illation (SKD), are necessary to achieve "ultimate" (albeit exponential) security confidence in the protocol. The closing section discusses the eventual vulnerability of quantum cryptosystems, when replaced in the realistic network environment, with its various forms of attack. The conclusion is that assuming all attacks are possible (as opposed to sheltering under "postulates" that these remain out of the picture), quantum cryptography represents just another, albeit very elegant, technique for key exchange. Despite its exponential degree of safety, QKD must be situated within the grander domain of global network security, where there cannot be "absolute" confidence in any cryptosystem. Message encryption, decryption, and code breaking This section reviews the elementary basics of cryptography principles, definitions, and jargon. There exist several instances where some private, confidential, sensitive, or even classified information must be conveyed through a communication channel. Examples abound, from online tax declaration and business reports on company's intranets to military and government communications. The driving concept for protecting such contents is cryptography, which comes from the Greek kryptos (secret) and graphein (writing). Thus, cryptography is the art of transforming a message into a self-contained secret, while ensuring a practical technique for the destinee to retrieve the information contents. The action of encryption is to transform an original text, or plaintext, into a ciphertext. Nowadays, "plaintexts" are raw computer data, namely blocks of bits, but the spirit is exactly similar. The word cipher refers to the coding algorithm or method of encryption. An equivalent, fancier, definition of cipher is cryptosystem. The reverse operation, i.e., resolving a ciphertext into the original plaintext, is called decryption. Decryption is based on the principle of a secret key. Throughout cryptographic history, the secret key could have been any method of encryption and decryption based on corr