# 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  Appendixes  Appendix A. Overview of the AES Block Cipher  Appendix B. Example Message Modification  Example Message Modification  Appendix C. Verifying the Integrity of Downloaded Files  Acronyms  References

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