Make rand_core optional (#447)

As proposed in #442 this makes `rand_core` an
optional feature that is not covered by the
SemVer public API stability guarantees.

Co-authored-by: Michael Rosenberg <michael@mrosenberg.pub>

CHANGELOG.md

@@ -5,6 +5,7 @@ major series.
 
 ## 4.x series
 
+* Make `rand_core` an optional feature
 * Add target u32/u64 backend overrides
 * Migrate documentation to docs.rs hosted
 * Fix backend documentation generation

Cargo.toml

@@ -25,7 +25,7 @@ exclude = [
 
 [package.metadata.docs.rs]
 rustdoc-args = ["--html-in-header", "docs/assets/rustdoc-include-katex-header.html", "--cfg", "docsrs"]
-features = ["serde", "simd_backend"]
+features = ["serde", "simd_backend", "rand_core"]
 
 [badges]
 travis-ci = { repository = "dalek-cryptography/curve25519-dalek", branch = "master"}
@@ -36,6 +36,7 @@ bincode = "1"
 criterion = { version = "0.4.0", features = ["html_reports"] }
 hex = "0.4.2"
 rand = "0.8"
+rand_core = { version = "0.6", default-features = false }
 
 [[bench]]
 name = "dalek_benchmarks"
@@ -43,7 +44,7 @@ harness = false
 
 [dependencies]
 cfg-if = "1"
-rand_core = { version = "0.6", default-features = false }
+rand_core = { version = "0.6", default-features = false, optional = true }
 digest = { version = "0.10", default-features = false }
 subtle = { version = "^2.2.1", default-features = false }
 serde = { version = "1.0", default-features = false, optional = true, features = ["derive"] }

Makefile

@@ -1,4 +1,4 @@
-FEATURES := simd_backend serde
+FEATURES := simd_backend serde rand_core
 
 doc:
 	cargo +nightly rustdoc --features "$(FEATURES)" -- --html-in-header docs/assets/rustdoc-include-katex-header.html --cfg docsrs

src/edwards.rs

@@ -1507,7 +1507,7 @@ mod test {
         // Construct random coefficients x0, ..., x_{n-1},
         // followed by some extra hardcoded ones.
         let xs = (0..n)
-            .map(|_| Scalar::random(&mut rng))
+            .map(|_| crate::mocks::MockScalar::random(&mut rng))
             // The largest scalar allowed by the type system, 2^255-1
             .chain(iter::once(Scalar::from_bits([0xff; 32])))
             .collect::<Vec<_>>();
@@ -1577,11 +1577,11 @@ mod test {
         let B = &crate::constants::ED25519_BASEPOINT_TABLE;
 
         let static_scalars = (0..128)
-            .map(|_| Scalar::random(&mut rng))
+            .map(|_| crate::mocks::MockScalar::random(&mut rng))
             .collect::<Vec<_>>();
 
         let dynamic_scalars = (0..128)
-            .map(|_| Scalar::random(&mut rng))
+            .map(|_| crate::mocks::MockScalar::random(&mut rng))
             .collect::<Vec<_>>();
 
         let check_scalar: Scalar = static_scalars

src/lib.rs

@@ -83,3 +83,6 @@ pub(crate) mod prelude;
 
 // Generic code for window lookups
 pub(crate) mod window;
+
+#[cfg(test)]
+pub(crate) mod mocks;

src/mocks.rs

@@ -0,0 +1,42 @@
+//! This is used mocking / proxying the below for tests:
+//! - random_test() to Scalar::random() depending on feature `rand_core`
+
+use crate::ristretto::RistrettoPoint;
+use crate::scalar::Scalar;
+
+use rand_core::{CryptoRng, RngCore};
+
+pub struct MockScalar;
+impl MockScalar {
+    #[cfg(feature = "rand_core")]
+    /// Proxy Scalar::random() for random_test
+    pub fn random<R: RngCore + CryptoRng + ?Sized>(rng: &mut R) -> Scalar {
+        Scalar::random(rng)
+    }
+
+    #[cfg(not(feature = "rand_core"))]
+    /// Mock Scalar::random() for random_test
+    pub fn random<R: RngCore + CryptoRng + ?Sized>(rng: &mut R) -> Scalar {
+        let mut scalar_bytes = [0u8; 64];
+        rng.fill_bytes(&mut scalar_bytes);
+        Scalar::from_bytes_mod_order_wide(&scalar_bytes)
+    }
+}
+
+pub struct MockRistrettoPoint;
+impl MockRistrettoPoint {
+    #[cfg(feature = "rand_core")]
+    /// Proxy RistrettoPoint::random() for random_test
+    pub fn random<R: RngCore + CryptoRng + ?Sized>(rng: &mut R) -> RistrettoPoint {
+        RistrettoPoint::random(rng)
+    }
+
+    #[cfg(not(feature = "rand_core"))]
+    /// Mock RistrettoPoint::random() for random_test
+    pub fn random<R: RngCore + CryptoRng + ?Sized>(rng: &mut R) -> RistrettoPoint {
+        let mut uniform_bytes = [0u8; 64];
+        rng.fill_bytes(&mut uniform_bytes);
+
+        RistrettoPoint::from_uniform_bytes(&uniform_bytes)
+    }
+}

src/montgomery.rs

@@ -374,6 +374,7 @@ mod test {
     use super::*;
     use crate::constants;
 
