Re: [babel] hmac info model elements

["STARK, BARBARA H" <[e-mail]>]

> >> > Toke said: "Bird uses an unencoded string..."
> >> > <bhs> I'm not sure what "unencoded" means here? But this sounds
> >> > like maybe the entered string is a "passphrase" / PSK, like what's
> >> > used for Wi-Fi, which is then used to derive the HMAC key using
> >> > Unicode encoding?
> >>
> >> Nope, no derivation, just the raw ASCII bytes from the string used as
> >> an HMAC key, zero-padded to the block size. Unless the supplied ASCII
> >> string is longer than the block size, in which case it is hashed
> >> first.
> >
> > ^^^^^^ ???? So you are saying an overlong key is transmuted into
> > something else entirely, not truncated?
> 
> Yup, that would appear to be the case:
> 
> https://gitlab.labs.nic.cz/labs/bird/blob/master/lib/mac.c#L105

I noticed the referenced code mentioned it could be used for OSPF (RFC 5709). So I went to see if RFC 5709 had anything to say about this. And it does:
   (1) PREPARATION OF KEY
       In this application, Ko is always L octets long.

       If the Authentication Key (K) is L octets long, then Ko is equal
       to K.  If the Authentication Key (K) is more than L octets long,
       then Ko is set to H(K).  If the Authentication Key (K) is less
       than L octets long, then Ko is set to the Authentication Key (K)
       with zeros appended to the end of the Authentication Key (K),
       such that Ko is L octets long.

I'd be fine with this scheme, but think we should probably specify it somewhere, like OSPF does. 
I notice OSPF makes no mention of arriving at the Authentication Key from a string (or hex). It just assumes its dealing with something that has a length measured in octets. 
WPA2 limits its passphrase strings to "printable ASCII" characters (32 - 126). Since OSPF doesn't seem to care how it comes by the Key octets, it has no such limit in its spec. I wonder if there are any characters that would make one of the babel implementations choke; if so we may want to limit what characters can be used if input as ASCII. WPA2 does also allow hex PSK, which has no limitation on usable values.
Barbara