To carry sensitive information, such as military or financial data, a system must be able to assure privacy. Microwave, satellite, and other wireless media, however, cannot be protected from the unauthorized reception (or interception) of transmissions. Even cable systems cannot always prevent unauthorized access. Cables pass through out-of-the-way areas (such as basements) that provide opportunities for malicious access to the cable and illegal reception of information.
It is unlikely that any system can completely prevent unauthorized access to trans¬mission media. A more practical way to protect information is to alter it so that only an authorized receiver can understand it. Data tampering is not a new issue, nor is it unique to the computer era. In fact, efforts to make information unreadable by unauthorized receivers date from Julius Caesar (100-44 B.C.). The method used today is called the encryption and decryption of information. Encryption means that the sender transforms the original information to another form and sends the resulting unintelligible message out over the network. Decryption reverses the encryption process in order to transform the message back to its original form.
Figure 1 shows the basic encryption/decryption process. The sender uses an encryption algorithm and a key to transform the plaintext (as the original message is called) into a cipher text (as the encrypted message is called). The receiver uses a decryption algorithm and a key to transform the cipher text back to the original plaintext.
Sender(Plain Text) --> Encryption Algorithm (ke) --> Cipher Text --> Decryption Algorithm (Kd)--> Receiver(Plain Text)
Figure 1
Encryption and decryption methods fall into 2 categories:
1. Conventional Method, and
2. Public key Method.
Conventional Method
In conventional encryption methods, the encryption key (Ke) and the decryption key (Kd) are the same and reamin secret. We can divide the conventional methods into 2 categories: Character-level encryption, and Bit-level encryption.
Public Key Method
In this method, every user has the same encryption algorithm and the key. The decryption algorithm and the key, however, are kept secret. Anyone can encrypt the information, but only an authorized receiver can decrypt it.