Resolve #268 by adding a Zeroize to DigestTranscript which writes a full block

This is a 'better-than-nothing' attempt to invalidate its state.

Also replaces black_box features with usage of the rustversion crate.

Cargo.lock

@@ -1658,6 +1658,7 @@ dependencies = [
  "ff-group-tests",
  "group 0.13.0",
  "rand_core 0.6.4",
+ "rustversion",
  "sha2 0.9.9",
  "subtle",
  "zeroize",
@@ -1995,6 +1996,7 @@ dependencies = [
  "k256",
  "multiexp",
  "rand_core 0.6.4",
+ "rustversion",
  "thiserror",
  "zeroize",
 ]
@@ -2798,7 +2800,10 @@ dependencies = [
  "blake2",
  "digest 0.10.6",
  "merlin 3.0.0",
+ "rustversion",
  "sha2 0.10.6",
+ "subtle",
+ "zeroize",
 ]
 
 [[package]]
@@ -5046,6 +5051,7 @@ dependencies = [
  "hex",
  "lazy_static",
  "rand_core 0.6.4",
+ "rustversion",
  "subtle",
  "zeroize",
 ]
@@ -5278,6 +5284,7 @@ dependencies = [
  "group 0.13.0",
  "k256",
  "rand_core 0.6.4",
+ "rustversion",
  "zeroize",
 ]
 

crypto/dalek-ff-group/Cargo.toml

@@ -13,12 +13,15 @@ all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
 
 [dependencies]
-rand_core = "0.6"
-digest = "0.10"
+rustversion = "1"
 
 zeroize = { version = "^1.5", features = ["zeroize_derive"] }
 subtle = "^2.4"
 
+rand_core = "0.6"
+
+digest = "0.10"
+
 ff = "0.13"
 group = "0.13"
 
@@ -29,6 +32,3 @@ curve25519-dalek = "^3.2"
 
 [dev-dependencies]
 ff-group-tests = { path = "../ff-group-tests" }
-
-[features]
-black_box = []

crypto/dalek-ff-group/src/lib.rs

@@ -38,14 +38,11 @@ use group::{
 mod field;
 pub use field::FieldElement;
 
-// Feature gated due to MSRV requirements
-#[cfg(feature = "black_box")]
-pub(crate) fn black_box<T>(val: T) -> T {
-  core::hint::black_box(val)
-}
-
-#[cfg(not(feature = "black_box"))]
-pub(crate) fn black_box<T>(val: T) -> T {
+// Use black_box when possible
+#[rustversion::since(1.66)]
+use core::hint::black_box;
+#[rustversion::before(1.66)]
+fn black_box<T>(val: T) -> T {
   val
 }
 

crypto/dleq/Cargo.toml

@@ -12,6 +12,8 @@ all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
 
 [dependencies]
+rustversion = "1"
+
 thiserror = { version = "1", optional = true }
 rand_core = "0.6"
 
@@ -41,7 +43,6 @@ std = []
 serialize = ["std"]
 
 # Needed for cross-group DLEqs
-black_box = []
 secure_capacity_difference = []
 experimental = ["std", "thiserror", "multiexp"]
 

crypto/dleq/src/cross_group/mod.rs

@@ -30,14 +30,11 @@ pub(crate) mod aos;
 mod bits;
 use bits::{BitSignature, Bits};
 
-// Feature gated due to MSRV requirements
-#[cfg(feature = "black_box")]
-pub(crate) fn black_box<T>(val: T) -> T {
-  core::hint::black_box(val)
-}
-
-#[cfg(not(feature = "black_box"))]
-pub(crate) fn black_box<T>(val: T) -> T {
+// Use black_box when possible
+#[rustversion::since(1.66)]
+use core::hint::black_box;
+#[rustversion::before(1.66)]
+fn black_box<T>(val: T) -> T {
   val
 }
 

crypto/ed448/Cargo.toml

@@ -13,6 +13,8 @@ all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
 
 [dependencies]
+rustversion = "1"
+
 lazy_static = "1"
 
 rand_core = "0.6"
@@ -30,6 +32,3 @@ crypto-bigint = { version = "0.5", features = ["zeroize"] }
 hex = "0.4"
 
 ff-group-tests = { path = "../ff-group-tests" }
-
-[features]
-black_box = []

crypto/ed448/src/backend.rs

@@ -1,13 +1,10 @@
 use zeroize::Zeroize;
 
-// Feature gated due to MSRV requirements
-#[cfg(feature = "black_box")]
-pub(crate) fn black_box<T>(val: T) -> T {
-  core::hint::black_box(val)
-}
-
-#[cfg(not(feature = "black_box"))]
-pub(crate) fn black_box<T>(val: T) -> T {
+// Use black_box when possible
+#[rustversion::since(1.66)]
+use core::hint::black_box;
+#[rustversion::before(1.66)]
+fn black_box<T>(val: T) -> T {
   val
 }
 

crypto/multiexp/Cargo.toml

@@ -13,6 +13,8 @@ all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
 
 [dependencies]
+rustversion = "1"
+
 zeroize = { version = "^1.5", features = ["zeroize_derive"] }
 
 ff = "0.13"
@@ -27,5 +29,4 @@ k256 = { version = "0.13", features = ["bits"] }
 dalek-ff-group = { path = "../dalek-ff-group" }
 
