المصدر الرابع لتقارير المرحلة الرابعة

Introduction:
* Cryptanalysis is the science of making encrypted data unencrypted.
A cryptographer will use cryptography to convert plaintext into ciphertext
and a cryptanalyst will use cryptanalysis to attempt to turn that ciphertext
back into plaintext. Both the cryptographer and the cryptanalyst are
cryptologists. Cryptography and cryptanalysis are the two sides of
cryptology.
It without use of the key. The other side of cryptography, cryptanalysis
is used to break codes by finding weaknesses within it. In addition to
being used by hackers with bad intentions, cryptanalysis is also often
used by the military. Cryptanalysis is also appropriately used by
designers of encryption systems to find, and subsequently correct, any
weaknesses that may exist in the system under design.
There are several types of attacks that a cryptanalyst may use to break
a code, depending on how much information they have. A ciphertext-only
attack is one where the cryptanalyst has a piece of ciphertext (text that
has already been encrypted), with no plaintext (unencrypted text). This is
probably the most difficult type of cryptanalysis, and calls for a bit of
guesswork. In a known-plaintext attack, the cryptanalyst has both a piece
of ciphertext and the corresponding piece of plaintext.
Other types of attacks may involve trying to derive a key through
trickery or theft. The "man-in-the-middle" attack is one example. In this
attack, the cryptanalyst places a piece of surveillance software in between
two parties that communicate. When the parties keys are exchanged for
secure communication, they exchange their keys with the attacker instead
of each other.
The ultimate goal of the cryptanalyst however, is to derive the key, so
that all ciphertext can be easily deciphered. A brute-force attack is one
way of doing so. In this type of attack, the cryptanalyst tries every
possible combination until the correct key is identified. Although using
longer keys make the derivation less statistically likely to be successful,
faster computers, continue to make brute-force attacks feasible.