# first lecture

```Education college for girls
Computer Science Dept.
Subject: cryptography
Year: 2011-2012
Lecture:1
Introduction to communications security
The need to keep certain messages secret has been appreciated for
thousands of years. Of course, people were not low to realize the
advantages to be gained from intercepting secret information, and this has
led to a continuous, fascinating battle between the 'codemakers' and the
'codebreakers'. The arena for this contest is the communications medium
which has changed considerably over the years. Society is now highly
dependent on our modern, fast and accurate means of transmitting
messages. As well as the long-established forms, such as the post and
courier services, we now have more technical and sophisticated media
such as radio, television, telephone, telex and high speed data links. There
are, however, a number of situations where the information is
confidential, and where an interceptor might able to benefit immensely
from the knowledge gained by monitoring the information circuit. In such
situations, the communicants must take steps to conceal and protect the
content of their message. Of course the amount of protection required will
vary. On some occasion it is sufficient to prevent a casual listener from
understanding the message, but there are other times when it is crucial
that even the most determined interceptor must not be able to deduce it.
If the communications were able to use a non- interceptible means of
transmission then, obviously, all their messages would be secure. But the
most common forms of communication don’t satisfy this requirement.
The method which comes nearest to satisfying it is, probably, the use of
courier. But this is likely to be very slow. Very expensive and, if the
number of messages requiring transmission is large, might even be
impossible. One alternative to a non- interceptible means of transmission
is to conceal the content of each message by transforming it before
transmission. This is the objective of a cipher system. The science of
designing such systems is called cryptography.
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Cryptography
Cryptography (or cryptology; from Greek kryptos, &quot;hidden,
secret&quot;; and gr&aacute;ph, &quot;writing&quot;, is the practice and study of hiding
information. Modern cryptography intersects the disciplines of
mathematics, computer science, and electrical engineering.
Cryptanalysis
Cryptanalysis (from the Greek krypt&oacute;s, &quot;hidden&quot;, and anal&yacute;ein, &quot;to
loosen&quot; or &quot;to untie&quot;) is the study of methods for obtaining the meaning
is normally required to do so. Typically, this involves finding a secret
key.
Basic terms
1- Plaintext (P) &amp; Ciphertext(C)
In cryptography, plaintext is information a sender wishes to transmit
to a receiver. Cleartext is, sometimes confusingly, often used as a
synonym. Before the computer era, plaintext most commonly meant
message text in the language of the communicating parties. Plaintext has
reference to the operation of cryptographic algorithms, usually encryption
algorithms, and is the input upon which they operate. Cleartext, by
contrast, refers to data that is transmitted or stored unencrypted (that is,
'in the clear').
Ciphertext (or cyphertext) is the result of encryption performed on
plaintext using an algorithm, called a cipher. Ciphertext is also known as
encrypted or encoded information because it contains a form of the
original plaintext that is unreadable by a human or computer without the
proper cipher to decrypt it. Decryption, the inverse of encryption, is the
process of turning ciphertext into readable plaintext.
2- Encryption and Decryption (E&amp;D)
In cryptography, encryption is the process of transforming
information (referred to as plaintext) using an algorithm (called cipher) to
make it unreadable to anyone except those possessing special knowledge,
usually referred to as a key. The result of the process is encrypted
information (in cryptography, referred to as ciphertext). In many contexts,
the word encryption also implicitly refers to the reverse process,
decryption (e.g. “software for encryption” can typically also perform
decryption), to make the encrypted information readable again (i.e. to
make it unencrypted).
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3- Key (K)
In cryptography, a key is a piece of information (a parameter) that
determines the functional output of a cryptographic algorithm or cipher.
Without a key, the algorithm would produce no useful result. In
encryption, a key specifies the particular transformation of plaintext into
ciphertext, or vice versa during decryption.
Keeping keys secret is one of the most difficult problems in practical
cryptography. An attacker who obtains the key ( by, for example, theft,
extortion ) can recover the original message from the encrypted data.
Encryption algorithms which use the same key for both encryption
and decryption are known as symmetric key algorithms. A newer class of
&quot; public key &quot; cryptographic algorithms was invented in the 1970s which
uses a pair of keys, one to encrypt and one to decrypt. These asymmetric
key algorithms allow one key to be made public while training the private
key is extremely difficult, even if the corresponding public key is known.
A user of public key technology can publish their public key, while
keeping their private key secret, allowing anyone to send them an
encrypted message.
Below the figure demonstrates the above basic terms of cryptography
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