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Re: [IPsec] Two questions about draft-ietf-ipsecme-chacha20-poly1305-00

Michael Richardson <mcr+ietf@sandelman.ca> Mon, 27 April 2015 15:15 UTC

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From: Michael Richardson <mcr+ietf@sandelman.ca>
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Date: Mon, 27 Apr 2015 11:14:55 -0400
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Subject: Re: [IPsec] Two questions about draft-ietf-ipsecme-chacha20-poly1305-00
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Yoav Nir <ynir.ietf@gmail.com> wrote:
    > Second issue is about UI advice. Some implementations (yes, mine is
    > included) allow the user to configure encryption algorithm, MAC
    > algorithm, and D-H group. There is no setting for PRF since such UIs
    > date back to IKEv1. The PRF is usually just taken from the setting for
    > MAC algorithm. This works fine as long as all supported MAC algorithms
    > are HMAC, XCBC, and CMAC. AES-GCM would have the same issue, but RFC
    > 5282 makes no mention of this issue. I’m wondering if we should
    > recommend to pair this algorithm in IKE with PRF_HMAC_SHA2_256.

So, in this case, if you wanted to not change your UI, maybe you would tell
the user to configure
    encryption-algorithm=Chacha20-Poly1305
    MAC=HMAC-SHA2
    DH=whatever

the MAC would not apply to IPsec at all?

I guess if we are deploying this algorithm with the concern that HMAC-SHA2/AES
might become weak, that it would seem odd to depend upon SHA2 as the PRF.
At least, users might not understand.

(noting that SHA2 != HMAC-SHA2, and also that the inputs to the PRF as not
very easily manipulated...)

--
Michael Richardson <mcr+IETF@sandelman.ca>, Sandelman Software Works
 -= IPv6 IoT consulting =-

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