Cryptography Reference
In-Depth Information
ciphertext should look like random text to an adversary. Even if the attacker has at
her disposal a piece of plaintext and the corresponding ciphertext, she should not
be able to find patterns connecting them that could be exploited to analyze other
ciphertexts.
As we have noticed, the systems studied in this chapter are all easy to cryptanalyze
by elementary means. There are many other classical ciphers that we have not studied
and most of them are also easy to break. There are also historical ciphers whose
cryptanalysis requires a much greater effort. This is the case, for example, for the
cipher implemented by the
Enigma machine
, used by the Germans duringWorldWar
II and cryptanalyzed by Polish and British cryptanalysts (see, for example, [184] for
details).
The cryptanalysis of these historical ciphers is a reflection of the fact that all
the publicly known encryption schemes originated before World War II—with the
exception of the one-time pad described in Chap.
3
—have essentially been broken.
After the introduction of public-key cryptography in the 1970s and the development
of encryption schemes based on hard computational problems, cryptanalysis has
become a much more difficult task but new cryptanalytic methods and attacks are
constantly being developed and refined; some of them are studied in subsequent
chapters. The recent publication of several topics integrally devoted to cryptanalysis
bears witness to this development, see for example [9, 106, 186, 190, 194, 201].
Finally, we mention the fact that our examples show that the classical ciphers we
have considered can be totally broken, i.e., the discussed attacks allow full recovery of
the plaintext or even the key. It is then clear that these ciphers are extremely insecure
but one should not draw the conclusion that an encryption scheme is secure whenever
such a total break is not possible. Far from it, we shall see in the following chapters
that much stricter conditions are required for an encryption scheme to be regarded as
secure, which essentially amount to demanding that no resource-bounded adversary
can obtain any information whatsoever about the plaintext.
