cryptography small details

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Introduction
Until modern times cryptography referred almost exclusively to encryption, which is the process of converting ordinary information (plaintext) into unintelligible gibberish (i.e., ciphertext).Decryption is the reverse, in other words, moving from the unintelligible ciphertext back to plaintext. A cipher (or cypher) is a pair of algorithms that create the encryption and the reversing decryption. The detailed operation of a cipher is controlled both by the algorithm and in each instance by a key. This is a secret parameter (ideally known only to the communicants) for a specific message exchange context. Keys are important, as ciphers without variable keys can be trivially broken with only the knowledge of the cipher used and are therefore less than useful for most purposes. Historically, ciphers were often used directly for encryption or decryption without additional procedures such as authentication or integrity checks.


Traditional Cryptography

Privacy is paramount when communicating sensitive information, and humans have invented some unusual ways to encode their conversations. In World War II, for example, the Nazis created a bulky machine called the Enigma that resembles a typewriter on steroids. This machine created one of the most difficult ciphers (encoded messages) of the pre-computer age.
Even after Polish resistance fighters made knockoffs of the machines -- complete with instructions on how the Enigma worked -- decoding messages was still a constant struggle for the Allies [source: Cambridge University]. As the codes were deciphered, however, the secrets yielded by the Enigma machine were so helpful that many historians have credited the code breaking as an important factor in the Allies' victory in the war.



Limitations of Traditional Cryptography

Both the secret-key and public-key methods of cryptology have unique flaws. Oddly enough, quantum physics can be used to either solve or expand these flaws.

The keys used to encode messages are so long that it would take a trillion years to crack one using conventional computers.


Uncertainity

Unlike in classical physics, the act of measurement is an integral part of quantum
mechanics. So it is possible to encode information into quantum properties of a photon in
such a way that any effort to monitor them disturbs them in some detectable way. The
effect arises because in quantum theory, certain pairs of physical properties are
complementary in the sense that measuring one property necessarily disturbs the other.
This statement is known as the Heisenberg uncertainty principle. The two complementary
properties that are often used in quantum cryptography are two types of photons
Polarization, e.g. rectilinear (vertical and horizontal) and diagonal (at 45° and 135°).