What is the meaning of the inscription on the back of a m'zuza?

Question

The string "כוזו במוכסז כוזו" is written on the back side of the parchment in many a doorpost scripture.

According to the Ra"sh (hilchos m'zuza 18)

וכן נהגו לכתוב באשכנז ובצרפת כנגד ה' אלקינו ה' מבחוץ שם של י"ד והם סמוכות לאותיות של ה' אלקינו ה

While the exact text does not appear here, by replacing each letter in God's names as they appear in the first pasuk of "Sh'ma'" with the "corresponding" letter (in this case the next one alphabetically), it is understandable that the Ra"sh was referring to the string above - "כוזו במוכסז כוזו". What does it mean and what is it doing there?

[Inspired by Cryptography in Judaism]

Answer 1

An historical answer, copied from the book 'Foundation and Origins of the Mishnah', by R. Reuven Margoliot (translation is my own):

We know of many instances when repressed mitzvot (by Roman or Greek persecution) where rechanneled in order not to forget that they existed, e.g. reading from the Prophets when reading from the Torah was prohibited, saying Shema publicly during kedusha of musaf instead of the being of shacharit, etc., the list goes on. We also know that putting up mezuzas was prohibited under Roman rule - as indicated by passages in the Mishna describing hiding mezuzas under the threshold, or in a cane that was then leaned against the doorway.

Therefore, a simple conclusion can be drawn: during the time that Jews were unable to keep the mitzva, they inscribed on there doorposts the short code כוזו במוכסז כוזו to remind themselves of the mitzva that was absent. Once the decree prohibiting the mitzva was repealed, that reminder was incorporated into the mitzva itself, just like we kept reading haftara and saying Shema in musaf.

I would add that the choice of those particular words is probably just because of the unusual and powerful combination of three ineffable names of God in a row, which are the centerpiece of the most central verse in all of Judaism.

Answer 2

This is totally my own thoughts. However I think there is truth to them.

Since the words are written on the outside, they are more prone to erasure from the elements. We do not want to directly cause the names of Hashem to be erased. Therefore we write כוזו במוכסז כוזו instead.

Answer 3

The letters are written there in order to provide protection for the residents of the house.

These letters share the g'matria (numerical value) of the words סיחון עוג - Sichon Og - two enemy kings in the time and place of the Torah. It was only after conquering these two easterly bordering kingdoms that B'nei Yisra'el were able to enter the land and the kings therefore were "blocking the doorstep" of the land. So we invoke protection of our real estate in that land by invoking their names, in the hope that we will be just as successful at entering, living on, and maintaining our (physical and spiritual) holdings as our ancestors were in vanquishing their enemies.

[This answer incorporates the root idea from Gershon Gold's comment as well as elements from the linked text in Aaron Shaffier's answer.]

Answer 4

This is one of the forms of tzeruf, letter exchange, that was taught to us by Moshe Rabbeinu like is explained in the opening words of Sefer HaMalchut Chochmat HaTzeruf by Rabbi David HaLevi and in Kuntress Sheirit Yosef on Sefer Ma'ayan HaChochmah by Rabbi Yosef Bar Shimshon Rafoel, Sha'ar 1:2.

Each form of tzeruf transformation indicates a different level of understanding and a step down from the source letters. The greater the transformation, the greater the descent. This is discussed in the opening chapters of the second section of the Tanya, Sha'ar HaYichud v'HaEmunah.

In this particular case, the type of tzeruf used is Semichah, which means in close proximity or touching. There is not much of a transformation. It contains the same number of letters and is in the same order.

The writing on the back of the klaf is the concept of going from front to back as it appears in the Torah, like the idea of how HaShem sustains the Sitra Achra begrudgingly, by tossing sustenance to it over His shoulder, to His back so to speak.

The letters for this type of transformation are the next letters in the order of the Alef-Beit from the source phrase. For example, the letter ב is somech to the letter א. The letter ל is somech to the letter כ.

So in this case, the unadulterated, source phrase is: יהוה אלהינו יהוה.

The Semicha transformation of that phrase is: כוזו במוכסז כוזו.