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Authentication for Fragments

Authentication for Fragments. Craig Partridge BBN Technologies craig@bbn.com. The Problem. Packet (Fragments). An Intermittent Link Comes Up. Router. Which Fragment Do You Send?. Why An Issue?. New network scenarios with intermittent (potentially) oversubscribed links

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Authentication for Fragments

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  1. Authentication for Fragments Craig Partridge BBN Technologies craig@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. Bytes 1..j Auth Unit P+1 Bytes k..n Auth Unit P “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

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