Subject: RE: [Isms] TLS / SSH scalability concerns

From: "Khanh Nguyen \(khanhvn\)" <[email-masked]>
Date: Mon, 16 May 2005 11:58:23 -0700

> From: McDonald, Ira [mailto:[email-masked]] 
>
> DTLS is explained (in current <draft-rescorla-dtls-04.txt>) 
> in terms of differences from TLS/1.1 (work-in-progress from 
> IETF TLS WG).

Thanks Ira for the info.  Looks like DTLS relies on TLS 1.1 
"explicit IV", which prepends a block of random data to each 
TLS data record.  As a side effect of this, records can be 
decrypted individually (no record chaining).  Note that 
TLS 1.1 & explicit IV is a proposal, not a standard (yet).

> 4.1.2.3. Block Cipher
> 
>    DTLS block cipher encryption and decryption are performed
>    exactly as with TLS 1.1.

Well... except the record sizes are different.  The extra 
block in each record is a small overhead for TLS, but can be 
a bigger one for DTLS, since TLS max record size is ~16Kb, 
while DTLS max rec size is limited by datagram MTU (~1.5Kb).
So in a sense, DTLS faces the same issue as USM: using a 
bulk cipher to encypt short datagram.

BTW, I also found this:
http://crypto.stanford.edu/~nagendra/papers/dtls.pdf

I haven't read thru it yet, but near the end, DTLS authors 
acknowledged that DTLS had more overhead than TLS.

Khanh

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From: "David T. Perkins" <[email-masked]>
Date: Mon, 16 May 2005 12:21:29 -0700 (PDT)

HI,

Have you compared DTLS using SHA and AES vs USM using SHA and AES?
Using a new security model will allow the definition of syntax
of the msgSecurityParameters field, since it probably no longer
needs many of the values needed to support USM (since they
are part of either session establishment or done by DTLS).

Regards,
/david t. perkins