It's definitely something they require. Here's the text from their SIM guide (under Section three > Generating the Unique Transaction Fingerprint) at http://developer.authorize.net/guides/SIM/

"The transaction authentication piece for the Server Integration Method is a “transaction fingerprint,” or a hash of merchant and transaction specific information using the HMAC-MD5 hashing algorithm. The transaction fingerprint must be generated on a per-transaction basis by a server-side script on the merchant’s Web server and inserted into the transaction request. The payment gateway uses the same mutually exclusive merchant information to decrypt the transaction fingerprint and authenticate the transaction."

That's interesting that you don't have to do this. How to you authenticate the transaction? -Mark
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>I have used the SIM method for Authorize.net and I never had to send a transaction fingerprint. Is this something you require or something they require?