PPP Support for MPPE (Microsoft Point to Point Encryption) ========================================================== 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: MPPE MPPE Key Derivation MPPC PPTP MS RADIUS Attributes You might be interested in PoPToP, a Linux PPTP server. You can find it at RADIUS support for MPPE is from Ralf Hofmann, . 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.)