# 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.

 Praise for 'Real 802.11 Security: Wi-Fi Protected Access and 802.11i'
 Preface
 Acknowledgments
 Part I: What Everyone Should Know
 Part II: The Design of Wi-Fi Security
 Part III: Wi-Fi Security in the Real World
 Acronyms
 References

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