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Re: [TLS] HKDF

Brian Smith <brian@briansmith.org> Wed, 25 March 2015 18:47 UTC

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Date: Wed, 25 Mar 2015 08:47:26 -1000
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From: Brian Smith <brian@briansmith.org>
To: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [TLS] HKDF
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Eric Rescorla <ekr@rtfm.com> wrote:
> As I mentioned in a previous message [0] during the interim we discussed
> moving from the TLS PRF to HKDF [RFC5869].
>
> The general sentiment was:
>
> - Move to HKDF
> - Specify both SHA-256 and SHA-384 (the latter being required for
>   Suite B)

Assuming that the choice of either SHA-256 or SHA-384 is fixed per
cipher suite, as is the case with TLS 1.2, switching to HKDF seems
like a good idea to me.

> This is also the time when we would want to look at adjusting
> the key expansion to separate keys and IVs (assuming we still
> have IVs).

If the IVs are secret (only shared between the two peers) then I don't
see the need to change how they are calculated from the keyblock.

Cheers,
Brian

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