Problem - Kevin Tafuro

Because public key encryption is so expensive, it is only really useful for processing
small pieces of data. As a result, there are two ways in which public key cryptography
is widely used: key exchange (done by encrypting a symmetric encryption key) and digital
signatures (done by encrypting a hash of the data to sign; see Recipes 7.12, 7.13
and 7.15).
When using digital signatures for authentication, a valid signature on a piece of data
proves that the signer has the correct secret key that corresponds to the public key
we have (of course, we then need to ensure that the public key really does belong to
the entity we want to authenticate). The signature also validates that the message
arrived without modification. However, it’s not a good idea to use digital signatures
for all of our message integrity needs because it is incredibly slow. You essentially
need public key cryptography to provide message integrity for a key exchange, and
while you’re doing that, you might as well use it to authenticate (the authentication
is often free). However, once you have a symmetric key to use, you should use MACs
to provide message integrity because they’re far more efficient.
The only time it makes sense to use a digital signature outside the context of initial
connection establishment is when there is a need for non-repudiation. That is, if you
wish to be able to demonstrate that a particular user “signed” a piece of data to a
third party, you must use public key–based algorithms. Symmetric key integrity
checks are not sufficient for implementing non-repudiation, because anyone who has
the shared secret can create valid message integrity values. There’s no way to bind
the output of the integrity check algorithm to a particular entity in the system. Public
key cryptography allows you to demonstrate that someone who has the private
key associated with a particular public key “signed” the data, and that the data
hasn’t changed since it was signed.
See Also
Recipes 7.12, 7.13, 7.15, 10.1
7.2 Selecting a Public Key Algorithm
Problem
You want to determine which public key algorithms you should support in your
application.
Solution
RSA is a good all-around solution. There is also nothing wrong with using Diffie-
Hellman for key exchange and DSA for digital signatures.
Selecting a Public Key Algorithm | 311
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