diff --git a/libraries/crypto/src/sha256.rs b/libraries/crypto/src/sha256.rs
index 09e32e0d..cf98b753 100644
--- a/libraries/crypto/src/sha256.rs
+++ b/libraries/crypto/src/sha256.rs
@@ -15,18 +15,11 @@
 use super::{Hash256, HashBlockSize64Bytes};
 use arrayref::{array_mut_ref, array_ref};
 use byteorder::{BigEndian, ByteOrder};
-use core::cell::Cell;
 use core::num::Wrapping;
 use zeroize::Zeroize;
 
 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],
@@ -46,7 +39,6 @@ impl Drop for Sha256 {
 
 impl Hash256 for Sha256 {
     fn new() -> Self {
-        assert!(!BUSY.replace(true));
         Sha256 {
             state: Sha256::H,
             block: [0; BLOCK_SIZE],
@@ -112,7 +104,6 @@ impl Hash256 for Sha256 {
         for i in 0..8 {
             BigEndian::write_u32(array_mut_ref![output, 4 * i, 4], self.state[i].0);
         }
-        BUSY.set(false);
     }
 }
 
diff --git a/libraries/opensk/src/api/crypto/rust_crypto.rs b/libraries/opensk/src/api/crypto/rust_crypto.rs
index c03932ad..59d6cea7 100644
--- a/libraries/opensk/src/api/crypto/rust_crypto.rs
+++ b/libraries/opensk/src/api/crypto/rust_crypto.rs
@@ -34,6 +34,8 @@ use aes::cipher::{
     BlockDecrypt, BlockDecryptMut, BlockEncrypt, BlockEncryptMut, KeyInit, KeyIvInit,
 };
 use alloc::vec::Vec;
+#[cfg(test)]
+use core::cell::RefCell;
 use core::convert::TryFrom;
 use hmac::digest::FixedOutput;
 use hmac::Mac;
@@ -43,6 +45,18 @@ use p256::ecdsa::signature::{SignatureEncoding, Signer, Verifier};
 use p256::ecdsa::{SigningKey, VerifyingKey};
 use p256::elliptic_curve::sec1::ToEncodedPoint;
 use sha2::Digest;
+#[cfg(test)]
+use std::sync::{Mutex, MutexGuard};
+
+// To be able to support hardware cryptography, we want to make sure we never compute multiple
+// sha256 in parallel. Note that calling `digest` or `digest_mut` is statically correct.
+// These variables track whether `new` was called but `finalize` wasn't called yet.
+#[cfg(test)]
+static BUSY: Mutex<()> = Mutex::new(());
+#[cfg(test)]
+thread_local! {
+    static BUSY_GUARD: RefCell<Option<MutexGuard<'static, ()>>> = RefCell::new(None);
+}
 
 pub struct SoftwareCrypto;
 pub struct SoftwareEcdh;
@@ -224,6 +238,11 @@ impl Sha256 for SoftwareSha256 {
     }
 
     fn new() -> Self {
+        #[cfg(test)]
+        BUSY_GUARD.with_borrow_mut(|guard| {
+            assert!(guard.is_none());
+            *guard = Some(BUSY.lock().unwrap());
+        });
         let hasher = sha2::Sha256::new();
         Self { hasher }
     }
@@ -236,6 +255,11 @@ impl Sha256 for SoftwareSha256 {
     /// Finalizes the hashing process, returns the hash value.
     fn finalize(self, output: &mut [u8; HASH_SIZE]) {
         FixedOutput::finalize_into(self.hasher, output.into());
+        #[cfg(test)]
+        BUSY_GUARD.with_borrow_mut(|guard| {
+            assert!(guard.is_some());
+            *guard = None;
+        });
     }
 }
 
diff --git a/libraries/opensk/src/api/crypto/software_crypto.rs b/libraries/opensk/src/api/crypto/software_crypto.rs
index 6ce9bbb1..89c73d24 100644
--- a/libraries/opensk/src/api/crypto/software_crypto.rs
+++ b/libraries/opensk/src/api/crypto/software_crypto.rs
@@ -22,9 +22,23 @@ use crate::api::crypto::{
 };
 use crate::api::rng::Rng;
 use alloc::vec::Vec;
+#[cfg(test)]
+use core::cell::RefCell;
 use crypto::Hash256;
+#[cfg(test)]
+use std::sync::{Mutex, MutexGuard};
 use zeroize::Zeroize;
 
+// To be able to support hardware cryptography, we want to make sure we never compute multiple
+// sha256 in parallel. Note that calling `digest` or `digest_mut` is statically correct.
+// These variables track whether `new` was called but `finalize` wasn't called yet.
+#[cfg(test)]
+static BUSY: Mutex<()> = Mutex::new(());
+#[cfg(test)]
+thread_local! {
+    static BUSY_GUARD: RefCell<Option<MutexGuard<'static, ()>>> = RefCell::new(None);
+}
+
 /// Cryptography implementation using our own library of primitives.
 ///
 /// Warning: The used library does not implement zeroization.
@@ -184,6 +198,11 @@ pub struct SoftwareSha256 {
 
 impl Sha256 for SoftwareSha256 {
     fn new() -> Self {
+        #[cfg(test)]
+        BUSY_GUARD.with_borrow_mut(|guard| {
+            assert!(guard.is_none());
+            *guard = Some(BUSY.lock().unwrap());
+        });
         let hasher = crypto::sha256::Sha256::new();
         Self { hasher }
     }
@@ -195,7 +214,12 @@ impl Sha256 for SoftwareSha256 {
 
     /// Finalizes the hashing process, returns the hash value.
     fn finalize(self, output: &mut [u8; HASH_SIZE]) {
-        self.hasher.finalize(output)
+        self.hasher.finalize(output);
+        #[cfg(test)]
+        BUSY_GUARD.with_borrow_mut(|guard| {
+            assert!(guard.is_some());
+            *guard = None;
+        });
     }
 }