diff --git a/embed/extmod/modtrezorcrypto/modtrezorcrypto-aes.h b/embed/extmod/modtrezorcrypto/modtrezorcrypto-aes.h
index 8060f745d..6c4a1bc69 100644
--- a/embed/extmod/modtrezorcrypto/modtrezorcrypto-aes.h
+++ b/embed/extmod/modtrezorcrypto/modtrezorcrypto-aes.h
@@ -101,7 +101,8 @@ STATIC mp_obj_t mod_trezorcrypto_AES_update(mp_obj_t self, mp_obj_t data) {
     if (buf.len == 0) {
         return mp_const_empty_bytes;
     }
-    uint8_t out[buf.len];
+    vstr_t vstr;
+    vstr_init_len(&vstr, buf.len);
     mp_obj_AES_t *o = MP_OBJ_TO_PTR(self);
     switch (o->mode & AESModeMask) {
         case ECB:
@@ -109,9 +110,9 @@ STATIC mp_obj_t mod_trezorcrypto_AES_update(mp_obj_t self, mp_obj_t data) {
                 mp_raise_ValueError("Invalid data length");
             }
             if ((o->mode & AESDirMask) == Encrypt) {
-                aes_ecb_encrypt(buf.buf, out, buf.len, &(o->ctx.encrypt_ctx));
+                aes_ecb_encrypt(buf.buf, (uint8_t *)vstr.buf, buf.len, &(o->ctx.encrypt_ctx));
             } else {
-                aes_ecb_decrypt(buf.buf, out, buf.len, &(o->ctx.decrypt_ctx));
+                aes_ecb_decrypt(buf.buf, (uint8_t *)vstr.buf, buf.len, &(o->ctx.decrypt_ctx));
             }
             break;
         case CBC:
@@ -119,26 +120,26 @@ STATIC mp_obj_t mod_trezorcrypto_AES_update(mp_obj_t self, mp_obj_t data) {
                 mp_raise_ValueError("Invalid data length");
             }
             if ((o->mode & AESDirMask) == Encrypt) {
-                aes_cbc_encrypt(buf.buf, out, buf.len, o->iv, &(o->ctx.encrypt_ctx));
+                aes_cbc_encrypt(buf.buf, (uint8_t *)vstr.buf, buf.len, o->iv, &(o->ctx.encrypt_ctx));
             } else {
-                aes_cbc_decrypt(buf.buf, out, buf.len, o->iv, &(o->ctx.decrypt_ctx));
+                aes_cbc_decrypt(buf.buf, (uint8_t *)vstr.buf, buf.len, o->iv, &(o->ctx.decrypt_ctx));
             }
             break;
         case CFB:
             if ((o->mode & AESDirMask) == Encrypt) {
-                aes_cfb_encrypt(buf.buf, out, buf.len, o->iv, &(o->ctx.encrypt_ctx));
+                aes_cfb_encrypt(buf.buf, (uint8_t *)vstr.buf, buf.len, o->iv, &(o->ctx.encrypt_ctx));
             } else {
-                aes_cfb_decrypt(buf.buf, out, buf.len, o->iv, &(o->ctx.encrypt_ctx));
+                aes_cfb_decrypt(buf.buf, (uint8_t *)vstr.buf, buf.len, o->iv, &(o->ctx.encrypt_ctx));
             }
             break;
         case OFB: // (encrypt == decrypt)
-            aes_ofb_crypt(buf.buf, out, buf.len, o->iv, &(o->ctx.encrypt_ctx));
+            aes_ofb_crypt(buf.buf, (uint8_t *)vstr.buf, buf.len, o->iv, &(o->ctx.encrypt_ctx));
             break;
         case CTR: // (encrypt == decrypt)
-            aes_ctr_crypt(buf.buf, out, buf.len, o->ctr, aes_ctr_cbuf_inc, &(o->ctx.encrypt_ctx));
+            aes_ctr_crypt(buf.buf, (uint8_t *)vstr.buf, buf.len, o->ctr, aes_ctr_cbuf_inc, &(o->ctx.encrypt_ctx));
             break;
     }
-    return mp_obj_new_bytes(out, sizeof(out));
+    return mp_obj_new_bytes((uint8_t *)vstr.buf, vstr.len);
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_2(mod_trezorcrypto_AES_update_obj, mod_trezorcrypto_AES_update);
 
