use hmac-drbg as prng in libcrux provider (#1545)

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Changed
+
+- (hpke-rs-libcrux) [#1545](https://github.com/celabshq/libcrux/issues/1545): Use DRBG as the PRNG in the HPKE provider
+
 ## [0.0.5] (2026-07-15)
 
 ### Fixed

crates/protocols/hpke/libcrux_provider/Cargo.toml

@@ -16,6 +16,7 @@ libcrux-hkdf.workspace = true
 libcrux-kem.workspace = true
 libcrux-aead.workspace = true
 libcrux-traits.workspace = true
+libcrux-hmac-drbg = { workspace = true, features = ["rand"] }
 # RustCrypto curves not available in libcrux
 # TODO: These should be replaced with libcrux implementations as soon as they
 # are available.

crates/protocols/hpke/libcrux_provider/src/lib.rs

@@ -34,7 +34,7 @@ pub struct HpkeLibcrux {}
 pub struct HpkeLibcruxPrng {
     #[cfg(feature = "deterministic-prng")]
     fake_rng: Vec<u8>,
-    rng: rand_chacha::ChaCha20Rng,
+    rng: libcrux_hmac_drbg::HmacSha256DrbgRng<UnwrapErr<SysRng>>,
 }
 
 impl Zeroize for HpkeLibcruxPrng {
@@ -358,15 +358,19 @@ impl HpkeCrypto for HpkeLibcrux {
         {
             let mut fake_rng = alloc::vec![0u8; 256];
             rand_chacha::ChaCha20Rng::from_rng(&mut UnwrapErr(SysRng)).fill_bytes(&mut fake_rng);
+            let rng = UnwrapErr(SysRng);
             HpkeLibcruxPrng {
                 fake_rng,
-                rng: rand_chacha::ChaCha20Rng::from_rng(&mut UnwrapErr(SysRng)),
+                rng: libcrux_hmac_drbg::HmacSha256DrbgRng::new(rng, &[0u8; 32]),
             }
         }
 
         #[cfg(not(feature = "deterministic-prng"))]
-        HpkeLibcruxPrng {
-            rng: rand_chacha::ChaCha20Rng::from_rng(&mut UnwrapErr(SysRng)),
+        {
+            let rng = UnwrapErr(SysRng);
+            HpkeLibcruxPrng {
+                rng: libcrux_hmac_drbg::HmacSha256DrbgRng::new(rng, &[0u8; 32]),
+            }
         }
     }
 

crates/protocols/hpke/src/lib.rs

@@ -507,6 +507,23 @@ impl<Crypto: HpkeCrypto> Hpke<Crypto> {
         }
     }
 
+    /// Set up the configuration for HPKE with a custom PRNG.
+    pub fn new_with_prng(
+        mode: Mode,
+        kem_id: KemAlgorithm,
+        kdf_id: KdfAlgorithm,
+        aead_id: AeadAlgorithm,
+        prng: Crypto::HpkePrng,
+    ) -> Self {
+        Self {
+            mode,
+            kem_id,
+            kdf_id,
+            aead_id,
+            prng,
+        }
+    }
+
     /// Set up an HPKE sender.
     ///
     /// For the base and PSK modes this encapsulates the public key `pk_r`