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Re: VoIP over IPsec
From: "Stephen Sprunk" <stephen () sprunk org>
Date: Tue, 18 Feb 2003 13:11:31 -0600


Thus spake "Vadim Antonov" <avg () kotovnik com>
In fact, a method to encrypt small parcels of data efficiently is
well-known for decades.  It is called "stream cypher" (surprise).
Besides LFSR-based and other stream cyphers, any block cypher
can be used in this mode. Its application to RTP is trivial and
straight-forward.  Just leave sequence number in clear text, so that
position in the stream is recoverable in case of packet loss.

Most stream modes are chained in some way to intentionally disrupt
decryption if part of the ciphertext is missing; that is why IPsec resets
the stream for each packet (currently).

When NIST was standardizing AES, they added CTR mode specifically to address
IPsec implementations.  I think there's already been a draft out of the IRTF
on how to modify IPsec for this, but it's not something I've followed
closely.

It also allows precomputation of the key stream, adding nearly zero
latency/jitter to the actual packet processing.

You fail to note that this requires precomputing and storing a keystream for
every SA on the encrypting device, which often number in the thousands.
This isn't feasible in a software implementation, and it's unnecessary in
hardware.

S

Stephen Sprunk         "God does not play dice."  --Albert Einstein
CCIE #3723         "God is an inveterate gambler, and He throws the
K5SSS        dice at every possible opportunity." --Stephen Hawking


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