dhkem: add no_std + refactor tests

Adds `no_std` support to `dhkem`, which involved moving `std`-dependent
tests from `lib/` to the `tests/` directory, which links them in a
separate crate that can access `std`.

Also adds `default-features = false` to each of the elliptic curve
crate dependencies.

Finally, adds a CI check that the crate can link on a `no_std` target.

.github/workflows/dhkem.yml

@@ -30,6 +30,26 @@ jobs:
     with:
       working-directory: ${{ github.workflow }}
 
+  no_std:
+    needs: set-msrv
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        rust:
+          - ${{needs.set-msrv.outputs.msrv}}
+          - stable
+        target:
+          - thumbv7em-none-eabi
+          - wasm32-unknown-unknown
+    steps:
+      - uses: actions/checkout@v4
+      - uses: RustCrypto/actions/cargo-cache@master
+      - uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{ matrix.rust }}
+          targets: ${{ matrix.target }}
+      - run: cargo build --no-default-features --target ${{ matrix.target }}
+
   test:
     needs: set-msrv
     runs-on: ubuntu-latest

.github/workflows/ml-kem.yml

@@ -23,6 +23,13 @@ jobs:
     with:
       msrv: 1.74.0
 
+  minimal-versions:
+    # temporarily disabled as requested by Tony (https://github.com/RustCrypto/KEMs/pull/15#pullrequestreview-2006378802)
+    if: false
+    uses: RustCrypto/actions/.github/workflows/minimal-versions.yml@master
+    with:
+      working-directory: ${{ github.workflow }}
+
   no_std:
     needs: set-msrv
     runs-on: ubuntu-latest
@@ -43,13 +50,6 @@ jobs:
           targets: ${{ matrix.target }}
       - run: cargo build --no-default-features --target ${{ matrix.target }}
 
-  minimal-versions:
-    # temporarily disabled as requested by Tony (https://github.com/RustCrypto/KEMs/pull/15#pullrequestreview-2006378802)
-    if: false
-    uses: RustCrypto/actions/.github/workflows/minimal-versions.yml@master
-    with:
-      working-directory: ${{ github.workflow }}
-
   test:
     needs: set-msrv
     runs-on: ubuntu-latest

dhkem/Cargo.toml

@@ -15,17 +15,17 @@ readme = "README.md"
 [dependencies]
 kem = "0.3.0-pre.0"
 rand_core = "0.6.4"
-x25519 = { version = "2.0.1", package = "x25519-dalek", optional = true }
-elliptic-curve = { version = "0.13.8", optional = true }
-bign256 = { version = "0.13.1", optional = true }
-k256 = { version = "0.13.3", optional = true }
-p192 = { version = "0.13.0", optional = true }
-p224 = { version = "0.13.2", optional = true }
-p256 = { version = "0.13.2", optional = true }
-p384 = { version = "0.13.0", optional = true }
-p521 = { version = "0.13.3", optional = true }
-sm2 = { version = "0.13.3", optional = true }
-zeroize = { version = "1.8.1", optional = true }
+x25519 = { version = "2.0.1", package = "x25519-dalek", optional = true, default-features = false }
+elliptic-curve = { version = "0.13.8", optional = true, default-features = false }
+bign256 = { version = "0.13.1", optional = true, default-features = false, features = ["arithmetic"] }
+k256 = { version = "0.13.3", optional = true, default-features = false, features = ["arithmetic"] }
+p192 = { version = "0.13.0", optional = true, default-features = false, features = ["arithmetic"] }
+p224 = { version = "0.13.2", optional = true, default-features = false, features = ["arithmetic"] }
+p256 = { version = "0.13.2", optional = true, default-features = false, features = ["arithmetic"] }
+p384 = { version = "0.13.0", optional = true, default-features = false, features = ["arithmetic"] }
+p521 = { version = "0.13.3", optional = true, default-features = false, features = ["arithmetic"] }
+sm2 = { version = "0.13.3", optional = true, default-features = false, features = ["arithmetic"] }
+zeroize = { version = "1.8.1", optional = true, default-features = false }
 
 [features]
 default = ["zeroize"]

dhkem/src/arithmetic.rs

@@ -1,9 +1,11 @@
 use crate::{DhDecapsulator, DhEncapsulator, DhKem};
-use elliptic_curve::ecdh::{EphemeralSecret, SharedSecret};
-use elliptic_curve::{CurveArithmetic, PublicKey};
+use core::marker::PhantomData;
+use elliptic_curve::{
+    ecdh::{EphemeralSecret, SharedSecret},
+    CurveArithmetic, PublicKey,
+};
 use kem::{Decapsulate, Encapsulate};
 use rand_core::CryptoRngCore;
-use std::marker::PhantomData;
 
 pub struct ArithmeticKem<C: CurveArithmetic>(PhantomData<C>);