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How to embed a digital signature with limited space

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!
Sep 24 '09 #1
3 2950
3,525 Expert 2GB
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.
Sep 24 '09 #2
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).
Sep 24 '09 #3
3,525 Expert 2GB
The code included in this thread seems to be on an 8 byte key.
Maybe it will help
Sep 24 '09 #4

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