diff --git a/embed/extmod/modtrezorcrypto/modtrezorcrypto-chacha20poly1305.h b/embed/extmod/modtrezorcrypto/modtrezorcrypto-chacha20poly1305.h
index 2626b3407..902aeb9c8 100644
--- a/embed/extmod/modtrezorcrypto/modtrezorcrypto-chacha20poly1305.h
+++ b/embed/extmod/modtrezorcrypto/modtrezorcrypto-chacha20poly1305.h
@@ -51,10 +51,11 @@ STATIC mp_obj_t mod_trezorcrypto_ChaCha20Poly1305_encrypt(mp_obj_t self, mp_obj_
     mp_obj_ChaCha20Poly1305_t *o = MP_OBJ_TO_PTR(self);
     mp_buffer_info_t in;
     mp_get_buffer_raise(data, &in, MP_BUFFER_READ);
-    uint8_t out[in.len];
-    chacha20poly1305_encrypt(&(o->ctx), in.buf, out, in.len);
+    vstr_t vstr;
+    vstr_init_len(&vstr, in.len);
+    chacha20poly1305_encrypt(&(o->ctx), in.buf, (uint8_t *)vstr.buf, in.len);
     o->plen += in.len;
-    return mp_obj_new_bytes(out, sizeof(out));
+    return mp_obj_new_bytes((uint8_t *)vstr.buf, vstr.len);
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_2(mod_trezorcrypto_ChaCha20Poly1305_encrypt_obj, mod_trezorcrypto_ChaCha20Poly1305_encrypt);
 
@@ -66,10 +67,11 @@ STATIC mp_obj_t mod_trezorcrypto_ChaCha20Poly1305_decrypt(mp_obj_t self, mp_obj_
     mp_obj_ChaCha20Poly1305_t *o = MP_OBJ_TO_PTR(self);
     mp_buffer_info_t in;
     mp_get_buffer_raise(data, &in, MP_BUFFER_READ);
-    uint8_t out[in.len];
-    chacha20poly1305_decrypt(&(o->ctx), in.buf, out, in.len);
+    vstr_t vstr;
+    vstr_init_len(&vstr, in.len);
+    chacha20poly1305_decrypt(&(o->ctx), in.buf, (uint8_t *)vstr.buf, in.len);
     o->plen += in.len;
-    return mp_obj_new_bytes(out, sizeof(out));
+    return mp_obj_new_bytes((uint8_t *)vstr.buf, vstr.len);
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_2(mod_trezorcrypto_ChaCha20Poly1305_decrypt_obj, mod_trezorcrypto_ChaCha20Poly1305_decrypt);
 
diff --git a/embed/extmod/modtrezorcrypto/modtrezorcrypto-random.h b/embed/extmod/modtrezorcrypto/modtrezorcrypto-random.h
index df48036b8..bf0bc2a18 100644
--- a/embed/extmod/modtrezorcrypto/modtrezorcrypto-random.h
+++ b/embed/extmod/modtrezorcrypto/modtrezorcrypto-random.h
@@ -31,9 +31,10 @@ STATIC mp_obj_t mod_trezorcrypto_random_bytes(mp_obj_t len) {
     if (l > 1024) {
         mp_raise_ValueError("Maximum requested size is 1024");
     }
-    uint8_t out[l];
-    random_buffer(out, l);
-    return mp_obj_new_bytes(out, sizeof(out));
+    vstr_t vstr;
+    vstr_init_len(&vstr, l);
+    random_buffer((uint8_t *)vstr.buf, l);
+    return mp_obj_new_bytes((uint8_t *)vstr.buf, vstr.len);
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(mod_trezorcrypto_random_bytes_obj, mod_trezorcrypto_random_bytes);