Improve bls::SecretKey privacy (#1164)

* Improve bls::SecretKey privacy

* Add missed file

* Remove more methods from bls::SecretKey

* Add as_bytes() to SecretKey, remove as_raw

* Remove as_raw

* Add back as_raw

* Address review comments

crypto/bls/src/plain_text.rs

@@ -0,0 +1,40 @@
+use zeroize::Zeroize;
+
+/// Provides wrapper around `Vec<u8>` that implements `Zeroize`.
+#[derive(Zeroize, Clone, PartialEq)]
+#[zeroize(drop)]
+pub struct PlainText(Vec<u8>);
+
+impl PlainText {
+    /// Instantiate self with `len` zeros.
+    pub fn zero(len: usize) -> Self {
+        Self(vec![0; len])
+    }
+
+    /// The byte-length of `self`
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    /// Returns a reference to the underlying bytes.
+    pub fn as_bytes(&self) -> &[u8] {
+        &self.0
+    }
+
+    /// Returns a mutable reference to the underlying bytes.
+    pub fn as_mut_bytes(&mut self) -> &mut [u8] {
+        &mut self.0
+    }
+}
+
+impl From<Vec<u8>> for PlainText {
+    fn from(vec: Vec<u8>) -> Self {
+        Self(vec)
+    }
+}
+
+impl AsRef<[u8]> for PlainText {
+    fn as_ref(&self) -> &[u8] {
+        &self.0
+    }
+}