 [features]
-black_box = []
 batch = ["rand_core"]

crypto/multiexp/src/lib.rs

@@ -22,14 +22,11 @@ pub use batch::BatchVerifier;
 #[cfg(test)]
 mod tests;
 
-// Feature gated due to MSRV requirements
-#[cfg(feature = "black_box")]
-pub(crate) fn black_box<T>(val: T) -> T {
-  core::hint::black_box(val)
-}
-
-#[cfg(not(feature = "black_box"))]
-pub(crate) fn black_box<T>(val: T) -> T {
+// Use black_box when possible
+#[rustversion::since(1.66)]
+use core::hint::black_box;
+#[rustversion::before(1.66)]
+fn black_box<T>(val: T) -> T {
   val
 }
 

crypto/schnorr/src/aggregate.rs

@@ -139,18 +139,12 @@ impl<C: Ciphersuite> SchnorrAggregate<C> {
 
 /// A signature aggregator capable of consuming signatures in order to produce an aggregate.
 #[allow(non_snake_case)]
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Zeroize)]
 pub struct SchnorrAggregator<C: Ciphersuite> {
   digest: DigestTranscript<C::H>,
   sigs: Vec<SchnorrSignature<C>>,
 }
 
-impl<C: Ciphersuite> Zeroize for SchnorrAggregator<C> {
-  fn zeroize(&mut self) {
-    self.sigs.zeroize();
-  }
-}
-
 impl<C: Ciphersuite> SchnorrAggregator<C> {
   /// Create a new aggregator.
   ///

crypto/transcript/Cargo.toml

@@ -13,6 +13,11 @@ all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
 
 [dependencies]
+rustversion = "1"
+
+subtle = "^2.4"
+zeroize = "^1.5"
+
 digest = "0.10"
 
 blake2 = { version = "0.10", optional = true }

crypto/transcript/src/lib.rs

@@ -2,6 +2,15 @@
 #![no_std]
 
 ///! A transcript trait valid over a variety of transcript formats.
+use zeroize::Zeroize;
+
+use digest::{
+  typenum::{
+    consts::U32, marker_traits::NonZero, type_operators::IsGreaterOrEqual, operator_aliases::GrEq,
+  },
+  core_api::BlockSizeUser,
+  Digest, Output, HashMarker,
+};
 
 #[cfg(feature = "merlin")]
 mod merlin;
@@ -12,13 +21,6 @@ pub use crate::merlin::MerlinTranscript;
 #[cfg(any(test, feature = "tests"))]
 pub mod tests;
 
-use digest::{
-  typenum::{
-    consts::U32, marker_traits::NonZero, type_operators::IsGreaterOrEqual, operator_aliases::GrEq,
-  },
-  Digest, Output, HashMarker,
-};
-
 /// A transcript trait valid over a variety of transcript formats.
 pub trait Transcript: Send + Clone {
   type Challenge: Send + Sync + Clone + AsRef<[u8]>;
@@ -134,6 +136,61 @@ impl<D: Send + Clone + SecureDigest> Transcript for DigestTranscript<D> {
   }
 }
 
+// Digest doesn't implement Zeroize
+// Implement Zeroize for DigestTranscript by writing twice the block size to the digest in an
+// attempt to overwrite the internal hash state/any leftover bytes
+impl<D: Send + Clone + SecureDigest> Zeroize for DigestTranscript<D>
+where
+  D: BlockSizeUser,
+{
+  fn zeroize(&mut self) {
+    // Update in 4-byte chunks to reduce call quantity and enable word-level update optimizations
+    const WORD_SIZE: usize = 4;
+
+    // block_size returns the block_size in bytes
+    // Use a ceil div in case the block size isn't evenly divisible by our word size
+    let words = (D::block_size() + (WORD_SIZE - 1)) / WORD_SIZE;
+    for _ in 0 .. (2 * words) {
+      self.0.update([255; WORD_SIZE]);
+    }
+
+    // Hopefully, the hash state is now overwritten to the point no data is recoverable
+    // These writes may be optimized out if they're never read
+    // Attempt to get them marked as read
+
+    #[rustversion::since(1.66)]
+    fn mark_read<D: Send + Clone + SecureDigest>(transcript: &mut DigestTranscript<D>) {
+      // Just get a challenge from the state
+      let mut challenge = core::hint::black_box(transcript.0.clone().finalize());
+      challenge.as_mut().zeroize();
+    }
+
+    #[rustversion::before(1.66)]
+    fn mark_read<D: Send + Clone + SecureDigest>(transcript: &mut DigestTranscript<D>) {
+      // Get a challenge
+      let challenge = transcript.0.clone().finalize();
+
+      // Attempt to use subtle's, non-exposed black_box function, by creating a Choice from this
+      // challenge
+
+      let mut read = 0;
+      for byte in challenge.as_ref() {
+        read ^= byte;
+      }
+      challenge.as_mut().zeroize();
+
+      // Since this Choice isn't further read, its creation may be optimized out, including its
+      // internal black_box
+      // This remains our best attempt
+      let mut choice = bool::from(subtle::Choice::from(read >> 7));
+      read.zeroize();
+      choice.zeroize();
+    }
+
+    mark_read(self)
+  }
+}
+
 /// The recommended transcript, guaranteed to be secure against length-extension attacks.
 #[cfg(feature = "recommended")]
 pub type RecommendedTranscript = DigestTranscript<blake2::Blake2b512>;