Authentication Algorithm for embedded RFID device

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Hi All,

I need to write algorithm (to code later in C) for an embedded RFID
I have a reader and a tag and I want to authenticate that the tag is an
original one ( there is no need to authenticate the reader for the

To do so the tags will have a secret number that is know in advance to
the reader. The secret number will be put in manufacturing.

The authentication will be as follow:
1. The reader will send a random number to the tag.
2. The tag will do a certain algorithm with his secret number and the
random number and will send the result to the reader.
3. The reader will authenticate the return number and will send an ACK.
4. The tag will then send is ID number, (The ID number is not the
secret number, the secret number is never transmitted between the tag
and the reader)

I need to fine a good and simple algorithm (for 8 bit microcontroller
with low power) to do the work as describe in stage 2.

Any suggestion? Any useful links? Source code?
BTW if you have a better scenario to achieve the authentication I'll be
happy to read about.

Thanks in Advance

Re: Authentication Algorithm for embedded RFID device
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AES can be implemented in an 8-bit micro and may provide enough
security for you. Look at the Linux CryptoAPI sources.

Re: Authentication Algorithm for embedded RFID device

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Lookup "asymmetric key exchange". A standard procedure in the internet.
Perhaps "SSL" could also be looked up.

Ing.Buero R.Tschaggelar -
& commercial newsgroups -

Re: Authentication Algorithm for embedded RFID device
A related idea is one which has been used many times to do exactly what
you want using Two linear feedback shift registers,the first LFSR is
seeded with the remote tags serial number and the reader can verify the
authenticity of the tag using a simple challenge/response algo the lfsr
can be thought of as producing a destinct sequence for each challenge
from the reader.This is basically the algo used in car alarm key fobs.I
cannot remember the name of the company which has patented the basic
technique at the moment sorry.

Re: Authentication Algorithm for embedded RFID device

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A modern and strong algorithm for an 8 bit processor is AES (Advanced
Encryption Standard) ,see  It may be an
advantage to load all the subkeys, in stead of having a key scheduler.
RC5 may also be a suitable candidate.

There seems to be a flaw in the protocol, because after the reader sends the
ack, a rogue tag may answer, because the steps in the protocol are not
A solution may be:
reader sends random challenge to the tag ,
tag aswers with: encrypted(challenge concatenated with serial number) reader
(this may also satisfy groups worried about privacy ;-)
reader decrypts response and if the returned challenge is the same, the
reader accepts the serial number

See the work of Burrows Needham and Abadi e.g. for the design of authentication protocols

See the work of Ross Anderson for
technical papers and juicy stories of protocol failures

Success, Wim Ton

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