Getting Your Authorize.Net Transaction Key

To use Credit Card Terminal, you’ll need your Transaction Key for Authorize.Net. If you don’t already have your Transaction Key, you have to get a new one from Authorize.Net.

Warning: Authorize.Net does not allow the retrieval of an existing transaction key. Following these instructions will get a new transaction key and disable any old transaction key. If you have other software, such as a shopping cart or web site, that uses the Authorize.Net Transaction Key, you will need to update that software with the new Transaction Key within 24 hours, otherwise credit card processing in the other software will be disabled. In this situation, you may wish to get your transaction key from the configuration for your existing software rather than obtaining a new key from Authorize.Net.

The following instructions for finding your Transaction Key are from Authorize.Net’s Merchant Integration Guide.

The Transaction Key is a 16-character alphanumeric value that is randomly generated in the Merchant Interface and works in conjunction with your API Login ID to authenticate you as an authorized user of the Authorize.Net Payment Gateway when submitting transactions from your Web site.

Like the API Login ID, the Transaction Key is a sensitive piece of account information that should only be shared on a need-to-know basis.

To obtain a Transaction Key:

1. Log into the Merchant Interface at https://secure.authorize.net
2. Select Settings under Account in the main menu on the left
3. Click API Login ID and Transaction Key in the Security Settings section
4. Enter the secret answer to the secret question you configured when you activated your user account
5. Click Submit

The Transaction Key for your account is displayed on a confirmation page.

IMPORTANT: Be sure to record your Transaction Key immediately in a secure manner or copy it immediately to a file in a secure location as it is not always visible in the Merchant Interface like the API Login ID. Once you navigate away from the confirmation page there will be no other way to access the Transaction Key in the Merchant Interface. You would have to generate a new Transaction Key.

It is highly recommended that you create a new Transaction Key regularly, such as every six months, to strengthen the security of your payment gateway account. You will then need to communicate the new Transaction Key to your Web developer immediately to update your Web site integration code. Failure to do so will result in a disruption in transaction processing.

来源：http://www.innerfence.com/apps/credit-card-terminal/getting-your-authorize-net-transaction-key