Unconditionally and Conditionally
Secure Encryption Schema
An encryption scheme is unconditionally secure if the
ciphertext generated by the scheme does not contain enough information to
determine uniquely the corresponding plaintext, no matter how much ciphertext
is available. That is, no matter how much time an opponent has, it is
impossible for him or her to decrypt the ciphertext simply because the required
information is not there. With the exception of a scheme known as the one-time
pad (described later in this chapter), there is no encryption algorithm that is
unconditionally secure. Therefore, all that the users of an encryption
algorithm can strive for is an algorithm that meets one or both of the
following criteria:
- The cost of breaking the cipher exceeds the value of the encrypted information.
- The time required to break the cipher exceeds the useful lifetime of the information.
