John Gilmore on ispec
https://www.mail-archive.com/[email protected]/msg12325.html

openvpn.net
https://openvpn.net/

Wikipedia - OpenVPN#Encryption
https://en.wikipedia.org/wiki/OpenVPN#Encryption

Cryptanalysis of Microsoft's PPTP Authentication Extensions (MS-CHAPv2)
https://www.schneier.com/academic/archives/1999/09/cryptanalysis_of_mic_1.html

spiegel.de - NSA leaks on ipsec
http://www.spiegel.de/media/media-35529.pdf