Old-School Dictionary / Codebook

Oblivious Code

oblivious code /uh-BLIV-ee-us kohd/ noun A keyed Polybius cipher that turns plain letters into paired coordinates, readable only by someone holding the same eight-digit key.

Definition

Polybius Square

A Polybius square is a 5-by-5 alphabet grid. Each letter is encoded as a row and column pair: row 1, column 1 becomes 11, row 5, column 5 becomes 55.

Because the English alphabet has 26 letters and the square has 25 cells, this version combines I/J in the same cell. During encoding, J is treated as I.

The eight-digit key deterministically shuffles the 25-letter alphabet so the same key always creates the same codebook.

Video

One-Minute Cipher Lesson

Backup Short: Math Code | Polybius Square Cipher

Cipher Tool

Keyed Encoder / Decoder

8-digit key

Enter exactly eight digits to build the square.

Encode Plaintext

Punctuation is stripped during encoding.

Decode Numeric Code

Only digits 1-5 and / are used for decoding.