What uses a key of the same length as the message where each bit or character from the plaintext is encrypted by a modular addition?
正解: B
In cryptography, the one-time pad (OTP) is a type of encryption that is impossible to crack if used correctly. Each bit or character from the plaintext is encrypted by a modular addition with a bit or character from a secret random key (or pad) of the same length as the plaintext, resulting in a ciphertext. If the key is truly random, at least as long as the plaintext, never reused in whole or part, and kept secret, the ciphertext will be impossible to decrypt or break without knowing the key. It has also been proven that any cipher with the perfect secrecy property must use keys with effectively the same requirements as OTP keys. However, practical problems have prevented one-time pads from being widely used.
First described by Frank Miller in 1882, the one-time pad was re-invented in 1917 and patented a couple of years later. It is derived from the Vernam cipher, named after Gilbert Vernam, one of its inventors. Vernam's system was a cipher that combined a message with a key read from a punched tape. In its original form, Vernam's system was vulnerable because the key tape was a loop, which was reused whenever the loop made a full cycle. One-time use came a little later when Joseph Mauborgne recognized that if the key tape were totally random, cryptanalysis would be impossible.
The "pad" part of the name comes from early implementations where the key material was
distributed as a pad of paper, so the top sheet could be easily torn off and destroyed after
use. For easy concealment, the pad was sometimes reduced to such a small size that a
powerful magnifying glass was required to use it. Photos show captured KGB pads that fit
in the palm of one's hand, or in a walnut shell. To increase security, one-time pads were
sometimes printed onto sheets of highly flammable nitrocellulose so they could be quickly
burned.
The following are incorrect answers:
A running key cipher uses articles in the physical world rather than an electronic algorithm.
In classical cryptography, the running key cipher is a type of polyalphabetic substitution
cipher in which a text, typically from a book, is used to provide a very long keystream.
Usually, the book to be used would be agreed ahead of time, while the passage to use
would be chosen randomly for each message and secretly indicated somewhere in the
message.
The Running Key cipher has the same internal workings as the Vigenere cipher. The
difference lies in how the key is chosen; the Vigenere cipher uses a short key that repeats,
whereas the running key cipher uses a long key such as an excerpt from a book. This
means the key does not repeat, making cryptanalysis more difficult. The cipher can still be
broken though, as there are statistical patterns in both the key and the plaintext which can
be exploited.
Steganography is a method where the very existence of the message is concealed. It is the
art and science of encoding hidden messages in such a way that no one, apart from the
sender and intended recipient, suspects the existence of the message. it is sometimes
referred to as Hiding in Plain Sight.
Cipher block chaining is a DES operating mode. IBM invented the cipher-block chaining
(CBC) mode of operation in 1976. In CBC mode, each block of plaintext is XORed with the
previous ciphertext block before being encrypted. This way, each ciphertext block depends
on all plaintext blocks processed up to that point. To make each message unique, an
initialization vector must be used in the first block.
Reference(s) used for this question:
HARRIS, Shon, All-In-One CISSP Certification Exam Guide, McGraw-Hill/Osborne, 2002,
chapter 8: Cryptography (page 555).
and
http://en.wikipedia.org/wiki/One-time_pad http://en.wikipedia.org/wiki/Running_key_cipher http://en.wikipedia.org/wiki/Cipher_block_chaining#Cipher-block_chaining_.28CBC.29
