diff --git a/src/kasa_crypt/_crypt_impl.pyx b/src/kasa_crypt/_crypt_impl.pyx
index b1dca93..12b6128 100644
--- a/src/kasa_crypt/_crypt_impl.pyx
+++ b/src/kasa_crypt/_crypt_impl.pyx
@@ -9,18 +9,20 @@ cdef extern from "crypt_wrapper.h":
     void _decrypt_into(const char * encrypted, char * unencrypted)
 
 cdef char* _decrypt(const char *encrypted):
-    cdef char* unencrypted = <char *> malloc(((strlen(encrypted)) + 1) * sizeof(char))
+    cdef Py_ssize_t n = strlen(encrypted)
+    cdef char* unencrypted = <char *> malloc((n + 1) * sizeof(char))
     if not unencrypted:
         return NULL  # malloc failed
     _decrypt_into(encrypted, unencrypted)
-    return unencrypted
+    return unencrypted[:n]
 
 cdef char* _encrypt(const char *unencrypted):
-    cdef char* encrypted = <char *> malloc(((strlen(unencrypted)) + 1) * sizeof(char))
+    cdef Py_ssize_t n = strlen(unencrypted)
+    cdef char* encrypted = <char *> malloc((n + 1) * sizeof(char))
     if not encrypted:
         return NULL  # malloc failed
     _encrypt_into(unencrypted, encrypted)
-    return encrypted
+    return encrypted[:n]
 
 def encrypt(string: str) -> bytes:
     return _encrypt(string.encode('utf-8'))