Make zeroize optional

Cargo.toml

@@ -20,7 +20,7 @@ features = ["nightly"]
 [dependencies]
 keccak = { version = "0.1.0", default-features = false }
 byteorder = { version = "1.2.4", default-features = false }
-zeroize = { version = "1", default-features = false, features = ["zeroize_derive"] }
+zeroize = { version = "1", optional = true, default-features = false, features = ["zeroize_derive"] }
 rand_core = { version = "0.5", default-features = false }
 hex = {version = "0.3", default-features = false, optional = true}
 
@@ -30,7 +30,8 @@ curve25519-dalek = "2"
 rand_chacha = "0.2"
 
 [features]
-default = ["std"]
+default = ["std", "zeroize"]
 nightly = []
 debug-transcript = ["hex"]
 std = ["rand_core/std", "byteorder/std"]
+zeroize = ["dep:zeroize"]

src/strobe.rs

@@ -3,7 +3,9 @@
 use core::ops::{Deref, DerefMut};
 
 use keccak;
-use zeroize::Zeroize;
+
+#[cfg(feature = "zeroize")]
+use zeroize::{Zeroize, ZeroizeOnDrop};
 
 /// Strobe R value; security level 128 is hardcoded
 const STROBE_R: u8 = 166;
@@ -22,15 +24,16 @@ fn transmute_state(st: &mut AlignedKeccakState) -> &mut [u64; 25] {
 /// This is a wrapper around 200-byte buffer that's always 8-byte aligned
 /// to make pointers to it safely convertible to pointers to [u64; 25]
 /// (since u64 words must be 8-byte aligned)
-#[derive(Clone, Zeroize)]
-#[zeroize(drop)]
+#[derive(Clone)]
+#[cfg_attr(feature = "zeroize", derive(Zeroize, ZeroizeOnDrop))]
 #[repr(align(8))]
 struct AlignedKeccakState([u8; 200]);
 
 /// A Strobe context for the 128-bit security level.
 ///
 /// Only `meta-AD`, `AD`, `KEY`, and `PRF` operations are supported.
-#[derive(Clone, Zeroize)]
+#[derive(Clone)]
+#[cfg_attr(feature = "zeroize", derive(Zeroize, ZeroizeOnDrop))]
 pub struct Strobe128 {
     state: AlignedKeccakState,
     pos: u8,

src/transcript.rs

@@ -1,5 +1,6 @@
 use rand_core;
-use zeroize::Zeroize;
+#[cfg(feature = "zeroize")]
+use zeroize::{Zeroize, ZeroizeOnDrop};
 
 use crate::strobe::Strobe128;
 
@@ -50,7 +51,8 @@ fn encode_usize_as_u32(x: usize) -> [u8; 4] {
 /// implement a proof system, see the documentation at
 /// [merlin.cool](https://merlin.cool), particularly the [Using
 /// Merlin](https://merlin.cool/use/index.html) section.
-#[derive(Clone, Zeroize)]
+#[derive(Clone)]
+#[cfg_attr(feature = "zeroize", derive(Zeroize, ZeroizeOnDrop))]
 pub struct Transcript {
     strobe: Strobe128,
 }