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Re: [jose] RFC 8037 "alg" quirkiness

Neil Madden <neil.e.madden@gmail.com> Wed, 16 September 2020 13:03 UTC

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From: Neil Madden <neil.e.madden@gmail.com>
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Date: Wed, 16 Sep 2020 14:02:55 +0100
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To: Ilari Liusvaara <ilariliusvaara@welho.com>
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Subject: Re: [jose] RFC 8037 "alg" quirkiness
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On 16 Sep 2020, at 13:30, Ilari Liusvaara <ilariliusvaara@welho.com> wrote:
> […]
> 
>> 8037 was done by CFRG, so probably best to ask there. 
> 
> 8037 was done by JOSE. The CFRG part (generic specification of
> EdDSA2) was 8032.

Good to know, thanks.

> 
>> I like this aspect of the spec. IMO “alg” as a header was a mistake.
>> By using a generic algorithm header, it forces implementors to
>> associate the specific details in metadata stored with the key, which
>> is much safer. 
> 
> Actually a few months ago I came up with idea of having a way to express
> "algorithm determined by the key" in JOSE/COSE.
> 
> - There have been some serious security issues in JWS implementations
>  that have been caused by using completely wrong algorithms.
> - In general, using the same key with multiple algorithms is not
>  cryptographically safe. There can be algorithm pairs which interact
>  badly (for instance, Ed25519 and the original Ed25519ph).

I wrote about this some time ago: https://neilmadden.blog/2018/09/30/key-driven-cryptographic-agility/ <https://neilmadden.blog/2018/09/30/key-driven-cryptographic-agility/> 

JWK already allows associating an algorithm with a key, so I have considered a small draft to deprecate the “alg” header in JOSE entirely. With this WG being disbanded however, there’s not a natural place to do that.

— Neil

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