vRtrRipInstanceAuthKey is the authentication key that should be
used for the RIP instance when vRtrRipInstanceAuthType has a value
other than noAuthentication(1). Modifying the value of
vRtrRipInstanceAuthType does not modify the vRtrRipInstanceAuthKey
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.