- slide 1 of 3
Encryption - Decryption
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)
There are several data encryption standards and data encryption algorithms. However, Encryption and decryption methods fall into 2 categories:
1. Conventional Method, and
2. Public key Method.
Conventional MethodIn conventional encryption methods, the encryption key (Ke) and the decryption key (Kd) are the same and remain 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.
- slide 2 of 3
In this method, encryption is done at the character level. There are two general methods for character-level encryption: substitution and transpositional.
The simplest form of character-level encryption is substitution ciphering. In monoalphabetic substitution, sometimes called the Caesar Cipher each character is replaced by another character in the set. The monoalphabetic encryption algorithm simply adds a number to the ASCII code of the character; the decryption algorithm simply subtracts the same number from the ASCII co. Ke and Kd are the same and define the added or subtracted value. To make it simple, we do not encode the space character. If the substituted character is beyond the last character (Z), we wrap it around.
Monoalphabetic substitution is very simple, but the code can be broken easily by snoopers. The reason is that the method cannot hide the natural frequencies: characters in the language being used. For example, in English, the most frequently used characters are E, T, O, and A. A snooper can easily break the code by finding which character is used the most and replace that one with the letter E. It can then find the next most frequent and replace it with T, and so on.
In polyalphabetic substitution, each occurrence of a character can have a different substitute. One polyalphabetic encryption technique is to find the position of the character in the text and use that value as the key. However, polyalphabetic substitution is not very secure either. The reason is that although "DEAR DEAR" is replaced by "EGDV JLIA", the order of characters in "EGDV" and "JLIA" is still the same; the code can easily be broken by a more experienced snooper.
An even more secure method is transpositional encryption, in which the original characters remain the same but the positions of these characters are interchanged to create the cipher text. The text is organized into a two-dimensional table, and the columns are interchanged according to a key. For example, we can organize the plaintext into an eleven-column table and then reorganize the columns according to a key that indicates the interchange rule. As you have guessed, transpositional encryption is not very secure either. The character frequencies are preserved and the snooper can find the plaintext through trial and error.
- slide 3 of 3
In Bit-level encryption techniques, data as text, graphics, audio, or video are divided into different blocks of bits and then each block is altered using either of the techniques: encoding/decoding, permutation, substitution, etc.
Types of Encryption - Conventional Methods
This series deals with various types of encryption techniques and security flaws in them and how each technique is better than other or rather different from other. The method used today to protect data is called the encryption and decryption of information.