[IPSec] previously suffered from a scaling problem in that that the system/network administrators from any pair of systems or networks that want to have encryption between them had to exchange some sort of PublicKey. 

  

 With OE, each system/network administrator puts the key(s) needed to create IPSEC tunnels with their networks into the [DNS]. Then when another OE enabled IPSEC gateway wants to send packets to a host on the Internet, a check is first done to see if there are any keys in the DNS for that host and if there are, they are fetched and an encrypted tunnel is set up transparently. If there are no keys, then the packets MAY (if the administrator of the sending network allows so in his security policy) be sent in the clear. 

  

  

 See the very readable [IETF draft for the Opportunistic Encryption specification|http://www.sandelman.ottawa.on.ca/SSW/freeswan/oeid/draft-richardson-ipsec-opportunistic.html]