the data is not only text, there are several types of data such as : audio , image and other multimedia , images have been vastly used in our life , while the more widely we use image , the more necessary their security will be.
it should to give real data to real person at real time , what actually the data has been sent should be exactly the data been received.
image security has became a prime concern in the current computer world.
the prime things that a security has to achieved are: confidentiality, integrity availability and authentication.
most security system use cryptography cause it allows various algorithm and techniqe particulary its too hard to break because of thier complixity
cryptography ensures that the data must be sent without any modifieds and only the autherized person can be able to open and see the data, and there are alot of cryptography techniques developed to accomplish secure channel.
cryptography is the sceince that studies the mathmatical techniques for the message against attack, there are basic term used in cryptography the following below are explain it :
-Plain Text : The primer message that the person wants to communicate with the other
-Cipher Text : The result of converting primer message to meaningless message
-key: is a numeric or alpha numeric text or may be a special symbol.
-Encryption : A process of converting original-plain Text into Cipher Text this process requires an encryption algorithm and a key.
-Decryption : It is a process of converting Cipher Text into Plain Text , this process requires a Decryption algorithm and a key.and it is reverse process of encryption.
Cryptography is very important since the security of encryption algorithm depends directly on it.
it is classified into two techniques:
Substitution : the letters of plain text are replaced by other character or any number or by symbols. such as , Caesar cipher.
transposition : some sort of permutation is performed on plaintext. such as, columnar method .
The CIA principle
the purpose of cryptography is provides the security to ensure non modification of data , the privacy of data and so on.
the CIA is a vastly used criteria for evaluation of information systems security, depend on the three keys of Confidentiality, Integrity and Availability of information. this three key principles which must be guaranteed in any type of secure system, If any one of these keys can be breached it can have serious consequences for the parties concerned. 
Confidentiality :the ability to hide information from unauthorised person to see it. and preventing access by or disclosure to unauthorized ones, and it is concern with the concept of data privacy.
Integrity: The ability to ensure that data is an accurate and unauthorized user cannot modify or alert the data.
Availability: refers to ensuring that authorized parties are able to access the information when needed.
Cryptography algorithms Classification
Cryptography Algorithm can be divided into the following parts:
only one key for both encryption and decryption, and it is also called secret key . both the sender and the receiver must know the same key , This technique works by :
1.The plaintext is encrypted by sender with the key to produce cipher text and it is sent to the receiver.
2.The receiver uses the same key to decrypt the cipher text and exists the primary plaintext.
There are two patterns in this method: stream cipher & Block cipher.
The stream ciphers generate a sequence of bits used as a key called a key stream, and the encryption is achievied by combining the key stream with the original text ..
A block cipher converts a fixed-length block of plaintext into a block of cipher text which is of the same length. In decryption, same secret key is used by applying the reverse transformation ..
Asymmetric Cryptography (PKC)
it requires one key for each process , thus it is used a pair of keys , two: one to encrypt the plaintext and one to decrypt the cipher text, and it doesn’t work without one or another.
The most common public key algorithm is the RSA , used for key exchange, digital signatures, or encryption of small blocks of data.. 
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