rust: add zeroizing str to cstr util function

Not using CString as it did not look to me like it was actually
preallocating for the null terminator, and reallocation would leave
unzeroed copies.

Author: benma

src/rust/bitbox02/src/keystore.rs

@@ -75,7 +75,7 @@ pub fn _unlock(password: &str) -> Result<zeroize::Zeroizing<Vec<u8>>, Error> {
     let mut seed_len: usize = 0;
     match unsafe {
         bitbox02_sys::keystore_unlock(
-            crate::util::str_to_cstr_vec(password)
+            crate::util::str_to_cstr_vec_zeroizing(password)
                 .unwrap()
                 .as_ptr()
                 .cast(),

src/rust/bitbox02/src/util.rs

@@ -42,7 +42,7 @@ pub fn truncate_str(s: &str, len: usize) -> &str {
 }
 
 /// Converts a Rust string to a null terminated C string by appending a null
-/// terminator.  Returns `Err(())` if the input already contians a null byte.
+/// terminator.  Returns `Err(())` if the input already contains a null byte.
 pub fn str_to_cstr_vec(input: &str) -> Result<Vec<core::ffi::c_char>, ()> {
     let cstr = alloc::ffi::CString::new(input)
         .or(Err(()))?
@@ -53,6 +53,23 @@ pub fn str_to_cstr_vec(input: &str) -> Result<Vec<core::ffi::c_char>, ()> {
     Ok(cstr.into_iter().map(|c| c as _).collect())
 }
 
+/// Converts a Rust string to a null terminated C string by appending a null
+/// terminator.  Returns `Err(())` if the input already contains a null byte.
+pub fn str_to_cstr_vec_zeroizing(
+    input: &str,
+) -> Result<zeroize::Zeroizing<Vec<core::ffi::c_char>>, ()> {
+    let bytes = input.as_bytes();
+    if bytes.contains(&0) {
+        return Err(());
+    }
+    let mut result: zeroize::Zeroizing<Vec<core::ffi::c_char>> =
+        zeroize::Zeroizing::new(vec![0; bytes.len() + 1]);
+    for (i, b) in bytes.iter().enumerate() {
+        result[i] = *b as _;
+    }
+    Ok(result)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -134,4 +151,18 @@ mod tests {
         );
         assert_eq!(str_to_cstr_vec("te\0st"), Err(()));
     }
+
+    #[test]
+    fn test_str_to_cstr_vec_zeroizing() {
+        assert_eq!(str_to_cstr_vec_zeroizing("").unwrap().as_slice(), &[0]);
+        assert_eq!(
+            str_to_cstr_vec_zeroizing("test").unwrap(),
+            b"test\0"
+                .iter()
+                .map(|c| *c as _)
+                .collect::<Vec<core::ffi::c_char>>()
+                .into(),
+        );
+        assert_eq!(str_to_cstr_vec_zeroizing("te\0st"), Err(()));
+    }
 }