[e2e] Re: [Tsvwg] Really End-to-end or CRC vs everything else ?

[vince_cavanna at agilent.com]

Jonathan and others,

Would a "keyed CRC32" instead of "keyed MD5" be appropriate for the purpose
of preventing unauthorized tampering of the protected packet by middle
boxes? A CRC is much further from being a one-way hash function than the
MD5, but once keyed, by including the shared secret in the data protected by
the CRC, it may be good enough to discourage tampering by middle boxes. The
benefit, of course, is that computing the CRC32 digest is much easier than
computing an MD5 digest. To foil such protection, Middle boxes would need
to, first, discover the shared secret and, second, remember the shared
secret - for each connection. Since, as David Reed has pointed out, middle
boxes do not store per-connection state, the second step alone may be a
large enough obstacle to render adequate the communication of the shared
"secret" in the clear and thus avoid the use of a secure key-exchange
protocol.

Vince

|-----Original Message-----
|From: Jonathan Stone [mailto:jonathan at DSG.Stanford.EDU]
|Sent: Friday, May 25, 2001 2:51 PM
|To: sommerfeld at orchard.arlington.ma.us
|Cc: David P. Reed; tsvwg at ietf.org; end2end
|Subject: Re: [e2e] Re: [Tsvwg] Really End-to-end or CRC vs everything
|else? 
|
|
|
|In message 
|<20010525212300.A8F062A4B at orchard.arlington.ma.us>Bill Sommerfeld wr
|ites
|>> [... md5 as an error-check function to defeat would-be 
|middleboxers...]
|>> 
|>> If you put that in the transport layer, won't that makes 
|communication
|>> without a shared-secret impossible? At least without using 
|some other
|>> transport protocol, to bootstrap a D-H or SPEKE or other initial key
|>> exchange.
|>
|>md5 is an unkeyed function, just like the CRC or internet checksum.
|>
|>hmac-md5 is a keyed function built out of md5 (it's one of two MAC
|>functions used with IPsec). [...]
|
|Yes, I know.  David Reed was explicitly suggesting md5 *with* a
|shared-secret (like hmac-md5) as an e2e integrity check in order to
|detect middle-boxes.  Using shared secrets at the transport protocol
|needs way -- perhaps unekeyed md5? -- to bootstrap the conversation.
|
|I hope just aying "md5" while referring to shared-secrets didn't
|cause confusion.
|
|
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