diff --git a/extmod/moduhashlib.c b/extmod/moduhashlib.c
index 195a24334..b170de9e5 100644
--- a/extmod/moduhashlib.c
+++ b/extmod/moduhashlib.c
@@ -60,9 +60,16 @@
 
 typedef struct _mp_obj_hash_t {
     mp_obj_base_t base;
-    char state[0];
+    bool final; // if set, update and digest raise an exception
+    uintptr_t state[0]; // must be aligned to a machine word
 } mp_obj_hash_t;
 
+static void uhashlib_ensure_not_final(mp_obj_hash_t *self) {
+    if (self->final) {
+        mp_raise_ValueError(MP_ERROR_TEXT("hash is final"));
+    }
+}
+
 #if MICROPY_PY_UHASHLIB_SHA256
 STATIC mp_obj_t uhashlib_sha256_update(mp_obj_t self_in, mp_obj_t arg);
 
@@ -78,6 +85,7 @@ STATIC mp_obj_t uhashlib_sha256_make_new(const mp_obj_type_t *type, size_t n_arg
     mp_arg_check_num(n_args, n_kw, 0, 1, false);
     mp_obj_hash_t *o = m_new_obj_var(mp_obj_hash_t, char, sizeof(mbedtls_sha256_context));
     o->base.type = type;
+    o->final = false;
     mbedtls_sha256_init((mbedtls_sha256_context *)&o->state);
     mbedtls_sha256_starts_ret((mbedtls_sha256_context *)&o->state, 0);
     if (n_args == 1) {
@@ -88,6 +96,7 @@ STATIC mp_obj_t uhashlib_sha256_make_new(const mp_obj_type_t *type, size_t n_arg
 
 STATIC mp_obj_t uhashlib_sha256_update(mp_obj_t self_in, mp_obj_t arg) {
     mp_obj_hash_t *self = MP_OBJ_TO_PTR(self_in);
+    uhashlib_ensure_not_final(self);
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(arg, &bufinfo, MP_BUFFER_READ);
     mbedtls_sha256_update_ret((mbedtls_sha256_context *)&self->state, bufinfo.buf, bufinfo.len);
@@ -96,6 +105,8 @@ STATIC mp_obj_t uhashlib_sha256_update(mp_obj_t self_in, mp_obj_t arg) {
 
 STATIC mp_obj_t uhashlib_sha256_digest(mp_obj_t self_in) {
     mp_obj_hash_t *self = MP_OBJ_TO_PTR(self_in);
+    uhashlib_ensure_not_final(self);
+    self->final = true;
     vstr_t vstr;
     vstr_init_len(&vstr, 32);
     mbedtls_sha256_finish_ret((mbedtls_sha256_context *)&self->state, (unsigned char *)vstr.buf);
@@ -110,6 +121,7 @@ STATIC mp_obj_t uhashlib_sha256_make_new(const mp_obj_type_t *type, size_t n_arg
     mp_arg_check_num(n_args, n_kw, 0, 1, false);
     mp_obj_hash_t *o = m_new_obj_var(mp_obj_hash_t, char, sizeof(CRYAL_SHA256_CTX));
     o->base.type = type;
+    o->final = false;
     sha256_init((CRYAL_SHA256_CTX *)o->state);
     if (n_args == 1) {
         uhashlib_sha256_update(MP_OBJ_FROM_PTR(o), args[0]);
@@ -119,6 +131,7 @@ STATIC mp_obj_t uhashlib_sha256_make_new(const mp_obj_type_t *type, size_t n_arg
 
 STATIC mp_obj_t uhashlib_sha256_update(mp_obj_t self_in, mp_obj_t arg) {
     mp_obj_hash_t *self = MP_OBJ_TO_PTR(self_in);
+    uhashlib_ensure_not_final(self);
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(arg, &bufinfo, MP_BUFFER_READ);
     sha256_update((CRYAL_SHA256_CTX *)self->state, bufinfo.buf, bufinfo.len);
@@ -127,6 +140,8 @@ STATIC mp_obj_t uhashlib_sha256_update(mp_obj_t self_in, mp_obj_t arg) {
 
