Re: QUIC-LB update: Eliminate block ciphers?

Martin Duke <> Wed, 06 October 2021 23:28 UTC

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From: Martin Duke <>
Date: Wed, 06 Oct 2021 16:27:59 -0700
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Subject: Re: QUIC-LB update: Eliminate block ciphers?
To: Phillip Hallam-Baker <>
Cc: Martin Thomson <>, IETF QUIC WG <>
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If the Block Cipher goes away, this will simply be the "encrypted" method.
No need to bikeshed the name for now.

On Wed, Oct 6, 2021 at 4:21 PM Phillip Hallam-Baker <>

> I think this is a different cryptographic construct and we should create a
> name for the generic. Something like Keyed Permutation.
> Rather than bikeshed the name here, I propose taking it to either CFRG or
> the Cryptography list (or both) to socialize the concept. It is quite
> possible that there is a prior nomenclature we should follow.
> It is not clear to me what the precise security properties required here
> are. For my particular application, they are fairly weak because I am only
> providing some traffic analysis resistance. I am not interested in
> plaintext recovery attack, but I do care about the attacker being able to
> discover that E(n), E(N+1) are a sequence.
> None of my systems are going to collapse if this primitive is broken but
> it might afford a foothold.
> On Wed, Oct 6, 2021 at 6:13 PM Martin Thomson <> wrote:
>> On Thu, Oct 7, 2021, at 07:02, Christian Huitema wrote:
>> > Phil,
>> >
>> > What we have in the current LB spec is called a "stream cipher", but
>> > that's a misnomer. What we have in the spec is actually a variable size
>> > block cipher, derived from AES-ECB using a construct similar to FFX.
>> > Your review of that algorithm would be appreciated.
>> Christian,
>> I would call this a Feistel network, but avoid talking about FFX.  FFX
>> has a bunch of guidance about the number of iterations of the network that
>> this ignores; to call this FFX or even imply that it is FFX isn't really
>> fair.  When you get right down to it, the real contribution in FFX is the
>> analysis that produces guidance on the number of iterations and the
>> inclusion of tweaks; if you use neither, then it's not really FFX.  As
>> additional iterations are necessary to maintain a security level, we need
>> to be careful about the claims we make in relation to security.