The
following figure, which is on the next page, illustrates the conventional
encryption process. The original “plaintext” is converted into apparently
random nonsense, called “ciphertext”. The encryption process consists of an
algorithm and a key.
The
key is a value independent of the plaintext. The algorithm will produce a
different output depending on the specific key being used at the time. Changing
the key changes the output of the algorithm, the ciphertext.
Once
the ciphertext is produced, it may be transmitted. Upon reception, the
ciphertext can be transformed back to the original plaintext by using a
decryption algorithm and the same key that was used for encryption.
The
security of conventional encryption depends on several factors:
1.
The
Encryption Algorithm. It must be powerful enough that it is impractical to
decrypt a message on the basis of the "ciphertext" alone.
2.
Secrecy
of the key- It was shown that the security of conventional encryption depends
on the secrecy of the key, not the secrecy of the algorithm.
Referring
to Fig. 1 above, with the message X and the encryption key K as input, the
encryption algorithm forms the ciphertext.
Y=Ek
(X)
The
intended receiver, in possession of the key is able to invert the
transformation
X=Dk
(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 X^.
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
K^.
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