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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 http://mi.yodeya.com/questions/6509/cryptography-in-judaism/6519#6519]

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See Yabia Omer (Y"D:8:28) for a discussion at length regarding Sheimot written on the outside of the mezuzah. – Barry Apr 29 '11 at 16:35

There are many explanations of this in the Kabbalah writings. Here is a link to a fascinating explanation of this for those who understand these type of things: Sefer Mezuzat Melachim

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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.

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Interesting, but why write Hashem's name out there at all? – WAF Mar 24 '11 at 19:16
As a extra Shemira – Gershon Gold Aug 16 '11 at 17:19
@GershonGold What does that mean? Writing God's name in random places affords protection to what and by what mechanism? – Double AA Oct 7 '13 at 23:18

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.]

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