--- a/src/undo.c
+++ b/src/undo.c
@@ -102,6 +102,8 @@ static void u_freeentry __ARGS((u_entry_
 #ifdef FEAT_PERSISTENT_UNDO
 static void corruption_error __ARGS((char *msg, char_u *file_name));
 static void u_free_uhp __ARGS((u_header_T *uhp));
+static size_t fwrite_crypt __ARGS((buf_T *buf UNUSED, char_u *ptr, size_t len, FILE *fp));
+static char_u *read_string_decrypt __ARGS((buf_T *buf UNUSED, FILE *fd, int len));
 static int serialize_header __ARGS((FILE *fp, buf_T *buf, char_u *hash));
 static int serialize_uhp __ARGS((FILE *fp, buf_T *buf, u_header_T *uhp));
 static u_header_T *unserialize_uhp __ARGS((FILE *fp, char_u *file_name));
@@ -661,7 +663,7 @@ nomem:
     return FAIL;
 }
 
-#ifdef FEAT_PERSISTENT_UNDO
+#if defined(FEAT_PERSISTENT_UNDO) || defined(PROTO)
 
 # define UF_START_MAGIC	    "Vim\237UnDo\345"  /* magic at start of undofile */
 # define UF_START_MAGIC_LEN	9
@@ -801,6 +803,62 @@ u_free_uhp(uhp)
     vim_free(uhp);
 }
 
+/*
+ * Like fwrite() but crypt the bytes when 'key' is set.
+ * Returns 1 if successful.
+ */
+    static size_t
+fwrite_crypt(buf, ptr, len, fp)
+    buf_T	*buf UNUSED;
+    char_u	*ptr;
+    size_t	len;
+    FILE	*fp;
+{
+#ifdef FEAT_CRYPT
+    char_u  *copy;
+    char_u  small_buf[100];
+    size_t  i;
+
+    if (*buf->b_p_key == NUL)
+	return fwrite(ptr, len, (size_t)1, fp);
+    if (len < 100)
+	copy = small_buf;  /* no malloc()/free() for short strings */
+    else
+    {
+	copy = lalloc(len, FALSE);
+	if (copy == NULL)
+	    return 0;
+    }
+    crypt_encode(ptr, len, copy);
+    i = fwrite(copy, len, (size_t)1, fp);
+    if (copy != small_buf)
+	vim_free(copy);
+    return i;
+#else
+    return fwrite(ptr, len, (size_t)1, fp);
+#endif
+}
+
+/*
+ * Read a string of length "len" from "fd".
+ * When 'key' is set decrypt the bytes.
+ */
+    static char_u *
+read_string_decrypt(buf, fd, len)
+    buf_T   *buf UNUSED;
+    FILE    *fd;
+    int	    len;
+{
+    char_u  *ptr;
+
+    ptr = read_string(fd, len);
+#ifdef FEAT_CRYPT
+    if (ptr != NULL || *buf->b_p_key != NUL)
+	crypt_decode(ptr, len);
+#endif
+    return ptr;
+}
+
     static int
 serialize_header(fp, buf, hash)
     FILE	*fp;