Build a custom payment gateway for Magento

We wanna have a custom payment gateway to accept credit card payment to our bank. This payment gateway required some features. 1. Using HTTP POST method. 2. An option to select test mode or normal mode.(Post to 2 different address) 3. There are only 8 parameters in this module. (MID)Merchant ID (CID)Child ID (TID)Terminal ID (ONO)Order # (TA)Total Price (U)Return URL (MV)MAC value (M)MAC Key And we will send 7 parameters to the bank. (MID), ex:"8089010411" (CID), must be empty (TID), "EC000001" or "EC000002" (ONO), ex:"100000300" (TA), ex:"30000" (U): return url (M), generated by a special rule. Combine all parameters in sequence with "&", like this. MID&CID&TID&ONO&TA&U&MV Then MD5 this string and we will get MAC Key(M). Attached file provided by our bank.(Is traditional Chinese). Sorry, we don't have english document now. But all necessary parameters can be find in page 2 and page 3.