Just as compression is technically a data representation function, and thus a Presentation Layer service, so encryption is technically a data representation function, and thus a Presentation Layer service as well. Encryption is the process of disguising data. Those who do not know the method (the “trick”) cannot decipher it; those who do know the trick, can.
To demonstrate a simple encryption scheme, suppose we want to encrypt a memo created on a word processor. Our encryption scheme replaces each letter of the text with the letter that follows it in the alphabet. “B” follows “A,” “M” follows “L,” etc. “A” follows “Z” in our scheme. Therefore, to encode “THE,” replace “T” with “U,” “H” with “I,” and “E” with “F,” forming “UIF.” We repeat this process for each word and end up with an encrypted memo. No one can read and understand this memo in its present state unless they know how to decode it. The recipient must know how to decode, or decrypt, it. The decoding scheme involves substituting each letter with the one preceding it in the alphabet, “Z” preceding “A.” In this example, deciphering reverses the process of enciphering, or encrypting. Other encryption schemes make this impossible, and there are ways to make the trick known.
There are many applications for encryption, including password security, authorization, authentication, data protection, and virus protection. Securing passwords is one of the most common applications of encryption. When you create a password, it is encrypted so others cannot see it. Encryption helps provide authorization to determine which resources you are allowed to use. It is also used to ensure authentication, determining whether you are really who you say you are. Encryption can be used to protect any data on your computer or any data you send to another computer. Virus protection programs use encryption to detect virus attacks on your software. They can generate a code based on the contents of the file, such as a byte count. They encrypt this code and attach it to the file. Any modification to this file makes the code incompatible with the file contents, thus signaling a change.
Many encryption methods exist today. The National Institute of Standards and Technology declared that the Data Encryption Standard (DES) must be used for all nonclassified government information. RSA Cryptology, from RSA Data Security, Inc., is another popular scheme. Although it is proprietary, the RSA encryption method may be licensed for use in other products and software. Pretty Good Privacy (PGP) is widely used in e-mail applications.
Encryption is a Presentation Layer service, although it can be performed at other layers. There are both hardware and software solutions to encryption. The whole field of encryption in networked environments is a vast subject all by itself. Complete discussions of encryption include issues of “right to privacy,” anonymity, law enforcement, confidentiality, and many others.
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