Restrict Sha256 to be used sequentially

Also remove useless HashBlockSize64Bytes bound.

libraries/crypto/src/sha256.rs

@@ -15,10 +15,17 @@
 use super::{Hash256, HashBlockSize64Bytes};
 use arrayref::{array_mut_ref, array_ref};
 use byteorder::{BigEndian, ByteOrder};
+use core::cell::Cell;
 use core::num::Wrapping;
 
 const BLOCK_SIZE: usize = 64;
 
+// To be able to support hardware cryptography, we want to make sure we never compute multiple
+// sha256 in parallel. (Note that almost all usage of Sha256 is through Hash256::hash which is
+// statically correct. There's only 2 low-level usages in the `hmac::hmac_256` and those are
+// sequential.) This variable tracks whether `new` was called but `finalize` wasn't called yet.
+const BUSY: Cell<bool> = Cell::new(false);
+
 pub struct Sha256 {
     state: [Wrapping<u32>; 8],
     block: [u8; BLOCK_SIZE],
@@ -27,6 +34,7 @@ pub struct Sha256 {
 
 impl Hash256 for Sha256 {
     fn new() -> Self {
+        assert!(!BUSY.replace(true));
         Sha256 {
             state: Sha256::H,
             block: [0; BLOCK_SIZE],
@@ -93,6 +101,7 @@ impl Hash256 for Sha256 {
         for i in 0..8 {
             BigEndian::write_u32(array_mut_ref![result, 4 * i, 4], self.state[i].0);
         }
+        BUSY.set(false);
         result
     }
 }