For compatibility, SNMPv3 supports three security models:
The default is USM. You can select the security model based on your network manager.
When a user is created, an authentication method is selected, and the authentication and privacy passwords or keys are entered.
When RSA Data Security, Inc. MD5 Message-Digest Algorithm authentication is specified, HMAC-MD5-96 is used to achieve authentication with a 16-octet key, which generates a 128-bit authorization code. This authorization code is inserted in the msgAuthenticationParameters field of SNMPv3 PDUs when the security level is specified as either AuthnoPriv or AuthPriv. Specifying SHA authentication uses the HMAC-SHA protocol with a 20-octet key for authentication.
For privacy, the user can select any one of the following supported privacy protocols: DES, 3DES, AES 128/192/256. In the case of DES, a 16-octet key is provided as input to DES-CBS encryption protocol which generates an encrypted PDU to be transmitted. DES uses bytes 1-7 to make a 56 bit key. This key (encrypted itself) is placed in msgPrivacyParameters of SNMPv3 PDUs when the security level is specified as AuthPriv.