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Re: [TLS] Truncated HMAC: what to do with the MAC key?

Benjamin Kaduk <bkaduk@akamai.com> Fri, 07 July 2017 16:27 UTC

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To: Andreas Walz <andreas.walz@hs-offenburg.de>, tls@ietf.org
References: <595F99DA020000AC00136830@gwia2.rz.hs-offenburg.de> <595F99DA020000AC00136830@gwia2.rz.hs-offenburg.de>
From: Benjamin Kaduk <bkaduk@akamai.com>
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Date: Fri, 07 Jul 2017 11:27:05 -0500
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Subject: Re: [TLS] Truncated HMAC: what to do with the MAC key?
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On 07/07/2017 09:25 AM, Andreas Walz wrote:
> Dear all,
>
> today I encountered something that confuses me: different TLS
> implementations do not seem to agree on how to implement truncated
> HMAC. All implementations I tested truncate the HMAC output correctly,
> but they seem to use different MAC keys. When truncated HMAC is
> negotiated:
>
> -> MatrixSSL does not change the length of the MAC key but zeros all
> its bytes beyond index 10,
> -> mbedTLS truncates the MAC key to length 10,
> -> WolfSSL does not touch the MAC key at all.
>
> From RFC 6066 I would infer that the MAC key should not be affected by
> the negotiation of the truncated HMAC extension (as WolfSSL is
> implementing it). Is that correct?

I agree with your reading of RFC 6066 (and RFC 2104).

-Ben

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