Character-Level Encryption

In this method, encryption is done at the character level. There are two general methods for character-level encryption: substitution and transpositional.

Substitutional

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.

Transpositional

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.

Bit-Level Encryption

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

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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)

Figure 1

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 Method

In 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.

Conventional Cryptography

In conventional cryptography, also called secret-key or symmetric-key encryption, one key is used both for encryption and decryption. Figure 1-2 is an illustration of the conventional encryption process.

Figure 1-2. Conventional encryption

Caesar's Cipher

A substitution cipher is an extremely simple example of conventional cryptography. A substitution cipher substitutes one piece of information for another. This is most frequently done by offsetting letters of the alphabet. In Julius Caesar's cipher, the algorithm is to offset the alphabet and the key is the number of characters to offset it.

For example, if we encode the word "SECRET" using Caesar's key value of 3, we offset the alphabet so that the 3rd letter down (D) begins the alphabet.

Plain Text        ABCDEFGHIJKLMNOPQRSTUVWXYZ

Cipher Text     DEFGHIJKLMNOPQRSTUVWXYZABC

Key Management and Conventional Encryption

Conventional encryption has benefits. It is very fast. It is especially useful for encrypting data that is not going anywhere. However, conventional encryption alone as a means for transmitting secure data can be quite expensive simply due to the difficulty of secure key distribution. The expense of secure channels and key distribution relegated its use only to those who could afford it, such as governments and large banks (or small children with secret decoder rings).

Recall a character from your favorite spy movie: the person with a locked briefcase handcuffed to his or her wrist. What is in the briefcase, anyway? It's probably not the missile launch code/ biotoxin formula/ invasion plan itself. It's the key that will decrypt the secret data.

For a sender and recipient to communicate securely using conventional encryption, they must agree upon a key and keep it secret between themselves. If they are in different physical locations, they must trust a courier, the Bat Phone, or some other secure communication medium to prevent the disclosure of the secret key during transmission. Anyone who overhears or intercepts the key in transit can later read, modify, and forge all information encrypted or authenticated with that key. The persistent problem with conventional encryption is key distribution: how do you get the key to the recipient without someone intercepting it?

And the minor problem with it is the storage of keys: when you want to communicate with a lot of people and you have one key for each partner, how do you manage so many keys?

Model of Conventional Cryptosystems

The following figure, which is on the next page, illustrates the conventional encryption process. The original “plaintext” is converted into apparently random nonsense, called “ciphertext”. The encryption process consists of an algorithm and a key.

The key is a value independent of the plaintext. The algorithm will produce a different output depending on the specific key being used at the time. Changing the key changes the output of the algorithm, the ciphertext.

Once the ciphertext is produced, it may be transmitted. Upon reception, the ciphertext can be transformed back to the original plaintext by using a decryption algorithm and the same key that was used for encryption.

 

The security of conventional encryption depends on several factors:

1.      The Encryption Algorithm. It must be powerful enough that it is impractical to decrypt a message on the basis of the "ciphertext" alone.

2.      Secrecy of the key- It was shown that the security of conventional encryption depends on the secrecy of the key, not the secrecy of the algorithm. 

Referring to Fig. 1 above, with the message X and the encryption key K as input, the encryption algorithm forms the ciphertext.

Y=E_k (X)

The intended receiver, in possession of the key is able to invert the transformation

X=D_k (Y)

An opponent, observing Y but not having access to K or X, may attempt to recover X or K or both X and K. It is assumed that the opponent knows the encryption (E) and decryption (D) algorithms.

If the opponent is interested in only this particular message, then the focus of the effort is to recover X by generating a plaintext estimate X^.

Often, however, the opponent is interested in being able to read future messages as well, in which case an attempt is made to recover K by generating an estimate K^.