Cryptography Reference
In-Depth Information
3.1 Aim and Methods. Some Basic Notions
You've already learned what plaintext, ciphertext, cryptography, and cryptanal-
ysis are. And you have heard of a cryptanalytic method, the plaintext attack.
It's about time I told you what we want to achieve with cryptanalysis:
Cryptanalysis is aimed at revealing as much information about the plaintext as
possible without knowing the secret key.
In the worst case, all we have is a ciphertext without any other information.
In this case, we even have to find the encryption method used. Sometimes we
succeed. For example, if the ciphertext consists of uppercase letters only, and
the character distribution coincides with the usual one in the English language,
but it has a shift of its maximum, then chances are that a Caesar cipher was
used. If the cryptanalysis produces a readable plaintext, we were right and won
everything: the method, the key, and the plaintext.
This sort of success can normally be expected only from classic encryption
methods. In the following discussion, we will assume that
the encryption
method is known
. This prerequisite is not unrealistic: in information society
we deal with massware — encryption programs, chip cards, cell phones, cipher-
ing devices. Every algorithm will be disclosed sooner or later. And critical users
will want to know exactly what method they use and how it was implemented
anyhow.
Under the prerequisite of knowing the method, winning the key is probably the
greatest possible success for a cryptanalyst. It has the benefit that he can replay
all encrypted messages just as fast as the receiver with no additional work — as
long as the key doesn't change.
The 'next smaller' success is recovering the plaintext. If that can be done fast
enough thanks to a weak method, then it will be sufficient; otherwise, it's not
bad either.
Another little success is some idea about the plaintext without knowing it
entirely. Using the negative pattern search, for example, finding that a certain
word is
not
included in the ciphertext can be interesting indeed. Moreover,
the length of a message and its addressee can sometimes provide information.
But that's not something we deal with in this topic. We are only interested in
revealing methods, keys, or plaintext.
Four Basic Methods for Cryptanalysis
There are four known methods for cryptanalysis:
