Chapter 12 Problem 12.1 from Cryptography and Network Security 6E - William Stal

Question

Chapter 12 Problem 12.1 from Cryptography and Network Security 6E - William Stallings

If F is an error-detection function, either internal or external use will provide error-detection capability. If any bit of the transmitted message is altered, this will be reflected in a mismatch of the received FCS and the calculated FCS, whether the FCS function is performed inside or outside the encryption function. Some codes also provide an error-correction capability. Depending on the nature of the function, if one or a small number of bits is altered in transit, the error-correction code contains sufficient redundant information to determine the errored bit or bits and correct them. Clearly, an error-correction code will provide error correction capability when used external to the encryption function. Will it also provide this capability if used internal to the encryption function?

Explanation / Answer

Here we are talking about the message authentication which is basically a procedure to verify the source of the message so that we can know about the authenticated message .
So message authentication not only verify the source of the messsage but also the sequencing and availablity .
Message authentication is basically divided into three functions

1. Message Encryption
2. Massage Authentication Code (MAC)
3. Hash Function

Message encryption deals with the codes means in this function the ciphertext is used for the message verification .
In this FCS (frame check sequence) is used to force the original message to make a particular pattern so it will be easily detected by the message authenticator .
FCS is a error- detecting code . In the error detection if calculate FCS value is exactly equal to the upcoming FCS value then the message will be called verified of authenticated . FCS is used as external to the encryption in the term of message verification and as well as the internal to the encryption function also .

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