piclist 2001\01\08\052117a >
Thread: 32bit block cipher
face BY : Roman Black email (remove spam text)

Germain Morbe wrote:
> Hi,
> does anybody know a short 32bit block cipher routine for the pic. The keylen
> need not be more than 64bit. I already saw TEA but its blocklen is 64 bit
> and key is 128. Therefore i think one could save codespace and even ram. My
> requirement is not extremely high sercurity but rather good mixing from one
> input bit to more (best 50%) output bits.

It is a federal offence for USA firms or citizens to
release cipher technology/information to other countries.
It is considered weapons technology or something and
it is one thing the Feds DO keep an eye on.
I think it might even be illegal for a US citizen to
discuss methods with non-US citizen.

However I'm Australian and not limited by US federal
laws in this regard. :o) I have not done this on a PIC
but have done it in an application on the PC.

Most encryption systems use a transpose/substitute
system, where the key is used to determine the
position the data is transposed to in the block,
could be a simple rotate by x bits for example.
With x determined by data in the key.
Then you follow with a substitution where the
same key, or a different part of it, is used to
substitute one data unit for another. If each data
unit is a byte (like an alpha character), that byte
would be incremented by a value determined by the
key. Or each nibble, 2 bits etc is substituted.

When this transpose/substitute cycle is carried out
for only a few cycles the encryption is pretty secure.
With a fast PIC you can do it many times, even have
the total number of cycles determined by the key also.
This becomes very difficult to solve, and is one
of the reasons the Feds don't like it!
It gets more complex but you said you didn't need
max security and it sounds like ram/speed are issues
(aren't they always!)

The big issue is always key security, much easier
to crack the delivery system and get the key, or
crack the encryption algorithm within the product,
than to crack a message itself. Have you read up on
public/private key systems?


PS. Although the US cannot export encryption products
to your country you can export your product to the US.

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<3A599423.2E8@ezy.net.au> 7bit

See also: www.piclist.com/techref/microchip/devices.htm?key=pic
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