Integrity checking NANOBSD images

We have a number of Soekris devices that we will be deploying 
remotely in semi- hostile physical environments.   The remote links 
are dialup so I dont have a lot of bandwidth available.  I want to do 
integrity checks of the images so that I can detect any tampering of 
the flash image.

If I upload a static sha256 binary to /tmp on the remote box (which 
is a RAM disk) and then do something like

e.g.
# ssh remote1.example.com "mkdir /tmp/rand-directory"
# scp /usr/local/bin/sha256 remote1.example.com:/tmp/rand-directory/sha256
# scp /usr/local/bin/dd remote1.example.com:/tmp/rand-directory/dd

# ssh remote1.example.com "/tmp/rand-directory/dd if=/dev/ad2s1a 
bs=4096k | /tmp/rand-directory/sha256"
120+1 records in
120+1 records out
505389056 bytes transferred in 169.727727 secs (2977646 bytes/sec)
955ebad583bfc0718eb28ac89563941407294d5c61a0c0f35e3773f029cc0685

Can I be reasonably certain the image has not been tampered with 
?  Or are there trivial ways to defeat this check ?

The flash is always mounted read-only, so in theory nothing should 
change with it.  Or do I need to cram on tripwire or similar programs 
onto the nanobsd image ?

         ---Mike

--------------------------------------------------------------------
Mike Tancsa,                                      tel +1 519 651 3400
Sentex Communications,                            mike@sentex.net
Providing Internet since 1994                    www.sentex.net
Cambridge, Ontario Canada                         www.sentex.net/mike

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