Example Message Modification

For this example, we take the message above (ASCII 'N') and modify the message so it becomes an ASCII y and we still ensure that the ICV remains valid using the process described in Chapter 15.

Stating the problem: Given P(x)= 01001110110, derive P'(x)=01111001XXX where XXX is a valid CRC.

First, the delta between the current message, M(x), and the desired message, N(x), is computed by the exclusive OR of the two values.

 M(x) = 01001110 N(x) = 01111001 D(x) = 00110111 

Now, the CRC is calculated for the delta value:

           ------------     1101 |00110111000              1101                  1100                  1101                    010 

The remainder of the division is the CRC of the delta. Now the delta and its CRC are exclusive OR'd with P(x):

 P(x)     01001110110 D(x)  00110111000                     01111001110 CRC(D)                 010 P'(x)  01111001100 

Now, P'(x) is shown to be a valid message; the remainder of the CRC calculation is 0.

             ------------        1101 | 01111001100               1101                   1000               1101                 1011                 1101                   1101                   1101                         0 

Because the remainder of the CRC calculation is 0, P'(x) has a valid CRC and the message has been successfully modified.