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Clean up RNG comparison table #100

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Clean up RNG comparison table #100

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91e4a11
Avoid statements about `StdRng` and `SmallRng`
vks May 17, 2026
2ddddf3
`rand_pcg` also supports jump-ahead now
vks May 17, 2026
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14 changes: 8 additions & 6 deletions src/guide-rngs.md
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Expand Up @@ -39,10 +39,10 @@ You may wish to refer to the [pcg-random] and [xoshiro] websites.

| name | full name | performance | memory | quality | period | features |
|------|-----------|-------------|--------|---------|--------|----------|
| [`SmallRng`] | (unspecified) | 11 GB/s | 16 bytes | ★★★☆☆ | ≥ `u32` * 2<sup>64</sup> | not portable |
| [`Pcg32`] | PCG XSH RR 64/32 (LCG) | 5 GB/s | 16 bytes | ★★★☆☆ | `u32` * 2<sup>64</sup> | — |
| [`Pcg64`] | PCG XSL 128/64 (LCG) | 7 GB/s | 32 bytes | ★★★☆☆ | `u64` * 2<sup>128</sup> | — |
| [`Pcg64Mcg`] | PCG XSL 128/64 (MCG) | 8 GB/s | 16 bytes | ★★★☆☆ | `u64` * 2<sup>126</sup> | — |
| [`SmallRng`] | (unspecified) | (unspecified) | (unspecified) | ★★★☆☆ | ≥ `u32` * 2<sup>64</sup> | not portable |
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@dhardy dhardy May 18, 2026

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The purpose of SmallRng is to be a fast, low-memory PRNG. Maybe we should use inspecific words like 'fast', 'small' here? Or 'competitive'?

| [`Pcg32`] | PCG XSH RR 64/32 (LCG) | 5 GB/s | 16 bytes | ★★★☆☆ | `u32` * 2<sup>64</sup> | jump-ahead |
| [`Pcg64`] | PCG XSL 128/64 (LCG) | 7 GB/s | 32 bytes | ★★★☆☆ | `u64` * 2<sup>128</sup> | jump-ahead |
| [`Pcg64Mcg`] | PCG XSL 128/64 (MCG) | 8 GB/s | 16 bytes | ★★★☆☆ | `u64` * 2<sup>126</sup> | jump-ahead |
| [`XorShiftRng`] | Xorshift 32/128 | 7 GB/s | 16 bytes | ★☆☆☆☆ | `u32` * 2<sup>128</sup> - 1 | — |
| [`Xoshiro256PlusPlus`] | Xoshiro256++ | 11 GB/s | 32 bytes | ★★★☆☆ | `u64` * 2<sup>256</sup> - 1 | jump-ahead |
| [`Xoshiro256Plus`] | Xoshiro256+ | 13 GB/s | 32 bytes | ★★☆☆☆ | `u64` * 2<sup>256</sup> - 1 | jump-ahead |
Expand Down Expand Up @@ -79,9 +79,10 @@ table since CSPRNGs may not have observable defects.

| name | full name | performance | initialization | memory | security (predictability) | forward secrecy |
|------|-----------|--------------|--------------|----------|----------------|-------------------------|
| [`StdRng`] | (unspecified) | 4.1 GB/s | fast | 136 bytes | widely trusted | no |
| [`StdRng`] | (unspecified) | (unspecified) | fast | (unspecified) | widely trusted | no |
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@dhardy dhardy May 18, 2026

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Again, performance is part of the selection criteria so could write 'fast'.

| [`ChaCha20Rng`] | ChaCha20 | 2.6 GB/s | fast | 136 bytes | [rigorously analysed](https://tools.ietf.org/html/rfc7539#section-1) | no |
| [`ChaCha8Rng`] | ChaCha8 | 5.8 GB/s | fast | 136 bytes | small security margin | no |
| [`ChaCha12Rng`] | ChaCha12 | 4.1 GB/s | fast | 136 bytes | [large security margin](https://eprint.iacr.org/2019/1492) | no |
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@dhardy dhardy May 18, 2026

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I measure the size as 320 bytes now. I recall that the buffer size has been increased to better support some SIMD backends. This size is untested in the stream-ciphers repo.

Update this to 320 with a footnote clarifying that it may change?

| [`ChaCha8Rng`] | ChaCha8 | 5.8 GB/s | fast | 136 bytes | [sufficient security margin](https://eprint.iacr.org/2019/1492) | no |
| [`Hc128Rng`] | HC-128 | 4.6 GB/s | slow | 4176 bytes | [recommended by eSTREAM](http://www.ecrypt.eu.org/stream/) | no |
| [`IsaacRng`] | ISAAC | 2.1 GB/s | slow | 2072 bytes | [unknown](https://burtleburtle.net/bob/rand/isaacafa.html) | unknown |
| [`Isaac64Rng`] | ISAAC-64 | 3.7 GB/s | slow | 4136 bytes| unknown | unknown |
Expand Down Expand Up @@ -322,6 +323,7 @@ by P. Hellekalek.
[`Xoshiro256Plus`]: https://docs.rs/rand_xoshiro/latest/rand_xoshiro/struct.Xoshiro256Plus.html
[`SplitMix64`]: https://docs.rs/rand_xoshiro/latest/rand_xoshiro/struct.SplitMix64.html
[`ChaCha20Rng`]: https://docs.rs/chacha20/latest/chacha20/struct.ChaCha20Rng.html
[`ChaCha12Rng`]: https://docs.rs/chacha20/latest/chacha20/struct.ChaCha12Rng.html
[`ChaCha8Rng`]: https://docs.rs/chacha20/latest/chacha20/struct.ChaCha8Rng.html
[`Hc128Rng`]: https://docs.rs/rand_hc/latest/rand_hc/struct.Hc128Rng.html
[`IsaacRng`]: https://docs.rs/rand_isaac/latest/rand_isaac/isaac/struct.IsaacRng.html
Expand Down