Authentication for Fragments Craig Partridge BBN Technologies

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

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

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

Datagram may be (re)fragmented at any point in the data and at any time (including during transmission Nice assumption Nice assumption –Can pre-empt fragments during transmission –Very general Apparently untenable Apparently untenable –Creates unauthenticatable fragments –Creates new style of attack on fragments Bytes 1..j Auth Unit P+1 Bytes k..n Auth Unit P Must fragment on boundaries determined by origin (ugh!) Must fragment on boundaries determined by origin (ugh!)

Fragments do not all follow same path Distributed Romanow-Floyd problem 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 Shared keys problematic –Every fragmentation point has private key with each origin? –Public key signatures are BIG Either 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?)

Can We Make Authentication Information Small Enough? An idea: send function definition, not signature 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… Such functions exist… –But either compact in representation OR strong enough to provide digital signature –NOT both (yet!) –Why this is a HOTNETS paper

While I Take Questions… This builds on prior work 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