PPTP
acronymnetworking
Point-to-Point Tunneling Protocol: the first widely deployed remote-access VPN, broken repeatedly since 1998 and retained only as a lesson.
It shipped in Windows in the mid-1990s and made remote access ordinary, which is its real contribution. It was also cryptanalysed almost immediately: Schneier and Mudge published against the original in 1998, and against the revised MS-CHAPv2 authentication in 1999, and by 2012 the remaining protection could be reduced to a single brute-force problem solvable by a rented service in hours. The flaw that matters is structural rather than incidental - the encryption keys were derived from the user's password, so the strength of the tunnel could never exceed the strength of a password. It is disabled or removed in current operating systems. It appears here because it is the clearest example in this catalogue of a protocol that was not repaired but replaced, and because equipment still offers it.
Also known as: pptp, point-to-point tunneling protocol, mppe