vRtrRipGroupAuthKey is the authentication key that should be
used for the interfaces in this group when vRtrRipInstanceAuthType
has a value other than noAuthentication(1). Modifying the value of
vRtrRipGroupAuthType does not modify the vRtrRipGroupAuthKey
value. If a string shorter than 16 octets is supplied, it will
be left-justified and padded to 16 octets, on the right, with
nulls (0x00).
Reading this object always results in an OCTET STRING of
length zero; authentication may not be bypassed by reading
the MIB object.