Subject: basic Qs re: crypto on multicast

From: George Michaelson <[email-masked]>
Date: Fri, 23 May 1997 09:33:10 +1000 (EST)

Ok, if there is a primer on crypto for multicast I'd love a pointer.

A few obvious questions/issues

	You can't do stream cyphers on an unreliable datastream
	so whats the efficiency and trust issues for block-only
	cyphers for the current range of multicast packet sizes?

	Most keying schemes for multiple recipients appear hugely
	inefficient if based on assymetric/public key. Is that too glib?

	Key distribution looks to be an issue. Has anybody done any
	work on this yet? (Van asked at least 3 years ago and I attempted
	to find suitably skilled people in OZ, no takers)

	How do the various embedded-in-ip crypto issues relate to multicast?

cheers

-George

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From: Atanu Ghosh <[email-masked]>
Date: Fri, 23 May 1997 17:22:00 +1000

>>>>> "George" == George Michaelson <[email-masked]> writes:

    George> You can't do stream cyphers on an unreliable datastream so
    George> whats the efficiency and trust issues for block-only
    George> cyphers for the current range of multicast packet sizes?

You could use a stream cipher like RC4. The RC4 stream is XORed with
the plaintext data stream to give the encrypted steam. The sequence
number in the packet is used to find the correct position in the RC4
stream. As long as the sequence number is available before decryption
in the incoming packet the scheme is not affected by loss. If you use
RC4 which is byte based as opposed to a block based scheme you are
unaffected by variable packet sizes.

I have used such a scheme successfully, but not for a multicast
application.

	Atanu.

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From: George Michaelson <[email-masked]>
Date: Fri, 23 May 1997 17:25:58 +1000

Over the lifetime of a multicast session, as people join and leave,
doesn't the startup time to re-create the RC4 history to get synced
in kill you? There are an *awful* lot of bytes in a videosteam...

How do you re-key? (orthogonal issue of course)

-George

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From: Atanu Ghosh <[email-masked]>
Date: Fri, 23 May 1997 17:41:04 +1000

>>>>> "George" == George Michaelson <[email-masked]> writes:

    George> Over the lifetime of a multicast session, as people join
    George> and leave, doesn't the startup time to re-create the RC4
    George> history to get synced in kill you? There are an *awful*
    George> lot of bytes in a videosteam...

This is a problem that I hadn't considered. You could request all the
internal RC4 state from the transmitter or another listener who is in
sync. Maybe request a key change?

    George> How do you re-key? (orthogonal issue of course)

Pass.

	Atanu.