Problem 2

A web-authentication token has the known format

Bytes 0-19 User name

Bytes 20-22 User rights

Byte 23 Check-byte

The check-byte is the exclusive-or of the preceding 23 bytes. The tokens are encrypted with a stream cipher and stored at the clients machine. An adversarial user has determined that her user-rights bytes are (in hex) 0x01 0x01 0x01 and would like to change them to 0x07 0x07 0x07. Show how she can alter the encrypted token so that the altered token still has a correct check-byte.