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.

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
 }