Authentication for Fragments

1
Authentication for Fragments

• Craig Partridge
• BBN Technologies
• craig_at_bbn.com

2
The Problem
Packet (Fragments)
An Intermittent Link Comes Up
Router
Which Fragment Do You Send?
3
Why An Issue?

• New network scenarios with intermittent
  (potentially) oversubscribed links
• A desire to send the most valuable traffic first
• Large native unit of authentication
• Mobigrams
• DTN bundles

4
Starting Assumptions

• Datagram may be (re)fragmented at any point in
  the data and at any time (including during
  transmission)
• Fragments do not all follow the same path

5
Datagram may be (re)fragmented at any point in
the data and at any time (including during
transmission

• Nice assumption
• Can pre-empt fragments during transmission
• Very general
• Apparently untenable
• Creates unauthenticatable fragments
• Creates new style of attack on fragments

• Must fragment on boundaries determined by origin
  (ugh!)

6
Fragments do not all follow same path

• Distributed Romanow-Floyd problem
• Fragment lost on path 1 means fragments on path 2
  now can only do harm, yet path 2 must treat them
  as valuable
• Shared keys problematic
• Every fragmentation point has private key with
  each origin?
• Public key signatures are BIG
• Either
• Each fragment is self authenticating (see PK is
  BIG)
• Or we distribute aggregated authentication
  information down all possible paths (can we make
  it small enough?)

7
Can We Make Authentication Information Small
Enough?

• An idea send function definition, not signature
• Implies result of function is known
• E.g. fragment 5 has digital hash of 5
• Such functions exist
• But either compact in representation OR strong
  enough to provide digital signature
• NOT both (yet!)
• Why this is a HOTNETS paper

8
While I Take Questions

• This builds on prior work
• Kent/Mogul, Fragmentation Considered Harmful
• Romanow/Floyd, Dynamics of TCP Traffic over ATM
  Networks
• Matthis/Heffner/Chandler, Fragmentation
  Considered Very Harmful
• Toilet paper authentication ideas in DTN list