modtrezorutils: add consteq

embed/extmod/modtrezorutils/modtrezorutils.c

@@ -12,6 +12,34 @@
 #include <string.h>
 #include "common.h"
 
+/// def consteq(sec: bytes, pub: bytes) -> bool:
+///     '''
+///     Compares the private information in `sec` with public, user-provided
+///     information in `pub`.  Runs in constant time, corresponding to a length
+///     of `pub`.  Can access memory behind valid length of `sec`, caller is
+///     expected to avoid any invalid memory access.
+///     '''
+STATIC mp_obj_t mod_trezorutils_consteq(mp_obj_t sec, mp_obj_t pub) {
+    mp_buffer_info_t secbuf;
+    mp_get_buffer_raise(sec, &secbuf, MP_BUFFER_READ);
+    mp_buffer_info_t pubbuf;
+    mp_get_buffer_raise(pub, &pubbuf, MP_BUFFER_READ);
+
+    size_t diff = secbuf.len - pubbuf.len;
+    for (size_t i = 0; i < pubbuf.len; i++) {
+        const uint8_t *s = (uint8_t *)secbuf.buf;
+        const uint8_t *p = (uint8_t *)pubbuf.buf;
+        diff |= s[i] - p[i];
+    }
+
+    if (diff == 0) {
+        return mp_const_true;
+    } else {
+        return mp_const_false;
+    }
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_2(mod_trezorutils_consteq_obj, mod_trezorutils_consteq);
+
 /// def memcpy(dst: bytearray, dst_ofs: int,
 ///            src: bytearray, src_ofs: int,
 ///            n: int) -> int:
@@ -68,6 +96,7 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(mod_trezorutils_halt_obj, 0, 1, mod_t
 
 STATIC const mp_rom_map_elem_t mp_module_trezorutils_globals_table[] = {
     { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_trezorutils) },
+    { MP_ROM_QSTR(MP_QSTR_consteq), MP_ROM_PTR(&mod_trezorutils_consteq_obj) },
     { MP_ROM_QSTR(MP_QSTR_memcpy), MP_ROM_PTR(&mod_trezorutils_memcpy_obj) },
     { MP_ROM_QSTR(MP_QSTR_halt), MP_ROM_PTR(&mod_trezorutils_halt_obj) },
 };