+    #[cfg(feature = "rand_core")]
     use rand_core::OsRng;
 
     #[test]
@@ -458,6 +459,7 @@ mod test {
     }
 
     #[test]
+    #[cfg(feature = "rand_core")]
     fn montgomery_ladder_matches_edwards_scalarmult() {
         let mut csprng: OsRng = OsRng;
 

src/ristretto.rs

@@ -113,7 +113,7 @@
 //! used to implement
 //!
 //! * `RistrettoPoint::random()`, which generates random points from an
-//! RNG;
+//! RNG - enabled by `rand_core` feature;
 //!
 //! * `RistrettoPoint::from_hash()` and
 //! `RistrettoPoint::hash_from_bytes()`, which perform hashing to the
@@ -165,6 +165,7 @@ use core::ops::{Add, Neg, Sub};
 use core::ops::{AddAssign, SubAssign};
 use core::ops::{Mul, MulAssign};
 
+#[cfg(feature = "rand_core")]
 use rand_core::{CryptoRng, RngCore};
 
 use digest::generic_array::typenum::U64;
@@ -501,7 +502,8 @@ impl RistrettoPoint {
     /// \mathrm{enc}( \[2\]P\_1), \ldots, \mathrm{enc}( \[2\]P\_n ) \\)
     /// in a batch.
     ///
-    /// ```
+    #[cfg_attr(feature = "rand_core", doc = "```")]
+    #[cfg_attr(not(feature = "rand_core"), doc = "```ignore")]
     /// # use curve25519_dalek::ristretto::RistrettoPoint;
     /// use rand_core::OsRng;
     ///
@@ -509,6 +511,7 @@ impl RistrettoPoint {
     /// # // See https://doc.rust-lang.org/book/documentation.html#documentation-as-tests
     /// # fn main() {
     /// let mut rng = OsRng;
+    ///
     /// let points: Vec<RistrettoPoint> =
     ///     (0..32).map(|_| RistrettoPoint::random(&mut rng)).collect();
     ///
@@ -662,6 +665,7 @@ impl RistrettoPoint {
         )
     }
 
+    #[cfg(feature = "rand_core")]
     /// Return a `RistrettoPoint` chosen uniformly at random using a user-provided RNG.
     ///
     /// # Inputs
@@ -1343,7 +1347,7 @@ mod test {
     fn four_torsion_random() {
         let mut rng = OsRng;
         let B = &constants::RISTRETTO_BASEPOINT_TABLE;
-        let P = B * &Scalar::random(&mut rng);
+        let P = B * &crate::mocks::MockScalar::random(&mut rng);
         let P_coset = P.coset4();
         for point in P_coset {
             assert_eq!(P, RistrettoPoint(point));
@@ -1670,7 +1674,7 @@ mod test {
         let mut rng = OsRng;
         let B = &constants::RISTRETTO_BASEPOINT_TABLE;
         for _ in 0..100 {
-            let P = B * &Scalar::random(&mut rng);
+            let P = B * &crate::mocks::MockScalar::random(&mut rng);
             let compressed_P = P.compress();
             let Q = compressed_P.decompress().unwrap();
             assert_eq!(P, Q);
@@ -1683,7 +1687,7 @@ mod test {
         let mut rng = OsRng;
 
         let mut points: Vec<RistrettoPoint> = (0..1024)
-            .map(|_| RistrettoPoint::random(&mut rng))
+            .map(|_| crate::mocks::MockRistrettoPoint::random(&mut rng))
             .collect();
         points[500] = RistrettoPoint::identity();
 
@@ -1702,11 +1706,11 @@ mod test {
         let B = &crate::constants::RISTRETTO_BASEPOINT_TABLE;
 
         let static_scalars = (0..128)
-            .map(|_| Scalar::random(&mut rng))
+            .map(|_| crate::mocks::MockScalar::random(&mut rng))
             .collect::<Vec<_>>();
 
         let dynamic_scalars = (0..128)
-            .map(|_| Scalar::random(&mut rng))
+            .map(|_| crate::mocks::MockScalar::random(&mut rng))
             .collect::<Vec<_>>();
 
         let check_scalar: Scalar = static_scalars

src/scalar.rs

@@ -152,6 +152,7 @@ use crate::prelude::*;
 
 use cfg_if::cfg_if;
 
+#[cfg(feature = "rand_core")]
 use rand_core::{CryptoRng, RngCore};
 
 use digest::generic_array::typenum::U64;
@@ -566,6 +567,7 @@ impl Zeroize for Scalar {
 }
 
 impl Scalar {
+    #[cfg(feature = "rand_core")]
     /// Return a `Scalar` chosen uniformly at random using a user-provided RNG.
     ///
     /// # Inputs
@@ -1404,7 +1406,7 @@ mod test {
     fn non_adjacent_form_random() {
         let mut rng = rand::thread_rng();
         for _ in 0..1_000 {
-            let x = Scalar::random(&mut rng);
+            let x = crate::mocks::MockScalar::random(&mut rng);
             for w in &[5, 6, 7, 8] {
                 non_adjacent_form_iter(*w, &x);
             }