This parameter is used to verify the passbacks for you. Depending on what parameter set you are using, this will either appear as ‘key’ or ‘x_MD5_Hash‘. The MD5 hash is also provided to help you verify the authenticity of INS posts. On INS posts the hash is returned in the md5_hash parameter.

One of the Md5 hash components involves a secret word that can be set by you. To set it, follow these directions:

Setting Your Secret Word:

1. Login to your account.
2. Click on “Look and Feel” found on your account homepage.
3. Enter your secret word into the data field labeled, “Your Secret Word (16 Character Limit)”. As labeled, the only limit is that it must be 16 characters or less.
4. Click “Save Changes” when you are finished.

Please read the related article below for more information about the MD5 Specifications.
How do I use the MD5 Hash?