Mirror Digits

Mirror digits are a graphic substitution cipher where each character is replaced by its image in a vertical mirror. It’s the classic childhood visual trick (writing a message backwards, reading it in a mirror), formalised in a table published on the dCode platform.

The dCode table covers 10 digits (0-9) and 10 Latin letters (A-J), 20 characters in total. Letters K-Z are not in the table and are marked as missing on CipherChronicle (rendered with a dash —). It’s therefore a limited cipher but visually immediate: the eye recognises most inverted characters (an inverted 3 remains recognisable, like a reversed E).

How does the cipher work?

The cipher relies on a monoalphabetic substitution: each cleartext character (digit 0-9 or letter A-J) is replaced by its mirror image. Same mechanic as the Caesar cipher (~50 BC), except the transformation is geometrically determined (vertical reflection) rather than chosen arbitrarily.

The table holds 20 glyphs (10 digits + 10 letters A-J). Letters K-Z are missing: the rendering uses a dash (—) to replace them, as for Bibi-binary, Friderici or Theraprism (see our entries).

Cryptographic strength: low. Monoalphabetic substitution → trivial frequency analysis. But reading is immediate once you look at the message in a real mirror — it’s a recreational rather than confidential cipher.

Historical and modern usage

• Children’s games — secret school messages.
• Escape rooms — using a physical mirror as a tool.
• Visual riddles — symmetry detection in cryptopuzzles.
• Geometric pedagogy — example of reflection transformation.

Related variants

• Caesar cipher — see our entry, alphabetic substitution.
• Atbash — see our entry, alphabetic mirror.
• Bibi-binary — see our entry, another cipher with missing letters.

What are the weaknesses?

• Frequency analysis — breaks it in a few dozen words.
• Mirror reading — instant decryption with a mirror.
• Missing K-Z — very partial alphabetic coverage.

The 20 glyphs

0

1

2

3

4

5

6

7

8

9

A

B

C

D

E

F

G

H

I

J