Adapt to chacha20 by hpenne · Pull Request #1642 · rust-random/rand

hpenne · 2025-06-15T07:06:11Z

Added a CHANGELOG.md entry

Summary

Replaced the dependency on rand_chacha with one on chacha20. Added some tests to std.rs to ensure that the output of StdRng did not change.

Fixes #934.

Motivation

Reduces total code size and the total amount of unsafe code.

Details

Changes to config.toml and some replacement of rand_chacha:: with chacha20::.

Added three new unit tests to std.rs. These were based on tests of IETF test vectors from rand_chacha, but the actual expected values had to be replaced, as the IETF test vectors are for ChaCha20 while rand uses ChaCha12. The expected values were generated by using rand_chacha (before chacha20 was used) to verify that the algorithm change did not affect the output.

Cargo.toml

dhardy · 2025-06-17T08:54:37Z

I opened #1643. This PR is useful as a draft but won't be merged in its current form (likely we'll want the MSRV/edition bump first as its own PR).

I'm not certain on the timeline yet; the main blocker is the chacha20 release; we also need to decide whether we are ready to merge breaking changes to rand yet.

CHANGELOG.md

dhardy · 2025-09-08T07:38:48Z

@hpenne could you rebase now that #1653 and #1654 are merged please?

hpenne · 2025-09-08T18:54:21Z

@dhardy I've rebased and the new tests failed as expected (the default ChaCha12 in chacha20 is the IETF variant). I tried to change to the "Legacy" variant, but chacha20 only exports ChaCha20Legacy, not the 12 round variant that we will need here. The ChaCha20Legacy type is just an alias, so I tried to use the underlying type directly to make a 12 round variant (ChaChaCore<R12, Legacy>) but that needs the Legacy struct which turns out to be private.

@tarcieri It seems that I might stuck here until someone either exports Legacy or adds a ChaCha12Legacy. It might be useful to do both, just in case.

newpavlov · 2025-09-08T18:59:52Z

that needs the Legacy struct which turns out to be private.

It's an oversight, we should export it. Could you create a PR? It also may be worth to add aliases for the legacy variants as well.

tarcieri · 2025-09-08T19:26:17Z

@hpenne I'm confused, why do you need a ChaCha12Legacy instead of just ChaCha12Rng?

hpenne · 2025-09-08T19:36:35Z

@hpenne I'm confused, why do you need a ChaCha12Legacy instead of just ChaCha12Rng?

@tarcieri It seemed to me that the Legacy types were the ones with a 64 bit counter, which is what is needed here. One of us is confused, I suppose. It might also be me.

hpenne · 2025-09-08T19:40:54Z

@tarcieri When I add the "legacy" feature to the chacha20 crate dependency in the rand cargo.toml, the chacha20 crate fails to build:

error[E0432]: unresolved import `cipher::StreamCipherCoreWrapper`
 --> /Users/hpenne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/chacha20-0.10.0-rc.0/src/chacha.rs:2:41
  |
2 |     IvSizeUser, KeyIvInit, KeySizeUser, StreamCipherCoreWrapper,
  |                                         ^^^^^^^^^^^^^^^^^^^^^^^
  |                                         |
  |                                         no `StreamCipherCoreWrapper` in the root
  |                                         help: a similar name exists in the module: `StreamCipherCore`

Seems to work fine when i build run tests in chacha20 itself. Odd.

tarcieri · 2025-09-08T19:49:24Z

@hpenne all of the RNG types now have a 64-bit counter as of the latest prerelease.

Also looks like you found a bug. Perhaps it was being hidden by feature unification? Strange.

tarcieri · 2025-09-08T19:51:11Z

@hpenne oh, you're using an out-of-date version! Please upgrade to v0.10.0-rc.1

…sions that are no longer needed.

hpenne · 2025-09-08T20:07:46Z

@hpenne oh, you're using an out-of-date version! Please upgrade to v0.10.0-rc.1

@tarcieri That was embarrassing. Works much better with the correct version. All tests pass now. The only strangeness that I am left with is that if I do not enable the "legacy" feature (the only enabled feature is "rng"), chacha20 fails to build with:

error[E0432]: unresolved import `crate::variants::Legacy`
  --> /Users/hpenne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/chacha20-0.10.0-rc.1/src/rng.rs:24:16
   |
24 |     variants::{Legacy, Variant},
   |                ^^^^^^ no `Legacy` in `variants`
   |
note: found an item that was configured out
  --> /Users/hpenne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/chacha20-0.10.0-rc.1/src/variants.rs:57:10
   |
57 | pub enum Legacy {}
   |          ^^^^^^
note: the item is gated behind the `legacy` feature
  --> /Users/hpenne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/chacha20-0.10.0-rc.1/src/variants.rs:56:7
   |
56 | #[cfg(feature = "legacy")]
   |       ^^^^^^^^^^^^^^^^^^

I'm not able to reproduce this when I build chacha20 locally from master, so perhaps I've done something wrong or you have already fixed this on master. I'll look closer tomorrow.

tarcieri · 2025-09-08T20:14:23Z

@hpenne aah that was RustCrypto/stream-ciphers#454 which has been fixed

I can cut an rc.2 if that helps

dhardy

Sorry, I did not think to appraise you of the chacha20 changes. I think we have all the pieces to make a rand pre-release now.

dhardy · 2025-09-09T06:43:35Z

examples/rayon-monte-carlo.rs

-            rng.set_stream(i);
+            rng.set_stream(u64::from(i));


i should be u64 already?

