Re: Using G-Expressions for public keys (substitutes and possibly more)

[Liliana Marie Prikler <liliana.prikler@gmail.com>]

Hi Ludo,

Am Donnerstag, den 21.10.2021, 22:13 +0200 schrieb Ludovic Courtès:
> Hi!
> 
> Liliana Marie Prikler <liliana.prikler@gmail.com> skribis:
> 
> > let's say I wanted to add my own substitute server to my
> > config.scm. 
> > At the time of writing, I would have to add said server's public
> > key to
> > the authorized-keys of my guix-configuration like so:
> >   (cons* (local-file "my-key.pub") %default-authorized-guix-keys)
> > or similarily with append.  This local-file incantation is however
> > pretty weak.  It changes based on the current working directory and
> > even if I were to use an absolute path, I'd have to copy both that
> > file
> > and the config.scm to a new machine were I to use the same
> > configuration there as well.
> 
> Note that you could use ‘plain-file’ instead of ‘local-file’ and
> inline the key canonical sexp in there.
Yes, but for that I'd have to either write a (multi-line) string
directly, which visibly "breaks" indentation of the rest of the file,
or somehow generate a string which adds at least one layer of
indentation.  The former is imo unacceptable, the latter merely
inconvenient.

> > However, it turns out that the format for said key files is some
> > actually pretty readable Lisp-esque stuff.  For instance, an ECC
> > key reads like
> >   (public-key (ecc (curve CURVE) (q #Q#)))
> > with spaces omitted for simplicity.
> > Were it not for the (q #Q#) bit, we could construct it using
> > scheme-file.  In fact, it is so simple that in my local config I
> > now do exactly that.
> 
> Yeah it’s frustrating that canonical sexps are almost, but not quite,
> Scheme sexps.  :-)
> 
> (gcrypt pk-crypto) has a ‘canonical-sexp->sexp’ procedure:
> 
> --8<---------------cut here---------------start------------->8---
> scheme@(guile-user)> ,use(gcrypt pk-crypto)
> scheme@(guile-user)> ,use(rnrs io ports)
> scheme@(guile-user)> (string->canonical-sexp
> 		      (call-with-input-file
> "etc/substitutes/ci.guix.info.pub"
> 			get-string-all))
> $18 = #<canonical-sexp 7fce7f4e8b40 | 15d96a0>
> scheme@(guile-user)> ,pp (canonical-sexp->sexp $18)
> $19 = (public-key
>   (ecc (curve Ed25519)
>        (q #vu8(141 21 111 41 93 36 176 217 168 111 165 116 26 132 15
> 242 210 79 96 247 182 196 19 72 20 173 85 98 89 113 179 148))))
> --8<---------------cut here---------------end--------------->8---
> 
> > (define-record-type* <ecc-key> ...)
> > (define-gexp-compiler (ecc-key-compiler (ecc-key <ecc-key>) ...)
> > ...)
> > 
> > (ecc-key
> >   (name "my-key.pub")
> >   (curve 'Ed25519)
> >   (q "ABCDE..."))
> > 
> > Could/should we support such formats out of the box?  WDYT?
> 
> With this approach, we’d end up mirroring all the canonical sexps
> used by libgcrypt, which doesn’t sound great from a maintenance POV.
Given that we can use canonical sexps, what about a single canonical-
sexp compiler then?  I'd have to think about this a bit more when I
have the time to, but having a way of writing the canonical sexp
"directly" would imo be advantageous.

> Would providing an example in the doc that uses ‘canonical-sexp-
> >sexp’ and its dual help?
I'm not sure whether it'd be in the doc or as a cookbook entry, but
providing an example would in my opinion definitely help.

I'll take a closer look at guile-gcrypt later.  Hopefully they have
scheme-ified constructors for everything, which would make this quite
simple.

Thanks,
Liliana