Re: swipe working on infinity.c2.org

[Phil Karn <karn@qualcomm.com>]

>When I talked to Phil Karn months ago about IP encryption, he was
>talking about encrypting each packet independently - I guess you have
>to do that with IP since it's not a reliable protocol.  But it sounded
>a little risky to me - maybe vulnerable to attack via known bits
>at the start of each encrypted section.  Encrypting at the TCP
>level would allow inter-packet mixing, but then you miss all the
>UDP protocols such as (old) NFS.

My unreleased KA9Q NOS version of SwIPe (I really need to converge to
ji/mab's version) adds a sequence number in the header just above IP
that is covered by the encryption (DES CBC). This acts as an IV that
ensures different ciphertext every time even when identical packets
are sent. The only part of the packet left in the clear is the IP
header. An eavesdropper has no knowledge of the application or the
transport protocol in use, or even if there's another IP datagram
buried inside the encrypted part (e.g., the swipe boxes are providing
a secure tunnel for other hosts).

These are all advantages of IP-level encryption over doing it above
TCP. The main disadvantage is overhead -- Van Jacobsen TCP/IP header
compression breaks.

Phil