 STATIC mp_obj_t uhashlib_sha256_digest(mp_obj_t self_in) {
     mp_obj_hash_t *self = MP_OBJ_TO_PTR(self_in);
+    uhashlib_ensure_not_final(self);
+    self->final = true;
     vstr_t vstr;
     vstr_init_len(&vstr, SHA256_BLOCK_SIZE);
     sha256_final((CRYAL_SHA256_CTX *)self->state, (byte *)vstr.buf);
@@ -160,6 +175,7 @@ STATIC mp_obj_t uhashlib_sha1_make_new(const mp_obj_type_t *type, size_t n_args,
     mp_arg_check_num(n_args, n_kw, 0, 1, false);
     mp_obj_hash_t *o = m_new_obj_var(mp_obj_hash_t, char, sizeof(SHA1_CTX));
     o->base.type = type;
+    o->final = false;
     SHA1_Init((SHA1_CTX *)o->state);
     if (n_args == 1) {
         uhashlib_sha1_update(MP_OBJ_FROM_PTR(o), args[0]);
@@ -169,6 +185,7 @@ STATIC mp_obj_t uhashlib_sha1_make_new(const mp_obj_type_t *type, size_t n_args,
 
 STATIC mp_obj_t uhashlib_sha1_update(mp_obj_t self_in, mp_obj_t arg) {
     mp_obj_hash_t *self = MP_OBJ_TO_PTR(self_in);
+    uhashlib_ensure_not_final(self);
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(arg, &bufinfo, MP_BUFFER_READ);
     SHA1_Update((SHA1_CTX *)self->state, bufinfo.buf, bufinfo.len);
@@ -177,6 +194,8 @@ STATIC mp_obj_t uhashlib_sha1_update(mp_obj_t self_in, mp_obj_t arg) {
 
 STATIC mp_obj_t uhashlib_sha1_digest(mp_obj_t self_in) {
     mp_obj_hash_t *self = MP_OBJ_TO_PTR(self_in);
+    uhashlib_ensure_not_final(self);
+    self->final = true;
     vstr_t vstr;
     vstr_init_len(&vstr, SHA1_SIZE);
     SHA1_Final((byte *)vstr.buf, (SHA1_CTX *)self->state);
@@ -196,6 +215,7 @@ STATIC mp_obj_t uhashlib_sha1_make_new(const mp_obj_type_t *type, size_t n_args,
     mp_arg_check_num(n_args, n_kw, 0, 1, false);
     mp_obj_hash_t *o = m_new_obj_var(mp_obj_hash_t, char, sizeof(mbedtls_sha1_context));
     o->base.type = type;
+    o->final = false;
     mbedtls_sha1_init((mbedtls_sha1_context *)o->state);
     mbedtls_sha1_starts_ret((mbedtls_sha1_context *)o->state);
     if (n_args == 1) {
@@ -206,6 +226,7 @@ STATIC mp_obj_t uhashlib_sha1_make_new(const mp_obj_type_t *type, size_t n_args,
 
 STATIC mp_obj_t uhashlib_sha1_update(mp_obj_t self_in, mp_obj_t arg) {
     mp_obj_hash_t *self = MP_OBJ_TO_PTR(self_in);
+    uhashlib_ensure_not_final(self);
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(arg, &bufinfo, MP_BUFFER_READ);
     mbedtls_sha1_update_ret((mbedtls_sha1_context *)self->state, bufinfo.buf, bufinfo.len);
@@ -214,6 +235,8 @@ STATIC mp_obj_t uhashlib_sha1_update(mp_obj_t self_in, mp_obj_t arg) {
 
 STATIC mp_obj_t uhashlib_sha1_digest(mp_obj_t self_in) {
     mp_obj_hash_t *self = MP_OBJ_TO_PTR(self_in);
+    uhashlib_ensure_not_final(self);
+    self->final = true;
     vstr_t vstr;
     vstr_init_len(&vstr, 20);
     mbedtls_sha1_finish_ret((mbedtls_sha1_context *)self->state, (byte *)vstr.buf);
@@ -247,6 +270,7 @@ STATIC mp_obj_t uhashlib_md5_make_new(const mp_obj_type_t *type, size_t n_args,
     mp_arg_check_num(n_args, n_kw, 0, 1, false);
     mp_obj_hash_t *o = m_new_obj_var(mp_obj_hash_t, char, sizeof(MD5_CTX));
     o->base.type = type;
+    o->final = false;
     MD5_Init((MD5_CTX *)o->state);
     if (n_args == 1) {
         uhashlib_md5_update(MP_OBJ_FROM_PTR(o), args[0]);
@@ -256,6 +280,7 @@ STATIC mp_obj_t uhashlib_md5_make_new(const mp_obj_type_t *type, size_t n_args,
 
