Encryption in boot process doesn't make sense to me.
What you want to protect is the data in the storage, not the boot image
(kernel).
Encryption of the storage is OS's bussiness, and the OS will ask you the
password before decrypting any data.

Don't assume the boot image is a Linux kernel!

The boot image in this case would be a proprietary piece of software for real-time network filtering.  The algorythm for which is patented technology.  A user-input encryption key doesn't make sense as the box is stand-alone and does not have an operator.  A simple encryption at least stops someone from removing the drive (or flash disk) from the system and reverse engineering it.  Granted, they can get the key from the ROM, but if they have to disassemble the ROM to do it, it might slow things down.

It would at least be better protection than a non-encrypted system being booted by the PC BIOS.

----
Evan K. Langlois
Melior, Inc.

iSecure - CyberWarfare Defense
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