x: Remove vestigial feature and update feature docs (#857)

* Clarify features in x25519 README

* Remove alloc feature from x25519 bc it was doing nothing

* Add pubkey constructor benchmark

* Correct code size claim on README

* Update changelog

curve25519-dalek/README.md

@@ -50,7 +50,7 @@ curve25519-dalek = ">= 5.0, < 5.2"
 | :---               |  :---:   | :---        |
 | `alloc`            |    ✓     | Enables Edwards and Ristretto multiscalar multiplication, batch scalar inversion, and batch Ristretto double-and-compress. |
 | `zeroize`          |    ✓     | Enables [`Zeroize`][zeroize-trait] for all scalar and curve point types. |
-| `precomputed-tables` |    ✓     | Includes precomputed basepoint multiplication tables. This speeds up `EdwardsPoint::mul_base` and `RistrettoPoint::mul_base` by ~4x, at the cost of ~30KB added to the code size. |
+| `precomputed-tables` |    ✓     | Includes precomputed basepoint multiplication tables. This speeds up `EdwardsPoint::mul_base` and `RistrettoPoint::mul_base` by ~4x, at the cost of ~400KB added to the code size. |
 | `rand_core`        |          | Enables `Scalar::random` and `RistrettoPoint::random`. This is an optional dependency whose version is not subject to SemVer. See [below](#public-api-semver-exemptions) for more details. |
 | `digest`           |          | Enables `RistrettoPoint::{from_hash, hash_from_bytes}` and `Scalar::{from_hash, hash_from_bytes}`. Also enables hash-to-curve methods `EdwardsPoint::{encode_to_curve, hash_to_curve}`. This is an optional dependency whose version is not subject to SemVer. See [below](#public-api-semver-exemptions) for more details. |
 | `serde`            |          | Enables `serde` serialization/deserialization for all the point and scalar types. |

x25519-dalek/CHANGELOG.md

@@ -4,6 +4,10 @@ Entries are listed in reverse chronological order.
 
 # 3.x Series
 
+## Unreleased
+
+* Removed `alloc` feature flag, which was doing nothing
+
 ## 3.0.0-pre.3
 
 * Upgrade `rand_core` to v0.10.0-rc-2

x25519-dalek/Cargo.toml

@@ -60,11 +60,10 @@ name = "x25519"
 harness = false
 
 [features]
-default = ["alloc", "precomputed-tables", "zeroize"]
+default = ["precomputed-tables", "zeroize"]
 getrandom = ["dep:getrandom"]
 zeroize = ["dep:zeroize", "curve25519-dalek/zeroize"]
 serde = ["dep:serde", "curve25519-dalek/serde"]
-alloc = ["curve25519-dalek/alloc", "serde?/alloc", "zeroize?/alloc"]
 precomputed-tables = ["curve25519-dalek/precomputed-tables"]
 reusable_secrets = []
 static_secrets = []

x25519-dalek/README.md

@@ -97,15 +97,28 @@ This example used the ephemeral DH API, which ensures that secret keys
 cannot be reused; Alice and Bob could instead use the static DH API
 and load a long-term secret key.
 
-# Installation
+# Use
 
-To install, add the following to your project's `Cargo.toml`:
+To import `x25519-dalek`, add the following to your project's `Cargo.toml`:
 
 ```toml
 [dependencies]
 x25519-dalek = "3.0.0-pre.3"
 ```
 
+# Feature Flags
+
+This crate is `#[no_std]` compatible with `default-features = false`.
+
+| Feature              | Default? | Description |
+| :---                 | :---     | :---        |
+| `zeroize`            | ✓        | Implements `Zeroize` and `ZeroizeOnDrop` for `EphemeralSecret`, `ReusableSecret`, and `StaticSecret` |
+| `precomputed-tables` | ✓        | Includes precomputed basepoint multiplication tables. This speeds up `PublicKey::from` by ~3x, at the cost of ~400KB added to the code size. |
+| `getrandom`          |          | Exposes the `random()` constructor for `EphemeralSecret`, `ReusableSecret`, and `StaticSecret` |
+| `reusable_secrets`   |          | Exposes the `ReusableSecret` struct |
+| `static_secrets`     |          | Exposes the `StaticSecret` struct |
+| `serde`              |          | Enables `serde` serialization/deserialization for `PublicKey` and `StaticSecret` |
+
 # MSRV
 
 Current MSRV is 1.85.

x25519-dalek/benches/x25519.rs

@@ -30,11 +30,20 @@ fn bench_diffie_hellman(c: &mut Criterion) {
     });
 }
 
+fn bench_pubkey_constructor(c: &mut Criterion) {
+    let bob_secret = EphemeralSecret::random_from_rng(&mut OsRng.unwrap_err());
+
+    c.bench_function("PublicKey::from", move |b| {
+        b.iter(|| PublicKey::from(&bob_secret))
+    });
+}
+
 criterion_group! {
     name = x25519_benches;
     config = Criterion::default();
     targets =
         bench_diffie_hellman,
+        bench_pubkey_constructor,
 }
 criterion_main! {
     x25519_benches,