encrypting pppd?

Anyone worked on, or know of a freely available, one of these beasts?

Ian Goldberg <iang@cs.berkeley.edu> wrote:

> -----BEGIN PGP SIGNED MESSAGE-----
>
> In article <961002.235706.1R8.rnr.w165w@sendai.scytale.com>,
> Roy M. Silvernail <roy@scytale.com> wrote:
> >In list.cypherpunks, vax@linkdead.paranoia.com writes:
> >
> >> Anyone worked on, or know of a freely available, one of these beasts?
> >
> >What threat model does this address?  It'd be link encryption, where the
> >best security is found in end-to-end encryption.
>
> pppd doesn't necessarily run over a modem; you can "tunnel" it over
> another IP connection.
>
> I have in fact done this very thing.  Use ssh to (encrypted) log in to
> the ppp server, and start pppd at each end.  It's been a while; I think
> I had to tweak something to make it work over a pty instead of a serial
> port.

Here's a little script for tunnelling SLIP over a ssh session on Linux:

(sleep 2;\
 /usr/local/bin/ssh -l username -e none remotehost</dev/ttyqe>&/dev/ttyqe)&
(echo "port ptyqe";\
 echo "wait ord: 15";\
 echo "send secretpw\\n";\
 echo "wait (shellprompt)> 60";\
 echo "send slip\\n";\
 echo get \$rmtip 192.168.0.1;\
 echo get \$locip 192.168.0.2;\
 echo "mode CSLIP";sleep 15)|/sbin/dip -tv

PPP is a little more complicated; I found it's easier if you put the ssh on
the pty instead of the tty side.  You can't do this from a shell script,
because the pty can only be opened once.  This will take care of that:

main(ac,av)int ac;char **av;{
  int fd;
  fd=open("/dev/tty",O_RDWR);
  ioctl(fd,TIOCNOTTY);
  close(fd);
  fd=open("/dev/ptyqf",O_RDWR);
  if(fd<=0) exit(1);
  dup2(fd,0);
  dup2(fd,1);
  dup2(fd,2);
  execl("/usr/local/bin/ssh","ssh","-l","username","-e","none","-c","idea",
        "-t","remotehost","mesg n;/usr/sbin/ppp -direct mylink",(char *)0);
  exit(1);
}

Do remember that you can't route the ip addresses that you're tunneling
inside the tunnel!  So be sure to assign some alternate ip addresses to
each end of the link.  192.168.x.x will work; those addresses aren't
assigned to anyone.

I used /dev/ttyqf for the tty, that is usually unused.  If you have more
than 30 users online then you'll need to increase it.

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In article <961002.235706.1R8.rnr.w165w@sendai.scytale.com>,
Roy M. Silvernail <roy@scytale.com> wrote:
>In list.cypherpunks, vax@linkdead.paranoia.com writes:
>
>> Anyone worked on, or know of a freely available, one of these beasts?
>
>What threat model does this address?  It'd be link encryption, where the
>best security is found in end-to-end encryption.

pppd doesn't necessarily run over a modem; you can "tunnel" it over another
IP connection.

I have in fact done this very thing.  Use ssh to (encrypted) log in to
the ppp server, and start pppd at each end.  It's been a while; I think I
had to tweak something to make it work over a pty instead of a serial port.

   - Ian

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> pppd doesn't necessarily run over a modem; you can "tunnel" it over another
> IP connection.
> 
> I have in fact done this very thing.  Use ssh to (encrypted) log in to
> the ppp server, and start pppd at each end.  It's been a while; I think I
> had to tweak something to make it work over a pty instead of a serial port.
> 
>    - Ian

You need to use my ssh tunneling patch:

ftp://suburbia.net/pub/mailinglists/best-of-security/179

-- 
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 torment us for own good  will torment us  without end,  for they do so with 
 the approval of their own conscience."    -   C.S. Lewis, _God in the Dock_ 
+---------------------+--------------------+----------------------------------+
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In list.cypherpunks, vax@linkdead.paranoia.com writes:

> Anyone worked on, or know of a freely available, one of these beasts?

What threat model does this address?  It'd be link encryption, where the
best security is found in end-to-end encryption.
- -- 
           Roy M. Silvernail     [ ]      roy@scytale.com
DNRC Minister Plenipotentiary of All Things Confusing, Software Division
PGP Public Key fingerprint =  31 86 EC B9 DB 76 A7 54  13 0B 6A 6B CC 09 18 B6
                Key available from pubkey@scytale.com

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In message <961002.235706.1R8.rnr.w165w@sendai.scytale.com>, Roy M. Silvernail 
writes:
>What threat model does this address?

snooping the link

>It'd be link encryption, where the
>best security is found in end-to-end encryption.

Encrypting at higher levels involves a different effort/cost
tradeoff that doesn't do much better at addressing the threat
mentioned above.  AFAIK, application-level involves modification
of every app we are interested in, and network or transport level
should probably best wait for IPv6.  I think link-layer is best
for what we need.

Come to think of it I've never seen papers on this kind of issue,
probably because I haven't looked.  Anyone got any URLs/bibliorefs
to a paper on the benefits of encryption or authentication at the
different levels of the OSI or other network models?

Thinking about it a bit more, if you only encrypt, say, telnet
then you've got a pretty predictable plaintext stream.  If you
encrypt the entire link level properly then it might be much harder
to isolate the nonvariant bits of the protocols since the port and
that kind of header info is not available to the attacker at that
level.