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What are the advantages and disadvantages of public key
cryptography compared with secret key cryptography ?
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Answer Posted / prashant
The primary advantage of public-key cryptography is
increased security and convenience: private keys never need
to transmitted or revealed to anyone. In a secret-key
system, by contrast, the secret keys must be transmitted
(either manually or through a communication channel), and
there may be a chance that an enemy can discover the secret
keys during their transmission.
Another major advantage of public-key systems is that they
can provide a method for digital signatures. Authentication
via secret-key systems requires the sharing of some secret
and sometimes requires trust of a third party as well. As a
result, a sender can repudiate a previously authenticated
message by claiming that the shared secret was somehow
compromised by one of the parties sharing the secret. For
example, the Kerberos secret-key authentication system
involves a central database that keeps copies of the secret
keys of all users; an attack on the database would allow
widespread forgery. Public-key authentication, on the other
hand, prevents this type of repudiation; each user has sole
responsibility for protecting his or her private key. This
property of public-key authentication is often called non-
repudiation.
A disadvantage of using public-key cryptography for
encryption is speed: there are popular secret-key
encryption methods that are significantly faster than any
currently available public-key encryption method.
Nevertheless, public-key cryptography can be used with
secret-key cryptography to get the best of both worlds. For
encryption, the best solution is to combine public- and
secret-key systems in order to get both the security
advantages of public-key systems and the speed advantages
of secret-key systems. The public-key system can be used to
encrypt a secret key which is used to encrypt the bulk of a
file or message. Such a protocol is called a digital
envelope, which is explained in more detail in Question 16
in the case of RSA.
Public-key cryptography may be vulnerable to impersonation,
however, even if users' private keys are not available. A
successful attack on a certification authority (see
Question 127) will allow an adversary to impersonate
whomever the adversary chooses to by using a public-key
certificate from the compromised authority to bind a key of
the adversary's choice to the name of another user.
In some situations, public-key cryptography is not
necessary and secret-key cryptography alone is sufficient.
This includes environments where secure secret-key
agreement can take place, for example by users meeting in
private. It also includes environments where a single
authority knows and manages all the keys, e.g., a closed
banking system. Since the authority knows everyone's keys
already, there is not much advantage for some to
be "public" and others "private." Also, public-key
cryptography is usually not necessary in a single-user
environment. For example, if you want to keep your personal
files encrypted, you can do so with any secret-key
encryption algorithm using, say, your personal password as
the secret key. In general, public-key cryptography is best
suited for an open multi-user environment.
Public-key cryptography is not meant to replace secret-key
cryptography, but rather to supplement it, to make it more
secure. The first use of public-key techniques was for
secure key exchange in an otherwise secret-key system
[DH76]; this is still one of its primary functions. Secret-
key cryptography remains extremely important and is the
subject of much ongoing study and research. Some secret-key
cryptosystems are discussed in the sections on block
ciphers and stream ciphers.
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