Consider the following attack. Suppose Bob wishes to impersonate Alice.
If Bob can convincingly sign messages as Alice, he can send a message
to Alice's bank saying ``I wish to withdraw $10,000 from my account.
Please send me the money.'' To carry out this attack, Bob generates a
key pair and sends the public key to a certifying authority saying
``I'm Alice. Here is my public key. Please send me a certificate.'' If
the CA is fooled and sends him such a certificate, he can then fool the
bank, and his attack will succeed. In order to prevent such an attack
the CA must verify that a certificate request did indeed come from its
purported author, i.e., it must require sufficient evidence that it is
actually Alice who is requesting the certificate. The CA may, for
example, require Alice to appear in person and show a birth
certificate. Some CAs may require very little identification, but the
bank should not honor messages authenticated with such low-assurance
certificates. Every CA must publicly state its identification
requirements and policies; others can then attach an appropriate level
of confidence to the certificates.
An attacker who discovers the private key of a certifying authority could
then forge certificates. For this reason, a certifying authority must take
extreme precautions to prevent illegitimate access to its private key. The
private key should be kept in a high-security box, known as a Certificate
Signing Unit, or CSU (see Question 3.3.8).
The certifying authority's public key might be the target of an
extensive factoring attack. For this reason, CAs should use very long
keys, preferably 1000 bits or longer, and should also change keys
regularly. Top-level certifying authorities are exceptions: it may not
be practical for them to change keys frequently because the key may be
written into software used by a large number of verifiers.
In another attack, Alice bribes Bob, who works for the certifying
authority, to issue to her a certificate in the name of Fred. Now Alice
can send messages signed in Fred's name and anyone receiving such a
message will believe it authentic because a full and verifiable
certificate chain will accompany the message. This attack can be
hindered by requiring the cooperation of two (or more) employees to
generate a certificate; the attacker now has to bribe two employees
rather than one. For example, in some of today's CSUs, three employees
must each insert a data key containing secret information in order to
authorize the CSU to generate certificates. Unfortunately, there may be
other ways to generate a forged certificate by bribing only one
employee. If each certificate request is checked by only one employee,
that one employee can be bribed and slip a false request into a stack
of real certificate requests. Note that a corrupt employee cannot
reveal the certifying authority's private key, as long as it is
Another attack involves forging old documents. Alice tries to factor the
modulus of the certifying authority. It takes her 15 years, but she finally
succeeds, and she now has the old private key of the certifying authority.
The key has long since expired, but she can forge a certificate dated 15
years ago attesting to a phony public key of some other person, say Bob;
she can now forge a document with a signature of Bob dated 15 year ago,
perhaps a will leaving everything to Alice. The underlying issue raised by
this attack is how to authenticate a signed document dated many years ago;
this issue is discussed in Question 3.3.17.
Note that these attacks on certifying authorities do not threaten the privacy of messages between users, as might result from an attack on a secret-key distribution center.