PPTP

The Point to Point Tunneling Protocol (PPTP) is used to secure PPP connections over a TCP/IP link. Microsoft released extensions to the PPTP authentication mechanism (MS-CHAPv2). Another Microsoft extension is the Microsoft Point to Point Encryption (MPPE). It should be noted, that these extensions are Microsoft-proprietary, nevertheless, we will have a short look at them.

MS-CHAPv2 uses some very complicated hashing-algorithms, and some senseless procedures like using the SHA-Algorithm on some data twice.

MPPE is an encryption algorithm, that is only as good as the passwords that are used for authentication as MPPE keys get derived from MS-CHAPv2.

As all this is very Microsoft-proprietary, it is hard to implement solutions in open source software (although there are Linux-Kernel Patches), beyond that fact that the encryption algorithm is not secure, as it is only based on the authentication data. PPTP is therefore not the protocol we are looking for.