Yes. I used to need to convert to u128 with the old chacha20. I had to change u64 when I rebased but failed to notice that I could simply remove the conversion. Will fix.

dhardy · 2025-09-09T06:44:39Z

Cargo.toml

-rand_chacha = { path = "rand_chacha", version = "0.9.0", default-features = false, optional = true }
+chacha20 = { version = "=0.10.0-rc.1", default-features = false, features = ["rng", "legacy"], optional = true }


We can depend on the "legacy" feature for now, but add a TODO comment to remove it on the next version.

dhardy · 2025-09-09T06:47:19Z

CHANGELOG.md


 ## [0.10.0 — Unreleased]
 ### Changes
+- The dependency on `rand_chacha` has been replaced with a dependency on `chacha20`. This changes the implementation behind `StdRng`, but the output remains the same. There may be some API breakage when using the ChaCha-types directly as these are now the ones in `chacha20` instead of `rand_chacha`.


Include the PR # please.

…:from in rayon-monte-carlo.rs. Added a ToDo in Cargo.toml to remove the "legacy" feature of chacha20.

tarcieri · 2025-09-09T17:53:10Z

Cargo.toml

+# ToDo: Remove the "legacy" feature from chacha20 when this is not longer necessary
+chacha20 = { version = "=0.10.0-rc.1", default-features = false, features = ["rng", "legacy"], optional = true }


I've released an rc.2 with the feature-related bugfix:

Suggested change

# ToDo: Remove the "legacy" feature from chacha20 when this is not longer necessary

chacha20 = { version = "=0.10.0-rc.1", default-features = false, features = ["rng", "legacy"], optional = true }

# ToDo: Remove the "legacy" feature from chacha20 when this is not longer necessary

chacha20 = { version = "=0.10.0-rc.2", default-features = false, features = ["rng"], optional = true }

Done. I removed the ToDo-comment as well, now that the "legacy" feature is no longer necessary.

… the ToDo.

dhardy reviewed Jun 15, 2025

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This was referenced Jun 16, 2025

Replacing rand_chacha with chacha20 #934

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Tracker: rand v0.10 #1643

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tarcieri reviewed Aug 11, 2025

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CHANGELOG.md Outdated Show resolved Hide resolved

This was referenced Aug 12, 2025

Bump the MSRV to 1.85 #1653

Merged

Added test vectors for StdRng, verified using rand_chacha. #1654

Merged

tarcieri mentioned this pull request Aug 13, 2025

chacha20: 64-bit counter support RustCrypto/stream-ciphers#334

Closed

hpenne added 3 commits September 8, 2025 20:00

Replaced rand_chacha with chacha20.

52b1bea

Added entry to CHANGELOG.md

ce3f8dc

Added fix for multiple versions of rand_core to benches.

e41e428

hpenne force-pushed the adapt_to_chacha20 branch from 0fbb567 to e41e428 Compare September 8, 2025 18:12

Fixed some breakage due to upstream API changes.

0c201cd

Updated to latest chacha20 release candidate. Removed some int conver…

9a7a59c

…sions that are no longer needed.

dhardy reviewed Sep 9, 2025

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dhardy approved these changes Sep 9, 2025

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Added PR number to the CHANGELOG.md entry. Removed a superfluous u64:…

9ee24b9

…:from in rayon-monte-carlo.rs. Added a ToDo in Cargo.toml to remove the "legacy" feature of chacha20.

tarcieri reviewed Sep 9, 2025

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Updated chacha20 to 0.10.0-rc.2, and removed the "legacy" feature and…

4490b41

… the ToDo.

dhardy approved these changes Sep 10, 2025

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dhardy merged commit 4afc333 into rust-random:master Sep 10, 2025
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tarcieri mentioned this pull request Sep 26, 2025

Implement rand::Distribution for Array RustCrypto/hybrid-array#151

Closed

		rand_chacha = { path = "rand_chacha", version = "0.9.0", default-features = false, optional = true }
		chacha20 = { version = "=0.10.0-rc.1", default-features = false, features = ["rng", "legacy"], optional = true }

		# ToDo: Remove the "legacy" feature from chacha20 when this is not longer necessary
		chacha20 = { version = "=0.10.0-rc.1", default-features = false, features = ["rng", "legacy"], optional = true }

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hpenne commented Jun 15, 2025

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hpenne commented Sep 8, 2025 •

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