Text that follows is based on the
Wikipedia page on cryptography!
Cryptography is the practice and study of hiding information. In modern times,
cryptography is considered to be a branch of both mathematics and computer
science, and is affiliated closely with information theory, computer security,
and
engineering. Cryptography is used in applications present in technologically
advanced societies; examples include the security of ATM cards, computer
passwords, and electronic commerce, which all depend on cryptography. Until
modern
times, cryptography referred almost exclusively to encryption, the process of
converting ordinary information (plaintext) into unintelligible gibberish (i.e.,
ciphertext). Decryption is the reverse, moving from unintelligible ciphertext to
plaintext. A cipher (or cypher) is a pair of algorithms which perform this
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 are
trivially breakable and 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. In
colloquial use, the term "code" is often used to mean any method of encryption
or
concealment of meaning. However, in cryptography, code has a more specific
meaning; it means the replacement of a unit of plaintext (i.e., a meaningful
word
or phrase) with a code word (for example, apple pie replaces attack at dawn).
Codes are no longer used in serious cryptography - except incidentally for such
things as unit designations (e.g., 'Bronco Flight' or Operation Overlord) -
since
properly chosen ciphers are both more practical and more secure than even the
best
codes, and better adapted to computers as well. Some use the terms cryptography
and cryptology interchangeably in English, while others use cryptography to
refer
specifically to the use and practice of cryptographic techniques, and cryptology
to refer to the combined study of cryptography and cryptanalysis. The Ancient
Greek scytale (rhymes with Italy), probably much like this modern
reconstruction,
may have been one of the earliest devices used to implement a cipher. Before the
modern era, cryptography was concerned solely with message confidentiality
(i.e.,
encryption) - conversion of messages from a comprehensible form into an
incomprehensible one, and back again at the other end, rendering it unreadable
by
interceptors or eavesdroppers without secret knowledge (namely, the key needed
for
decryption of that message). In recent decades, the field has expanded beyond
confidentiality concerns to include techniques for message integrity checking,
sender/receiver identity authentication, digital signatures, interactive proofs,
and secure computation, amongst others. The earliest forms of secret writing
required little more than local pen and paper analogs, as most people could not
read. More literacy, or opponent literacy, required actual cryptography. The
main
classical cipher types are transposition ciphers, which rearrange the order of
letters in a message (e.g., 'help me' becomes 'ehpl em' in a trivially simple
rearrangement scheme), and substitution ciphers, which systematically replace
letters or groups of letters with other letters or groups of letters (e.g., 'fly
at once' becomes 'gmz bu podf' by replacing each letter with the one following
it
in the alphabet). Simple versions of either offered little confidentiality from
enterprising opponents, and still don't. An early substitution cipher was the
Caesar cipher, in which each letter in the plaintext was replaced by a letter