The value to be used as the Authentication Key in Level 1 Link State
Packets whenever the value of usdIsisSysAreaAuthenticationKeyType has a
value of plaintext or hmacMd5. A modification of
usdIsisSysAreaAuthenticationKeyType does not modify the
usdIsisSysAreaAuthenticationPwd value.
Reading this object always results in an OCTET STRING of length zero;
authentication may not be bypassed by reading the MIB object.