py/mpz: Do sign extension in mpz_as_bytes for negative values.

Signed-off-by: Damien George <damien@micropython.org>

1 files changed, 9 insertions, 1 deletions

diff --git a/py/mpz.c b/py/mpz.c
index b3d880679..751593053 100644
--- a/py/mpz.c
+++ b/py/mpz.c
@@ -1609,7 +1609,6 @@ bool mpz_as_uint_checked(const mpz_t *i, mp_uint_t *value) {
     return true;
 }
 
-// writes at most len bytes to buf (so buf should be zeroed before calling)
 void mpz_as_bytes(const mpz_t *z, bool big_endian, size_t len, byte *buf) {
     byte *b = buf;
     if (big_endian) {
@@ -1641,6 +1640,15 @@ void mpz_as_bytes(const mpz_t *z, bool big_endian, size_t len, byte *buf) {
             }
         }
     }
+
+    // fill remainder of buf with zero/sign extension of the integer
+    if (big_endian) {
+        len = b - buf;
+    } else {
+        len = buf + len - b;
+        buf = b;
+    }
+    memset(buf, z->neg ? 0xff : 0x00, len);
 }
 
 #if MICROPY_PY_BUILTINS_FLOAT