Not sure what you're asking here? The URL's you connect to at Authorize.net are all https://, which means they're automatically encrypted as long as you have support for connecting via https. Are you using the AIM API or sending XML directly?
i am sending XML request to Authorize.net . that xml request i want to encrypt so when i will send request to authorize.net we ll ensure that request what i have sent is secure enough. can i make it encrypt using any encryption algo so authorize.net can accept it ?
All data sent to Authorize.Net is sent over SSL so it already is encrypted.