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<div style="direction: ltr;font-family: Tahoma;color: #000000;font-size: 10pt;">Scott<br>
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The SHA256 authentication/integrity digest results in a huge overhead regardless of the protocol used. While we don't usually 'encourage' people to truncate SHA digests, it can be done when wire overhead is a major issue. See NIST SP 800-107 for info on truncation
(http://csrc.nist.gov/publications/nistpubs/800-107-rev1/sp800-107-rev1.pdf)<br>
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And as Dennis says, elliptic curve saves many bits over RSA.<br>
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Howie<br>
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<font style="font-family:Verdana" size="2"><span style="font-weight:bold"></span></font><span style="font-weight:bold">Howard Weiss</span><br>
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<div style="direction: ltr;" id="divRpF281096"><font color="#000000" face="Tahoma" size="2"><b>From:</b> Iannicca, Dennis C. (GRC-LCA0) [dennis.c.iannicca@nasa.gov]<br>
<b>Sent:</b> Tuesday, June 30, 2015 2:23 PM<br>
<b>To:</b> Burleigh, Scott C (JPL-312B)[Jet Propulsion Laboratory]; Weiss, Howard; Mayer, Jeremy P. (JSC-OT/ESA)[EUROPEAN SPACE AGENCY]; sis-dtn@mailman.ccsds.org<br>
<b>Subject:</b> Re: [Sis-dtn] Bundle Signing And Encryption With CMS<br>
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<div>On 6/30/15 1:26 PM, "Burleigh, Scott C (312B)" <<a href="mailto:scott.c.burleigh@jpl.nasa.gov" target="_blank">scott.c.burleigh@jpl.nasa.gov</a>> wrote:</div>
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<p class="MsoNormal"><span style="font-size:11pt; font-family:Calibri,sans-serif; color:rgb(31,73,125)">I agree, Jeremy, it’s terrific that you could do this so quickly. And I agree that the overheads are not bad, but to my mind they are still a little troubling.
I can imagine an SBSP Block Integrity Block ciphersuite that would use a one-time, randomly generated SHA256 key to generate a SHA256 digest over the payload (shipped in the BIB’s results field); would include that key in the BIB’s ciphersuite parameters;
and would also provide an elliptic-curve digital signature for that key (computed using the sender’s private key, to be verified using the sender’s pre-placed public key) as an additional ciphersuite parameter. I think that would come to 256 bits for the
SHA256 digest plus 256 bits for the SHA256 key, plus 320 bits for the ECDS, for a total of 832 bits = 104 bytes. Even allowing for a little additional BIB structural overhead, this is still less than a sixth of the overhead measured for the CMS signing option.</span></p>
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<div>Scott,</div>
<div><span class="Apple-tab-span" style="white-space:pre"></span>CMS would allow you to use ECDSA for signatures in lieu of RSA if you wanted to reduce the overhead seen in these examples.</div>
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<div>--</div>
<div>Dennis Iannicca</div>
<div>NASA Glenn Research Center</div>
<div>21000 Brookpark Road, MS 54-1</div>
<div>Cleveland, OH 44135</div>
<div>216-433-6493</div>
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