x: Remove vestigial features and update feature docs

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 `diffie_hellman()` by ~4x, at the cost of ~30KB 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,