The Edge SWG (Proxy SG) currently only supports the standard AES128 encryption method for SNMP.
EDGE SWG 7.3, 7.4
When configuring SNMP on the Edge SWG, there is an option to use AES encryption, as shown below:
Some SNMP vendors may require you to select a specific version of AES (e.g AES192, AES128 or AES256). When selecting "AES" on the ProxySG, you should select "AES128" on your SNMP receiving software.