the topic is ... .... .... ...
No', exactly that's not sensational, but the model of conventional Cryptograpy..
yuppp, take a look at 'her';;)
A source produces a message in plaintext, X = [ X 1 , X 2 , ..., X M ]. The M elements of X are letters in some finite alphabet. Traditionally, the alphabet usually consisted of the 26 capital letters . Nowadays, the binary alphabet {0, 1} is typically used. For encryption, a key of the form K = [ K 1 , K 2 , ..., K J ] is generated. If the key is generated at the message source, then it must also be provided to the destination by means of some secure channel. Alternatively, a third party could generate the key and securely deliver it to both source and destination.
With the message
X
and the encryption key
K
as input, the encryption algorithm forms the ciphertext
Y
= [
Y
1
,
Y
2
, ...,
Y
N
]. We can write this as
Y = E( K , X )
This notation indicates that
Y
is produced by using encryption algorithm E as a function of the plaintext
X
, with the specific function determined by the value of the key
K
.
The intended receiver, in possession of the key, is able to invert the transformation:
X
= D(
K
,
Y
)
An opponent, observing
Y
but not having access to
K
or
X
, may attempt to recover
X
or
K
or both
X
and
K
. It is assumed that the opponent knows the encryption (E) and
decryption (D) algorithms. If the opponent is interested in only this
particular message, then the focus of the effort is to recover
X
by generating a plaintext estimate
. Often, however, the opponent is interested in being able to read
future messages as well, in which case an attempt is made to recover
K
by generating an estimate
.
. Often, however, the opponent is interested in being able to read
future messages as well, in which case an attempt is made to recover
K
by generating an estimate
.
Cryptography
Cryptographic systems are characterized along three independent dimensions:
-
The type of operations used for transforming plaintext to ciphertext. All encryption algorithms are based on two general principles: substitution, in which each element in the plaintext (bit, letter, group of bits or letters) is mapped into another element, and transposition, in which elements in the plaintext are rearranged. The fundamental requirement is that no information be lost (that is, that all operations are reversible). Most systems, referred to as product systems , involve multiple stages of substitutions and transpositions.
-
The number of keys used. If both sender and receiver use the same key, the system is referred to as symmetric, single-key, secret-key, or conventional encryption. If the sender and receiver use different keys, the system is referred to as asymmetric, two-key, or public-key encryption.
-
The way in which the plaintext is processed . A block cipher processes the input one block of elements at a time, producing an output block for each input block. A stream cipher processes the input elements continuously, producing output one element at a time, as it goes along.
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