provide a try_random_mod method

Author: baloo

Cargo.lock

@@ -23,7 +23,7 @@ subtle = { version = "2.6", default-features = false }
 der = { version = "0.8.0-rc.1", optional = true, default-features = false }
 hybrid-array = { version = "0.2", optional = true }
 num-traits = { version = "0.2.19", default-features = false }
-rand_core = { version = "0.9", optional = true, default-features = false }
+rand_core = { version = "0.9.1", optional = true, default-features = false }
 rlp = { version = "0.6", optional = true, default-features = false }
 serdect = { version = "0.3", optional = true, default-features = false }
 zeroize = { version = "1", optional = true, default-features = false }
@@ -81,3 +81,7 @@ harness = false
 [[bench]]
 name = "int"
 harness = false
+
+[patch.crates-io]
+# https://github.com/rust-random/rand/pull/1593
+rand_core = { git = "https://github.com/fjarri/rand.git", branch = "sized" }

Cargo.toml

@@ -2,7 +2,7 @@
 
 use super::Limb;
 use crate::{Encoding, NonZero, Random, RandomMod};
-use rand_core::RngCore;
+use rand_core::{RngCore, TryRngCore};
 use subtle::ConstantTimeLess;
 
 impl Random for Limb {
@@ -35,4 +35,25 @@ impl RandomMod for Limb {
             }
         }
     }
+
+    fn try_random_mod<R: TryRngCore + ?Sized>(
+        rng: &mut R,
+        modulus: &NonZero<Self>,
+    ) -> Result<Self, R::Error> {
+        let mut bytes = <Self as Encoding>::Repr::default();
+
+        let n_bits = modulus.bits() as usize;
+        let n_bytes = (n_bits + 7) / 8;
+        let mask = 0xffu8 >> (8 * n_bytes - n_bits);
+
+        loop {
+            rng.try_fill_bytes(&mut bytes[..n_bytes])?;
+            bytes[n_bytes - 1] &= mask;
+
+            let n = Limb::from_le_bytes(bytes);
+            if n.ct_lt(modulus).into() {
+                return Ok(n);
+            }
+        }
+    }
 }

src/limb/rand.rs

@@ -420,6 +420,20 @@ pub trait RandomMod: Sized + Zero {
     /// leak anything about the output value aside from it being less than
     /// `modulus`.
     fn random_mod<R: RngCore + ?Sized>(rng: &mut R, modulus: &NonZero<Self>) -> Self;
+
+    /// Generate a random number which is less than a given `modulus`.
+    ///
+    /// This uses rejection sampling.
+    ///
+    /// As a result, it runs in variable time that depends in part on
+    /// `modulus`. If the generator `rng` is cryptographically secure (for
+    /// example, it implements `CryptoRng`), then this is guaranteed not to
+    /// leak anything about the output value aside from it being less than
+    /// `modulus`.
+    fn try_random_mod<R: TryRngCore + ?Sized>(
+        rng: &mut R,
+        modulus: &NonZero<Self>,
+    ) -> Result<Self, R::Error>;
 }
 
 /// Compute `self + rhs mod p`.

src/traits.rs

@@ -36,9 +36,19 @@ impl RandomBits for BoxedUint {
 impl RandomMod for BoxedUint {
     fn random_mod<R: RngCore + ?Sized>(rng: &mut R, modulus: &NonZero<Self>) -> Self {
         let mut n = BoxedUint::zero_with_precision(modulus.bits_precision());
-        random_mod_core(rng, &mut n, modulus, modulus.bits());
+        random_mod_core(rng, &mut n, modulus, modulus.bits())
+            .expect("RngCore used as TryRngCore failed");
         n
     }
+
+    fn try_random_mod<R: TryRngCore + ?Sized>(
+        rng: &mut R,
+        modulus: &NonZero<Self>,
+    ) -> Result<Self, R::Error> {
+        let mut n = BoxedUint::zero_with_precision(modulus.bits_precision());
+        random_mod_core(rng, &mut n, modulus, modulus.bits())?;
+        Ok(n)
+    }
 }
 
 #[cfg(test)]

src/uint/boxed/rand.rs

@@ -83,35 +83,46 @@ impl<const LIMBS: usize> RandomBits for Uint<LIMBS> {
 impl<const LIMBS: usize> RandomMod for Uint<LIMBS> {
     fn random_mod<R: RngCore + ?Sized>(rng: &mut R, modulus: &NonZero<Self>) -> Self {
         let mut n = Self::ZERO;
-        random_mod_core(rng, &mut n, modulus, modulus.bits_vartime());
+        random_mod_core(rng, &mut n, modulus, modulus.bits_vartime())
+            .expect("RngCore used as TryRngCore failed");
         n
     }
+
+    fn try_random_mod<R: TryRngCore + ?Sized>(
+        rng: &mut R,
+        modulus: &NonZero<Self>,
+    ) -> Result<Self, R::Error> {
+        let mut n = Self::ZERO;
+        random_mod_core(rng, &mut n, modulus, modulus.bits_vartime())?;
+        Ok(n)
+    }
 }
 
 /// Generic implementation of `random_mod` which can be shared with `BoxedUint`.
 // TODO(tarcieri): obtain `n_bits` via a trait like `Integer`
-pub(super) fn random_mod_core<T, R: RngCore + ?Sized>(
+pub(super) fn random_mod_core<T, R: TryRngCore + ?Sized>(
     rng: &mut R,
     n: &mut T,
     modulus: &NonZero<T>,
     n_bits: u32,
-) where
+) -> Result<(), R::Error>
+where
     T: AsMut<[Limb]> + AsRef<[Limb]> + ConstantTimeLess + Zero,
 {
     #[cfg(target_pointer_width = "64")]
-    let mut next_word = || rng.next_u64();
+    let mut next_word = || rng.try_next_u64();
     #[cfg(target_pointer_width = "32")]
-    let mut next_word = || rng.next_u32();
+    let mut next_word = || rng.try_next_u32();
 
     let n_limbs = n_bits.div_ceil(Limb::BITS) as usize;
 
     let hi_word_modulus = modulus.as_ref().as_ref()[n_limbs - 1].0;
     let mask = !0 >> hi_word_modulus.leading_zeros();
-    let mut hi_word = next_word() & mask;
+    let mut hi_word = next_word()? & mask;
 
     loop {
         while hi_word > hi_word_modulus {
-            hi_word = next_word() & mask;
+            hi_word = next_word()? & mask;
         }
         // Set high limb
         n.as_mut()[n_limbs - 1] = Limb::from_le_bytes(hi_word.to_le_bytes());
@@ -120,15 +131,16 @@ pub(super) fn random_mod_core<T, R: RngCore + ?Sized>(
             // Need to deserialize from little-endian to make sure that two 32-bit limbs
             // deserialized sequentially are equal to one 64-bit limb produced from the same
             // byte stream.
-            n.as_mut()[i] = Limb::from_le_bytes(next_word().to_le_bytes());
+            n.as_mut()[i] = Limb::from_le_bytes(next_word()?.to_le_bytes());
         }
         // If the high limb is equal to the modulus' high limb, it's still possible
         // that the full uint is too big so we check and repeat if it is.
         if n.ct_lt(modulus).into() {
             break;
         }
-        hi_word = next_word() & mask;
+        hi_word = next_word()? & mask;
     }
+    Ok(())
 }
 
 #[cfg(test)]