PPP Support for MPPE (Microsoft Point to Point Encryption)
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Frank Cusack		frank@google.com
Mar 19, 2002


DISCUSSION

MPPE is Microsoft's encryption scheme for PPP links.  It is pretty much
solely intended for use with PPP over Internet links -- if you have a true
point to point link you have little need for encryption.  It is generally
used with PPTP.

MPPE is negotiated within CCP (Compression Control Protocol) as option
18.  In order for MPPE to work, both peers must agree to do it.  This
complicates things enough that I chose to implement it as strictly a binary
option, off by default.  If you turn it on, all other compression options
are disabled and MPPE *must* be negotiated successfully in both directions
(CCP is unidirectional) or the link will be disconnected.  I think this is
reasonable since, if you want encryption, you want encryption.  That is,
I am not convinced that optional encryption is useful.

While PPP regards MPPE as a "compressor", it actually expands every frame
by 4 bytes, the MPPE overhead (encapsulation).

Because of the data expansion, you'll see that ppp interfaces get their
mtu reduced by 4 bytes whenever MPPE is negotiated.  This is because
when MPPE is active, it is *required* that *every* packet be encrypted.
PPPD sets the mtu = MIN(peer mru, configured mtu).  To ensure that
MPPE frames are not larger than the peer's mru, we reduce the mtu by 4
bytes so that the network layer never sends ppp a packet that's too large.

There is an option to compress the data before encrypting (MPPC), however
the algorithm is patented and requires execution of a license with Hifn.
MPPC as an RFC is a complete farce.  I have no further details on MPPC.

Some recommendations:

- Use stateless mode.  Stateful mode is disabled by default.  Unfortunately,
  stateless mode is very expensive as the peers must rekey for every packet.
- Use 128-bit encryption.
- Use MS-CHAPv2 only.

Reference documents:

    <http://www.ietf.org/rfc/rfc3078.txt> MPPE
    <http://www.ietf.org/rfc/rfc3079.txt> MPPE Key Derivation
    <http://www.ietf.org/rfc/rfc2118.txt> MPPC
    <http://www.ietf.org/rfc/rfc2637.txt> PPTP
    <http://www.ietf.org/rfc/rfc2548.txt> MS RADIUS Attributes

You might be interested in PoPToP, a Linux PPTP server.  You can find it at
<http://www.poptop.org/>

RADIUS support for MPPE is from Ralf Hofmann, <ralf.hofmann@elvido.net>.


BUILDING THE PPPD

The userland component of PPPD has no additional requirements above
those for MS-CHAP and MS-CHAPv2.  The kernel, however, requires SHA-1
and ARCFOUR.  Public domain implementations of these are provided.

Until such time as MPPE support ships with kernels, you can use
the Linux 2.2 or 2.4 implementation that comes with PPPD.  Run the
ppp/linux/mppe/mppeinstall.sh script, giving it the location to your
kernel source.  Then add the CONFIG_PPP_MPPE option to your config and
rebuild the kernel.  The ppp_mppe.o module is added, and the ppp.o module
(2.2) or ppp_generic.o (2.4) is modified (unfortunately).  You'll need
the new ppp.o/ppp_generic.o since it does the right thing for the 4
extra bytes problem discussed above.


CONFIGURATION

See pppd(8) for the MPPE options.  Under Linux, if your modutils is earlier
than 2.4.15, you will need to add

    alias ppp-compress-18 ppp_mppe

to /etc/modules.conf.  (A patch for earlier versions of modutils is included
with the kernel patches.)