Two-square cipher

The two-square cipher is the simplest member of the Delastelle family: two 5×5 grids each with its own key, placed side by side (horizontal variant) or stacked (vertical).

Principle

Two 5×5 grids, each filled with a keyword then the remaining alphabet (I/J merged):

Grid 1 (KEYS)         Grid 2 (EXAMPLE)
K E Y S A             E X A M P
B C D F G             L B C D F
H I L M N             G H I K N
O P Q R T             O Q R S T
U V W X Z             U V W Y Z

Rules (horizontal variant)

For each plain digram (P₁, P₂):

• Find P₁ in grid 1, P₂ in grid 2.
• If P₁ and P₂ are in the same row: swap them (each takes the other’s column).
• Otherwise (different rows): form a rectangle. C₁ = grid 1, row of P₁, column of P₂. C₂ = grid 2, row of P₂, column of P₁.

Decryption

The method is symmetric: applying the same rules to the ciphertext reproduces the plaintext.

Variants

• Four-square cipher — the more robust 4-grid version (2 keyed, 2 standard).
• Three-square cipher — intermediate, three grids.
• Playfair — single-grid digram cipher.

Strengths and weaknesses

Strengths

• Double letters are no problem (no need to inject an X, unlike Playfair).
• Simpler to use than the four-square cipher, with comparable resistance on short messages.

Weaknesses

• Only two keys → keyspace bounded by what two words can carry.
• Digram analysis still effective.
• Horizontal variant: if P₁ and P₂ share a row, C₁ = P₂ and C₂ = P₁ — pure swap, easy to detect.

In CipherChronicle

Two-square is a gentle entry into the Delastelle family: the geometric mechanic is identical to four-square but with half as many grids to juggle. A good stepping stone to Playfair or Four Square.