Encryption and Decryption

Encryption is the process of translating plain text data (plaintext) into something that appears to be random and meaningless (ciphertext). Decryption is the process of converting ciphertext back to plaintext.

To encrypt more than a small amount of data, symmetric encryption is used. A symmetric key is used during both the encryption and decryption processes. To decrypt a particular piece of ciphertext, the key that was used to encrypt the data must be used.

The goal of every encryption algorithm is to make it as difficult as possible to decrypt the generated ciphertext without using the key. If a really good encryption algorithm is used, there is no technique significantly better than methodically trying every possible key. For such an algorithm, the longer the key, the more difficult it is to decrypt a piece of ciphertext without possessing the key.

It is difficult to determine the quality of an encryption algorithm. Algorithms that look promising sometimes turn out to be very easy to break, given the proper attack. When selecting an encryption algorithm, it is a good idea to choose one that has been in use for several years and has successfully resisted all attacks.

When we use the Internet, we're not always just clicking around and passively taking in information, such as reading news articles or blog posts -- a great deal of our time online involves sending others our own information. Ordering something over the Internet, whether it's a book, a CD or anything else from an online vendor, or signing up for an online account, requires entering in a good deal of sensitive personal information. A typical transaction might include not only our names,e-mail addresses and physical address and phone number, but also passwords and personal identification numbers (PINs).

The incredible growth of the Internet has excited businesses and consumers alike with its promise of changing the way we live and work. It's extremely easy to buy and sell goods all over the world while sitting in front of a laptop. But security is a major concern on the Internet, especially when you're using it to send sensitive information between parties.

Let's face it, there's a whole lot of information that we don't want other people to see, such as:

·         Credit-card information

·         Social Security numbers

·         Private correspondence

·         Personal details

·         Sensitive company information

·         Bank-account information ­

Information security is provided on computers and over the Internet by a variety of methods. A simple but straightforward security method is to only keep sensitive information on removable storage media like portable flash memory drives or external hard drives. But the most popular forms of security all rely onencryption, the process of encoding information in such a way that only the person (or computer) with thekey can decode it.

Decryption is the reverse operation of encryption. For secret-key encryption, you must know both the key and IV that were used to encrypt the data. For public-key encryption, you must know either the public key (if the data was encrypted using the private key) or the private key (if the data was encrypted using the public key).

Symmetric Decryption

The decryption of data encrypted with symmetric algorithms is similar to the process used to encrypt data with symmetric algorithms. The CryptoStreamclass is used with symmetric cryptography classes provided by the .NET Framework to decrypt data read from any managed stream object.

The following example illustrates how to create a new instance of the RijndaelManaged class and use it to perform decryption on a CryptoStream object. This example first creates a new instance of the RijndaelManaged class. Next it creates a CryptoStream object and initializes it to the value of a managed stream called MyStream. Next, the CreateDecryptor method from the RijndaelManaged class is passed the same key and IV that was used for encryption and is then passed to the CryptoStream constructor. Finally, the CryptoStreamMode.Read enumeration is passed to the CryptoStream constructor to specify read access to the stream.