 STATIC mp_obj_t uhashlib_md5_update(mp_obj_t self_in, mp_obj_t arg) {
     mp_obj_hash_t *self = MP_OBJ_TO_PTR(self_in);
+    uhashlib_ensure_not_final(self);
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(arg, &bufinfo, MP_BUFFER_READ);
     MD5_Update((MD5_CTX *)self->state, bufinfo.buf, bufinfo.len);
@@ -264,6 +289,8 @@ STATIC mp_obj_t uhashlib_md5_update(mp_obj_t self_in, mp_obj_t arg) {
 
 STATIC mp_obj_t uhashlib_md5_digest(mp_obj_t self_in) {
     mp_obj_hash_t *self = MP_OBJ_TO_PTR(self_in);
+    uhashlib_ensure_not_final(self);
+    self->final = true;
     vstr_t vstr;
     vstr_init_len(&vstr, MD5_SIZE);
     MD5_Final((byte *)vstr.buf, (MD5_CTX *)self->state);
@@ -283,6 +310,7 @@ STATIC mp_obj_t uhashlib_md5_make_new(const mp_obj_type_t *type, size_t n_args,
     mp_arg_check_num(n_args, n_kw, 0, 1, false);
     mp_obj_hash_t *o = m_new_obj_var(mp_obj_hash_t, char, sizeof(mbedtls_md5_context));
     o->base.type = type;
+    o->final = false;
     mbedtls_md5_init((mbedtls_md5_context *)o->state);
     mbedtls_md5_starts_ret((mbedtls_md5_context *)o->state);
     if (n_args == 1) {
@@ -293,6 +321,7 @@ STATIC mp_obj_t uhashlib_md5_make_new(const mp_obj_type_t *type, size_t n_args,
 
 STATIC mp_obj_t uhashlib_md5_update(mp_obj_t self_in, mp_obj_t arg) {
     mp_obj_hash_t *self = MP_OBJ_TO_PTR(self_in);
+    uhashlib_ensure_not_final(self);
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(arg, &bufinfo, MP_BUFFER_READ);
     mbedtls_md5_update_ret((mbedtls_md5_context *)self->state, bufinfo.buf, bufinfo.len);
@@ -301,6 +330,8 @@ STATIC mp_obj_t uhashlib_md5_update(mp_obj_t self_in, mp_obj_t arg) {
 
 STATIC mp_obj_t uhashlib_md5_digest(mp_obj_t self_in) {
     mp_obj_hash_t *self = MP_OBJ_TO_PTR(self_in);
+    uhashlib_ensure_not_final(self);
+    self->final = true;
     vstr_t vstr;
     vstr_init_len(&vstr, 16);
     mbedtls_md5_finish_ret((mbedtls_md5_context *)self->state, (byte *)vstr.buf);
diff --git a/tests/extmod/uhashlib_final.py b/tests/extmod/uhashlib_final.py
new file mode 100644
index 000000000..f562cc178
--- /dev/null
+++ b/tests/extmod/uhashlib_final.py
@@ -0,0 +1,35 @@
+try:
+    import uhashlib
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+
+for algo_name in ("md5", "sha1", "sha256"):
+    algo = getattr(uhashlib, algo_name, None)
+    if not algo:
+        continue
+
+    # Running .digest() several times in row is not supported.
+    h = algo(b"123")
+    h.digest()
+    try:
+        h.digest()
+        print("fail")
+    except ValueError:
+        # Expected path, don't print anything so test output is the
+        # same even if the algorithm is not implemented on the port.
+        pass
+
+    # Partial digests are not supported.
+    h = algo(b"123")
+    h.digest()
+    try:
+        h.update(b"456")
+        print("fail")
+    except ValueError:
+        # Expected path, don't print anything so test output is the
+        # same even if the algorithm is not implemented on the port.
+        pass
+
+print("done")
diff --git a/tests/extmod/uhashlib_final.py.exp b/tests/extmod/uhashlib_final.py.exp
new file mode 100644
index 000000000..19f86f493
--- /dev/null
+++ b/tests/extmod/uhashlib_final.py.exp
@@ -0,0 +1 @@
+done