Point-to-Point Tunneling Protocol was developed by a consortium founded by Microsoft for creating VPN over dialup networks, and as such has long been the standard protocol for internal business VPN. It is a VPN protocol only, and relies on various authentication methods to provide security (with MS-CHAP v2 being the most common). Available as standard on just about every VPN capable platform and device, and thus being easy to set up without the need to install additional software, it remains a popular choice both for businesses and VPN providers. It also has the advantage of requiring a low computational overhead to implement (i.e. it’s quick).
However, although now usually only found using 128-bit encryption keys, in the years since it was first bundled with Windows 95 OSR2 back in 1999, a number of security vulnerabilities have come to light, the most serious of which is the possibility of unencapsulated MS-CHAP v2 Authentication. Using this exploit, PPTP has been cracked within 2 days, and although Microsoft has patched the flaw (through the use of PEAP authentication), it has itself issued a recommendation that VPN users should use L2TP/IPsec or SSTP instead.
Knowing that PPTP was insecure anyway, it came as no surprise to anybody that the NSA almost certainly decrypts PPTP encrypted communications as standard. Perhaps more worrying is that the NSA has (or is in the process of) almost certainly decrypted the vast amounts of older data it has stored, which was encrypted back when even security experts considered PPTP to be secure.
L2TP and L2TP/IPsec
Layer 2 Tunnel Protocol is a VPN protocol that on its own does not provide any encryption or confidentiality to traffic that passes through it. For this reason it is usually implemented with the IPsec encryption suite (similar to a cipher, as discussed below) to provide security and privacy.
L2TP/IPsec is built-in to all modern operating systems and VPN capable devices, and is just as easy and quick to set up as PPTP (in fact it usually uses the same client). Problems can arise however, because the L2TP protocol uses UDP port 500, which is more easily blocked by NAT firewalls, and may therefore require advanced configuration (port forwarding) when used behind a firewall (this is unlike SSL which can use TCP port 443 to make it indistinguishable from normal HTTPS traffic).
IPsec encryption has no major known vulnerabilities, and if properly implemented may still be secure. However, Edward Snowden’s revelations have strongly hinted at the standard being compromised by the NSA, and as John Gilmore (security specialist and founding member of the Electronic Frontier Foundation) explains in this post, it is likely that it has been been deliberately weakened during its design phase.
Relatively minor compared to the last point, but probably worth mentioning, is that because L2TP/IPsec encapsulates data twice it is not as efficient as SSL based solutions (such as OpenVPN and SSTP, and is therefore slightly slower.
OpenVPN is a fairly new open source technology that uses the OpenSSL library and SSLv3/TLSv1 protocols, along with an amalgam of other technologies, to provide a strong and reliable VPN solution. One of its major strengths is that it is highly configurable, and although it runs best on a UDP port, it can be set to run on any port, including TCP port 443. This makes traffic on it impossible to tell apart from traffic using standard HTTPS over SSL (as used by for example Gmail), and it is therefore extremely difficult to block.
Another advantage of OpenVPN is that the OpenSSL library used to provide encryption supports a number of cryptographic algorithms (e.g. AES, Blowfish, 3DES, CAST-128, Camellia and more), although VPN providers almost exclusively use either AES or Blowfish. 128-bit Blowfish is the default cypher built in to OpenVPN, and although it is generally considered secure, it does have known weaknesses, and even its creator was quoted in 2007 as saying ‘at this point, though, I’m amazed it’s still being used. If people ask, I recommend Twofish instead’.
AES is the newer technology, has no known weaknesses, and thanks to its adoption by the US government for use in protecting ‘secure’ data, is generally considered the ‘gold standard’ when it comes to encryption. The fact that it has a 128-bit block size rather than Blowfish’s 64-bit block size also means that it can handle larger (over 1 GB) files better than Blowfish. However, both ciphers are NIST certified, which while not widely recognized as problem, we have issues with. See below for a discussion about this.
How fast OpenVPN performs depends on the level of encryption employed, but it is generally faster than IPsec.
OpenVPN has become the default VPN connection type, and while natively supported by no platform, is widely supported on most through third party software (including both iOS and Android).
Compared to PPTP and L2TP/IPsec, OpenVPN can be a bit fiddly to set up. When using generic OpenVPN software in particular (such as the standard open source OpenVPN client for Windows), it is necessary to not only download and install the client, but also to download and setup additional configuration files. Many VPN providers get around this configuration problem by supplying customized VPN clients.
Perhaps most importantly in light of the information obtained from Edward Snowden, it seems OpenVPN has not been compromised or weakened by the NSA, and is also (thanks to its use of ephemeral key exchanges, as we will discuss later) immune to NSA attacks on RSA key encryption. Although no-one knows the full capabilities of the NSA for sure, both the evidence and the mathematics strongly point to OpenVPN, if used in conjunction with a strong cipher, being the only VPN protocol that can be considered truly secure.