[lamps] Key usages for composite keys

[Tim Hollebeek <[e-mail]>]

 

Hello,

 

This is a little off topic and possibly getting ahead of things a little
bit, but it came up in a discussion with our engineers so I thought I'd
float it here.

 

It's possible some of this might fit in somewhere in the composite signature
draft, and if the authors agree and can find an appropriate spot for it I'll
draft some text.

Otherwise maybe it goes in "PQC for Engineers" or one of the other PQUIP
drafts.

 

It's a pretty simple point, but it's not documented anywhere and it should
be:

 

RSA can be used for both signing and key encapsulation.

ML-DSA can be used for signing.

Composite<RSA, ML-DSA> can be used for . ?

 

It's pretty obvious that a composite key can only do what each individual
component key can do, so the key usage is at most the intersection of the
valid key usages of the component keys.

 

So the correct answer is "signing".

 

But this analysis and answer should be written down somewhere so everyone
doesn't keep having the same question, especially since some people might
come to the wrong answer.

 

-Tim