managers don't have the time to really
understand security. They read the trade press. The trade press,
influenced by PKI vendors, sings the praises of PKIs. And PKI vendors
know what busy people need: a minimal-impact solution. "Here, buy
this one thing and it will make you secure." So that's what they
offer. Reality falls far short of this promise, but then, this is a
business and the prominent voices are those with something to sell.
Caveat emptor.
Bruce Schneier is the author of Applied Cryptography, the Blowfish and
Twofish encryption algorithms, and dozens of research papers and
articles on cryptography and computer security. He is CTO of
Counterpane Internet Security, Inc., a managed security service
company offering leading-edge expertise in the fields of intrusion
detection and prevention, preemptive threat discovery, forensic
research, and organizational IT systems analysis.
You can subscribe to his free monthly e-mail newsletter, Crypto-Gram,
at http://www.counterpane.com
Carl M. Ellison is a Senior Security Architect for Intel Corporation,
with special focus on cryptography, cryptographic access control and
public key certificates. Prior to the focus on cryptography, his
earlier professional computer science career focused on system design
with special emphasis on distributed and networked systems.
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R. A. Hettinga <mailto: [EMAIL PROTECTED]>
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'
