Cryptography is an indispensable tool used to protect information in computing systems. It is used everywhere and by billions of people worldwide on a daily basis. It is used to protect data at rest and data in motion. Cryptographic systems are an integral part of standard protocols, most notably the Transport Layer Security (TLS) protocol, making it relatively easy to incorporate strong encryption into a wide range of applications. While extremely useful, cryptography is also highly brittle. The most secure cryptographic system can be rendered completely insecure by a single specification or programming error. No amount of unit testing will uncover security vulnerability in a cryptosystem.
The word cryptography comes from the Greek words κρυπτο (hidden or secret) and γραφη (writing). Oddly enough, cryptography is the art of secret writing. More generally, people think of cryptography as the art of mangling information into apparent unintelligibility in a manner allowing a secret method of unmangling. The basic service provided by cryptography is the ability to send information between participants in a way that prevents others from reading it. This kind of cryptography can provide other services, such as
Integrity checking—reassuring the recipient of a message that the message has not been altered since it was generated by a legitimate source
Authentication — Verifying someone’s (or something’s) identity But back to the traditional use of cryptography, A message in its original form is known as plaintext or cleartext. The mangled information is known as ciphertext. The process for producing ciphertext from plaintext is known as encryption. The reverse of encryption is called decryption.
Figure 1: Cryptography Process
Here Figure 1 shows the cryptography process of encryption and decryption. Cryptography is the art of achieving security by encoding messages to make them non-readable. Cryptography is the practice and study of hiding information. In modern times cryptography is considered a branch of both mathematics and computer science and is affiliated closely with information theory, computer security and engineering. Cryptography is used in applications present in technologically advanced societies; examples include the security of ATM cards, computer passwords and electronic commerce, which all depend on cryptography.
Classification of Cryptography
Cryptography can be divided into three major category based on the use of key is depicted in figure 2.
Figure 2: Cryptography Classification
- Symmetric Encryption (Private Key Encryption): In this type of encryption same key is used at the time of encryption and decryption. The key distribution has to be made before the transmission of the information starts. The key plays a very important role in this type of encryption. Symmetric encryption is known as secret key or single key, The receiver uses the same key which the sender uses to encrypt the data to decrypt the message,. This system was the only system used before discovering and developing the public key. A safe way of data transfer must be used to moving the secret key between the sender and the receiver in symmetric encryption. Figure 4 shows how the system works. Symmetric encryption occurs either by substitution transposition technique, or by a mixture of both. Substitution maps each plaintext element into cipher text element, but transposition transposes the positions of plaintext elements. Example: DES, 3DES, BLOWFISH, AES etc.
- Asymmetric Encryption (Public Key Encryption): In this type of encryption different key is being used for encryption and decryption process. Two different key is generated at once and one key is distributed to other side before the transmission starts; Asymmetric encryption is the opposite of symmetric encryption in safety, since it doesn’t require sharing the secret key between the sender and the receiver. And this is the main difference between symmetric and asymmetric encryption, the sender has the public key of the receiver. Because the receiver has his own secret key which is extremely difficult or impossible to know through the public key, no shared key is needed; the receiver is responsible for establishing his private and public key, and the receiver sends the public key to all senders by any channel he needs, even unsecured channels to send his public key, asymmetric key can use either the public or secret key to encrypt the data. Also it can use any of the keys in decryption, asymmetric encryption can be used to implement the authentication and non-repudiation security services, and also it can be used for digital signature and other application that never is implemented using symmetric encryption. Example: RSA algorithm.
- Hybrid Encryption (Combination of private and public): Hybrid encryption is a mode of encryption that merges two or more encryption systems. It incorporates a combination of asymmetric and symmetric encryption to benefit from the strengths of each form of encryption. These strengths are respectively defined as speed and security. Hybrid encryption is considered a highly secure type of encryption as long as the public and private keys are fully secure. A hybrid encryption scheme is one that blends the convenience of an asymmetric encryption scheme with the effectiveness of a symmetric encryption scheme. Hybrid encryption is achieved through data transfer using unique session keys along with symmetrical encryption. Public key encryption is implemented for random symmetric key encryption. The recipient then uses the public key encryption method to decrypt the symmetric key. Once the symmetric key is recovered, it is then used to decrypt the message. The combination of encryption methods has various advantages. One is that a connection channel is established between two users’ sets of equipment. Users then have the ability to communicate through hybrid encryption. Asymmetric encryption can slow down the encryption process, but with the simultaneous use of symmetric encryption, both forms of encryption are enhanced.
 Manoj Kumar Pandey, Mrs. Deepty Dubey, “Survey Paper: Cryptography The art of hiding Information”, International Journal of Advanced Research in Computer Engineering & Technology (IJARCET), PP. 3168-3171,Volume 2, December 2013.
 Mohammed AbuTaha, MousaFarajallah and Radwan Tahboub, “Survey Paper: Cryptography is the Science of Information Security”, International Journal of Computer Science and Security (IJCSS), PP. 298- 309, Volume (5), 2011.
 Prakash Kuppuswamy, and Saeed Q. Y. Al-Khalidi, “Hybrid Encryption/Decryption Technique Using New Public Key and Symmetric Key Algorithm”, MIS Review: An International Journal, Volume 19, No. 2, PP. 1-13, March 2014.