chacha20: adopt the new Generator API from rand_core

The new API force the output to be a straight array, but there is a
custom drop implementation where we can put the Zeroize of the output.

Author: baloo

Cargo.lock

@@ -10,3 +10,6 @@ members = [
 
 [profile.dev]
 opt-level = 2
+
+[patch.crates-io]
+rand_core = { git = "https://github.com/rust-random/rand_core.git", branch = "drop" }

Cargo.toml

@@ -10,7 +10,7 @@ use core::fmt::Debug;
 
 use rand_core::{
     CryptoRng, RngCore, SeedableRng,
-    block::{BlockRng, BlockRngCore, CryptoBlockRng},
+    block::{BlockRng, CryptoGenerator, Generator},
 };
 
 #[cfg(feature = "zeroize")]
@@ -146,35 +146,6 @@ pub type StreamId = U32x2;
 /// The arrays should be in little endian order.
 pub type BlockPos = U32x2;
 
-/// The results buffer that zeroizes on drop when the `zeroize` feature is enabled.
-#[derive(Clone)]
-pub struct BlockRngResults([u32; BUFFER_SIZE]);
-
-impl AsRef<[u32]> for BlockRngResults {
-    fn as_ref(&self) -> &[u32] {
-        &self.0
-    }
-}
-
-impl AsMut<[u32]> for BlockRngResults {
-    fn as_mut(&mut self) -> &mut [u32] {
-        &mut self.0
-    }
-}
-
-impl Default for BlockRngResults {
-    fn default() -> Self {
-        Self([0u32; BUFFER_SIZE])
-    }
-}
-
-#[cfg(feature = "zeroize")]
-impl Drop for BlockRngResults {
-    fn drop(&mut self) {
-        self.0.zeroize();
-    }
-}
-
 const BUFFER_SIZE: usize = 64;
 
 // NB. this must remain consistent with some currently hard-coded numbers in this module
@@ -334,7 +305,7 @@ macro_rules! impl_chacha_rng {
                 self.core.fill_bytes(dest)
             }
         }
-        impl CryptoBlockRng for $ChaChaXCore {}
+        impl CryptoGenerator for $ChaChaXCore {}
         impl CryptoRng for $ChaChaXRng {}
 
         #[cfg(feature = "zeroize")]
@@ -535,13 +506,17 @@ macro_rules! impl_chacha_rng {
             }
         }
 
-        impl BlockRngCore for $ChaChaXCore {
-            type Item = u32;
-            type Results = BlockRngResults;
+        impl Generator for $ChaChaXCore {
+            type Output = [u32; BUFFER_SIZE];
 
             #[inline]
-            fn generate(&mut self, r: &mut Self::Results) {
-                self.0.generate(&mut r.0);
+            fn generate(&mut self, r: &mut Self::Output) {
+                self.0.generate(r);
+            }
+
+            #[cfg(feature = "zeroize")]
+            fn drop(&mut self, output: &mut Self::Output) {
+                output.zeroize();
             }
         }
     };