Add CryptoRngProvider trait #4

wez · 2025-04-13T04:25:48Z

This makes it a bit easier to plug in an RNG when not building on a platform that has an OsRng.

This commit defines an interface that allows an embedding application to set an extension trait which can process the functions that require a CryptoRng. It does this without infecting the type signature of essentially every part of sunset. The implementation is 100% safe when building with the default getrandom feature enabled, but when that feature is omitted and the new assignment interface is added, there is some unsafe code to manage replacing the pointer to that extension trait. This is not likely totally sound in every scenario, but it happens to work for me. There's probably some cribbing that could be done from the log::logger() and log::set_logger_racy() functions to make this more-safe.

It resolves to 'a anyway, so let's just set it to that

mkj · 2025-04-14T02:50:21Z

Thanks for the patch. I'm not quite sure whether it makes sense setting the RNG here or using getrandom/custom feature. What kind of platform are you targetting? For the picow example I'm using a custom RNG with getrandom, that seemed to work OK? (that's still getrandom 0.2, it looks like 0.3 would be similar, though maybe a bit more messy needing to work with raw pointers [1]).

sunset/embassy/demos/picow/src/main.rs

Lines 69 to 71 in 5a91df0

    
           // RNG initialised early, all crypto relies on it 
        
           caprand::setup(&mut p.PIN_10).unwrap(); 
        
           getrandom::register_custom_getrandom!(caprand::getrandom);

Using getrandom has the advantage that it'll hopefully work with any other crypto-consuming parts of an application too (ongoing discussion of something similar in std or core [2]). That said maybe the approach in this PR makes sense - happy to hear your thoughts.

[1] https://docs.rs/getrandom/0.3.2/getrandom/index.html#custom-backend
[2] rust-lang/rust#130703 (comment)

wez · 2025-04-15T01:41:25Z

Oh, I somehow missed that getrandom had its own extension mechanism. Yeah, I can hook that up.
The reason I went down this path was because OsRng was defined in a different crate and was not supported, but looking at it with fresh eyes, I can see that it uses getrandom, so that's a double comprehension fail on my part :-/
In my defense, I got lost in a ton of tabs and examples.

I'm running on an rp2350 which actually has hardware TRNG available.

https://fosstodon.org/@wez/114325695508602149 shows sunset working on a picocalc on a pico 2w!

wez · 2025-04-15T01:47:44Z

Ah, the other issue is that getrandom 0.2 doesn't compile for this platform:

error: target is not supported, for more information see: https://docs.rs/getrandom/#unsupported-targets
   --> /Users/wez/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/getrandom-0.2.15/src/lib.rs:347:9
    |
347 | /         compile_error!("target is not supported, for more information see: \
348 | |                         https://docs.rs/getrandom/#unsupported-targets");
    | |________________________________________________________________________^

error[E0433]: failed to resolve: use of unresolved module or unlinked crate `imp`
   --> /Users/wez/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/getrandom-0.2.15/src/lib.rs:398:9
    |
398 |         imp::getrandom_inner(dest)?;
    |         ^^^ use of unresolved module or unlinked crate `imp`
    |
    = help: if you wanted to use a crate named `imp`, use `cargo add imp` to add it to your `Cargo.toml`

I think 0.3 gets further along, but there are some other dependencies to run down, and I'm not sure if those are solvable with the current version of embassy-rp. I'll dig in a bit more.

mkj · 2025-04-15T02:09:30Z

Ah awesome with the picocalc. I'd been eyeing those off, maybe I should get one!
Let me know if you have any ideas/questions with it. Better terminal emulation is something I was wondering about...

Ah, the other issue is that getrandom 0.2 doesn't compile for this platform:

It works if you build with getrandom/custom feature.

sunset/embassy/demos/picow/Cargo.toml

Line 52 in 5a91df0

getrandom = { version = "0.2", features = ["custom"] }

I'll have a look how 0.3 goes when I get a chance.

wez · 2025-04-15T02:13:29Z

I looked at 0.3, but it's blocked on dalek-cryptography/curve25519-dalek#729 which is incomplete right now; I tried using a git ref to that in sunset, but it's not quite there.

I did confirm just now that 0.2 with the custom feature works!

What I'll do is amend this PR to be just the elided lifetime warning, because the rest of this is redundant!

wez · 2025-04-15T02:15:59Z

Actually, let's just do a separate branch pr, that way I can keep the one around in my fork for posterity!
#6

wez added 2 commits April 12, 2025 21:27

fix elided lifetime has a name warning

40e22c9

It resolves to 'a anyway, so let's just set it to that

wez force-pushed< 8000 /a> the embed_rng branch from da73ea2 to 40e22c9 Compare April 13, 2025 04:27

wez closed this Apr 15, 2025

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