How to embed a digital signature with limited space

[tmoloy]

I am using RFID tags to store some data which will then be read by a 3rd party. I need to include a digital signature (or some variation) along with the data so that the 3rd party can verify the authenticity of the tag.

Straightforward enough. But the problem is that I am constrained by the RFID tag's 32-byte memory array. My application is written in C# (.NET 3.5) and the available digital signature class (DSACryptoServiceProvider) results in a 40 byte signature.

Assume that I need 24 of the 32 bytes to store my data. Is there some way to come up with a 8 byte signature?

Any insight is much appreciated!

[RedSon]

Which signature algorithm are you using? I'm sure there is one that has a signature less than 64 bit signature.

[Glenton]

This should be pretty straightfoward.
1. Make a hash function to convert your 24 bit data into 8 bit data (call this H).
2. Encrypt the 8 bit with your private key (this creates another 8 bits called E).
3. Send the data and E on the RFID.
The hash function and your public key can be made public.

Verification by receiving party is then:
1. Put data through hash function (to create H).
2. Decrypt E with your public key (to create H).
3. If the H's match you're done.

See this diagram

Good luck!