Re: [TLS] Pull Request: Removing the AEAD explicit IV

Michael StJohns <msj@nthpermutation.com> Wed, 18 March 2015 15:31 UTC

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Date: Wed, 18 Mar 2015 11:31:44 -0400
From: Michael StJohns <msj@nthpermutation.com>
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To: Adam Langley <agl@imperialviolet.org>
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Subject: Re: [TLS] Pull Request: Removing the AEAD explicit IV
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On 3/17/2015 2:21 PM, Adam Langley wrote:
> On Tue, Mar 17, 2015 at 11:06 AM, Michael StJohns
> <msj@nthpermutation.com>; wrote:
>> That works for the currently defined pure AEAD modes (CCM/GCM at least,
>> haven't looked at chacha).   Will that work in all cases for constructed
>> AEAD modes?  (e.g. where we build an AEAD mode out of an encryption mode and
>> an integrity mode and derive the E and I keys from a single key internal to
>> the constructed mode - say  AES-CBC with CMAC for example).  Short answer is
>> probably not.
> I believe that it will, although I welcome counterexamples.
>
> The AEAD interface requires only an nonce (i.e. unique, but not
> unpredictable) and, under this proposal, that's all that TLS will
> provide.
>
> CBC modes require an IV (i.e. unpredictable). If you wished to make an
> AES-CBC-HMAC-SHA256 AEAD, for example, then you would do something
> like this:
>
> Define the key length to be twice as large as the AES key and split it
> internally into a CBC key and an IV key. For each record, generate the
> IV by AES(key = IV_key, block = nonce) then encrypt the plaintext
> using CBC with that IV and the CBC key. (And you might need extra key
> bytes for the HMAC too, but that's beside the point here.) Basically
> you can generate the IVs using AES-CTR as a PRNG.

I wouldn't use AES-CTR to generate IVs for AES-CBC, but I take your 
point.    I was actually asking about more than CBC.  See below.


This is what I got from SP800-38A appendix C
> For the CBC and CFB modes, the IVs must be unpredictable. In 
> particular, for any given plaintext, it must not be possible to 
> predict the IV that will be associated to the plaintext in advance of 
> the generation of the IV.

> There are two recommended methods for generating unpredictable IVs. 
> The first method is to apply the forward cipher function, under the 
> same key that is used for the encryption of the plaintext, to a nonce. 
> The nonce must be a data block that is unique to each execution of the 
> encryption operation. For example, the nonce may be a counter, as 
> described in Appendix B, or a message number. The second method is to 
> generate a random data block using a FIPS-approved random number 
> generator.

But that's what's defined for CBC and will work with the per-session 
nonce with per-message sequence number as described above.

While at this time we've only got defined cipher suites for CBC, CCM and 
GCM, (and I should have stated my comment more to this point), will 
there never be other potential modes that provide some desired security 
characteristics not currently present that also require explicit 
per-message IVs?

Eliminating the message-wrapping down to the AEAD construct is one thing 
as you can build AEAD modes out of other modes.  I wonder that we're 
constraining the protocol so much that other, useful, modes might be 
excluded in the future.   E.g. will any of the modes on this list 
http://csrc.nist.gov/groups/ST/toolkit/BCM/modes_development.html end up 
in TLS cipher suites at some point?  Of these, which require per-message 
IVs that can't be generated as above?

>
>
> Cheers
>
> AGL
>
>