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!

[tlhintoq]

Well, a byte can hold from 0-255. If you have 8 bytes free you have 256^8 = 1.84467441 × 10e19 possible combinations.
I would think you and the 3rd party should be able to come up with some kind of signature in there someplace.

Though I would think some type of custom checksum would be more useful. Anyone can read/write an RFID tag. If the signature is the same on all tags regardless of data then what's the point?

But if you create a custom checksum for the rest of the data that only you and the 3rd party understand then it changes based on the data and is therefore harder to forge.

[tmoloy]

Let me add a little more to clarify...

The data portion of the array (24 bytes) will be unique for every RFID tag. That way, the digital signature will be unique for every tag. I simply need to implement a public/private key system like this.
I'm trying to find a signing algorithm that produces an n-byte signature (8 in my case).

[tlhintoq]

The code included in this thread seems to be on an 8 byte key.
Maybe it will help