Cryptography is the science that studies how to keep that data or messages stay safe when delivered, from the sender to the receiver without interference from third parties. According to Bruce Scheiner in his book "Applied Cryptography", cryptography is the science and art of keeping the message in order to remain safe (secure).

Cryptography is a branch of mathematics that has many functions in the data security. Cryptography is the process of taking the message and use some function to generate the cryptographic material (a digest or encrypted message).

Cryptography, in general is a science and an art to maintain the confidentiality of news [Bruce Schneier - Applied Cryptography]. In addition there is also an understanding of the meaning of the study of mathematical techniques related to aspects of information security such as data confidentiality, data authenticity, data integrity, and authentication of data [A. Menezes, P. van Oorschot and S. Vanstone - Handbook of Applied Cryptography]. Not all aspects of information security is handled by cryptography.

There are four fundamental goal of cryptography is the science which is also an aspect of information security, namely:

Confidentiality is a service used to keep the contents of the information of any person or authority except the secret key to unlock / peeling of information that has been encoded.

Data integrity, is associated with the maintenance of data changes illegally. To maintain the integrity of the data, the system must have the ability to detect data manipulation by parties who are not entitled to, including insertion, deletion, and substitution of other data into actual data.

Authentication, is related to the identification / recognition, either as a whole or the information system itself. The two parties must communicate introduced themselves. Information transmitted through a channel should be authenticated authenticity, data contents, delivery time, and others.

Non-repudiation., Or anti-denial is an attempt to prevent the denial of the delivery / creation of an information by the send / make.

The concept of cryptography itself has long been used by humans for example in Egypt and Roman civilization although still very simple. The principles underlying cryptographic namely:

**-Confidelity:**A service that sent the message content remains confidential and is not known by the other party (unless the sending party, the receiving party / parties have permission). Generally this is done by creating a mathematical algorithm that is able to transform the data to be difficult to read and understand.

**Data integrity:**A service that is able to recognize / detect any manipulation (deletion, alteration or addition) unauthorized data (by others).

**Authentication:**Namely services related to identification. Both authentication parties involved in data transmission and authentication of the authenticity of the data / information.

**Non-repudiation:**A service that can prevent a party to the previous action denying (denying that the message originated itself).

In contrast to classical cryptography which focuses on the power of secrecy algorithm used (which means that if the algorithm used is known then the message is clear "leaked" and it can be known by anyone who knows the algorithm), modern cryptography is focused on the secrecy of the key used in the algorithm (by the user) so that the algorithms can be distributed to the public without fear of loss of confidentiality for the wearer.

**Here are the terms used in the field of cryptography:**

Plaintext (M) is the message to be sent (containing original data).

Ciphertext (C) is a ter-encrypt the message (encrypted) which is the result of encryption.

Encryption (function E) is the process of changing plaintext into ciphertext.

Decryption (function D) is the inverse of the encryption that is changing ciphertext into plaintext, so that the data in the form of initial / original.

The key is a secret number that is used in the encryption and decryption process.

Cryptography itself consists of two main processes namely the process of encryption and decryption process. As mentioned above, the process of changing plaintext into ciphertext encrypted (using a specific key) so that the contents of the information in the message is difficult to understand.

The development of communications has prompted people to hide information in its possession of another person for reasons of security and privacy.

Someone is trying to develop and create a cryptographic code called Cryptographer. While someone who tried to break the code is called cryptanalists.

Cryptography has been known since 4000 years ago. Cryptography former is an attempt to change the message by adding or changing certain characters. Now, it has been found that the latest methods of cryptography: quantum cryptography.

**Physical Cryptography.**

Physical cryptography include methods to swap places (transposition) certain characters in the message.

Substitution ciphers is a cryptographic method by substituting letters / symbols specific to the character / symbol to another.

Eg, "a": "x" and "b": "y", then

"how are you": "xpx kxyxr".

Problems: (a) safe and (b) whether the "x" in the message truly "x" or the result of substitution?

Transposition ciphers is a cryptographic method to exchange or randomize word / word part of the message with certain rules.

example:

"I Love You": "aI eVol uYO"

That is the process of steganography to hide messages in other messages in the hope that the real message could not be detected.

example:

"I Love You": "I very love your house"

Hybrid System That combination of the above three methods. Example: Enigma machine used in World War II Germany to send a message to the boat "U" it, using a combination of substitution and transposition.

**Quantum Cryptography.**

Quantum cryptography is a relatively new method of encryption. In early 2002, its application is restricted to laboratories and secret government project. The process is based on the Heisenberg Uncertainty Principle is actually used in modern atomic theory.

In quantum cryptography, a message is sent by using cascading photons. If the receiver knows the sequence and polarity of the photons so he could translate the message, and if someone is trying to peek messages, some photons will change polarity. This will inform the recipient that the message has been read by other people, and he could ask the sender to resend the message with a different polarity photons.

Quantum cryptography allows for encryption that can not be solved. However the costs involved are very expensive and are still used in limited applications.

Password algorithm is an algorithm which is used to perform cryptographic purposes. The algorithm should have the power to do (put forward by Shannon):

confusion (confusion), from the text of the light so it is difficult to be reconstructed directly without using the decryption algorithm

diffusion / melting (Difusion), of the text so that the bright light of the characteristics of the text is lost.

so it can be used to secure information. In the implementation of a algoritmas password must pay attention to the quality of service / Quality of Service or QoS of the whole system in which he implemented. The algorithm is a powerful password password strength algorithm lies in the key, not the secrecy of the algorithm itself. Techniques and methods for testing the reliability of the algorithm is kriptanalisys password.

Basic mathematical processes underlying encryption and decryption is a relation between two sets that contain elements that light text / plaintext and cipher text containing elements / ciphertext. Encryption and decryption is a function of transformation between these associations. When light text elements denoted by P, the cipher text elements denoted by C, is for the encryption process is denoted by E, decryption with the notation D.

Encryption: E (P) = C

Decryption: D (C) = P or D (E (P)) = P

In general, based on common key, password algorithm can be divided into:

symmetric-key / symetric-key, often referred to as conventional password algorithm since the algorithm is generally applied to the classic password

asymmetric-key / asymmetric-key

Under the direction of implementation can be divided into:

classic classical cryptography algorithms password

Modern cryptography algorithms modern password

Based on key secrecy can be divided into:

secret-key encryption algorithm secret-key

public-key encryption algorithm is public-key

In symmetric-key schemes, a secret key is used together to perform encryption and decryption process. While in the lock-asimentris system used a different key pair, commonly called the public key (the public key) and private key (private key), used for encryption and decryption process. When the light of the text element is encrypted using the private key generates the cipher text elements can only didekripsikan by using a private key pairs. Vice versa, if the private key is used for encryption, the decryption process must use public key partner.

This scheme is an algorithm that uses different keys for encryption and decryption process. This scheme is also known as public key cryptography system as an encryption key to be made known by the public (public-key) or can be known to anyone, but for the decryption process can only be performed by authorized which has a secret key to decrypt it, so-called private-key . Is analogous to that post office box can only be opened by a postman who has the key but anyone can enter letters into the box. The advantage of this model algorithms, to correspond confidentially with many secret keys are not needed as much as the number of parties, enough to create two keys, ie public key for the korensponden to encrypt a message, and the private key to decrypt the message. In contrast to the symmetric-key scheme, the number of keys made is as much as the number of parties with whom to correspond.

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