Re: [TLS] Randomization of nonces

Watson Ladd <watsonbladd@gmail.com> Tue, 16 August 2016 14:57 UTC

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From: Watson Ladd <watsonbladd@gmail.com>
Date: Tue, 16 Aug 2016 07:51:16 -0700
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To: Martin Thomson <martin.thomson@gmail.com>
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Subject: Re: [TLS] Randomization of nonces
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On Mon, Aug 15, 2016 at 9:56 PM, Martin Thomson
<martin.thomson@gmail.com> wrote:
> On 16 August 2016 at 09:46, Paterson, Kenny <Kenny.Paterson@rhul.ac.uk> wrote:
>> Sadly, you can't implement XGCM using an existing AES-GCM API, because of
>> the way the MAC (which is keyed) is computed over the ciphertext in the
>> standard GCM scheme.
>
>
> Is there a reason why you can't simply XOR the plaintext stream that
> is fed to AES-GCM?
>
> We set N = N XOR HKDF(IKM, salt, label[N], 12), which the paper shows
> improves things.  If we also set P = P XOR repeat(HKDF(IKM, salt,
> label[P], 16)) would we gain any of the advantages of XCAU?  That
> change could be made without needing a new algorithm.

Yes. XOR two adjacent blocks, and you get something that is a function
purely of the key.

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