Three unintelligible “words,” comprised of fourteen Hebrew letters, can be seen written upside-down on the back of a mezuzah scroll: כוזו במוכסז כוזו. These letters are actually an encrypted form of the Divine names written in the third, fourth, and fifth words of the Shema Yisrael. They are written upside-down so that they will correspond to their matching letter on the front of the mezuzah.

To “hack” this code you must decipher the letters using a method of substitution known as a one-shift “Caesar cipher,” meaning that these letters are shifted one letter up from the letter they are meant to encrypt. (Reportedly, Julius Caesar used similar codes to encrypt his documents.)

This custom is not universal. Sephardic authorities do not allow any other writing anywhere on the mezuzah aside from the customary Divine Name Sha-dai (see Mezuzah Maven “What’s in a Name” in Ohrnet Vayeitzei). These authorities base their hesitation on Rambam’s sharp rejection of similar practices:

It is a common custom to write Sha-dai, on the outside of a mezuzah opposite the empty space left between the two passages. There is no difficulty in this, since [the addition is made] outside.

Those, however, who write the names of angels, other sacred names, verses, or forms, on the inside, are among those who do not have a portion in the World-to-Come. Not only do these fools nullify the mitzvah, but furthermore, they make from a great mitzvah — reflecting G-d’s unity, and the love of Him and the service of Him — a talisman for their own benefit. They, in their foolish conception, think that this will help them regarding the vanities of the world.

In a similar vein, Rambam writes elsewhere:

A person who whispers an incantation over a wound and then recites a verse from the Torah…., or who places a Torah scroll or tefillin over a baby so that it will sleep, is considered to be a soothsayer or one who casts spells. Furthermore, such people are included among those who deny the Torah, because they relate to the words of the Torah as if they are cures for the body, when, in fact, they are cures for the soul, as Proverbs 3:22 states: “And they shall be life for your soul.”

Although Rambam would acknowledge the protective power that comes in the merit of the mitzvah itself, he scorns those who portray the mezuzah as nothing more than a tool to achieve material aims.

Curiously, however, the insertion of these mysterious letters has been accepted in Ashkenazic practice from at least the eleventh century. The authorities justify this custom by pointing out that Rambam referred to those who made additions to the inside of the parchment, whereas these letters, like the Name Sha-dai, are specifically written on the backside to accentuate that they are not meant to be considered part of the mitzvah itself.

Even so, Ashkenazic custom disallows writing any of the other angelic names, codes, and runes mentioned by Rambam on either side of the parchment. Yet this was not always the case. Before Rambam’s view was accepted as normative halacha, symbolic runes and Biblical passages were often inserted in the margins of the parchment at strategic places in an effort to “supercharge” the protective power of the mezuzah. Many of these symbols are actually representations of letters from the ancient Paleo-Hebrew alphabet found in archaeological inscriptions and coins in Israel.

• Sources: Rambam, Hilchot Sefer Torah U’Mezuzah 5:4 and Hilchot Avodat Kochavim 11:12; Shulchan Aruch Yoreh De’ah 288:15

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