Four-square cipher encoder and decoder

Use it free in the browser for history-of-crypto lessons, digraph puzzle design, and reversible worksheet keys, or call the API at $0.002 per successful request when pipelines need reproducible four-square transforms without re-implementing the square layout.

What the four-square cipher is and what this tool returns

The JSON response includes result for the transformed uppercase string, encoding fixed to four_square, mode set to encrypt or decrypt, key1_letters and key2_letters showing the normalized keyword streams, unique-letter views of each key, normalized_input after extraction and padding, a padded flag, digraphs count, length fields, letters_processed, the alphabet string, the four flattened squares, and a formula string that restates the digraph mapping for documentation and tests. There is no streaming path and no silent fallback to Playfair or Vigenère: either you supply non-empty text with at least one letter and two keywords that each contain at least one letter, or the request fails with invalid_input. If you need AES, Vigenère, columnar transposition, or Morse, use those dedicated endpoints; this landing answers only the classical four-square digraph contract that crypto history chapters and CTF warm-ups already describe.

How text, keys, and mode are read and how results stay reproducible

Only alphabetic characters inside each key are kept, uppercased, and mapped with J to I; digits and symbols in a key are ignored. Every digraph is processed with the fixed square layout, so encrypting then decrypting with the same pair of keywords always restores the normalized padded letter stream. The preferred payload fields are text for the message, key1 and key2 for the two keywords, and optional mode for encrypt or decrypt. Text accepts aliases such as input, value, s, message, plaintext, and ciphertext so forms and SDK clients can reuse existing column names. Key1 accepts keyword1, key_1, k1, upper_key, and right_key; key2 accepts keyword2, key_2, k2, lower_key, and left_key. If after that filter no letter remains for either keyword, or if a keyword string is empty, the engine rejects the request with invalid_input and a clear message that the keyword must contain at least one A–Z letter. Mode defaults to encrypt when omitted and also accepts short aliases such as enc, encode, e for encrypt and dec, decode, d for decrypt; any other string fails validation. Missing text, empty text, text with no letters, non-string types, and unknown modes all raise invalid_input so a blank form does not look like a successful no-op. Pair the free browser run on this page with the priced API when you need the identical mathematical result inside an automated pipeline without re-implementing the four squares yourself. Invalid inputs return invalid_input and are not charged, which keeps validation cheap while successful transforms remain a fixed item price of $0.002.

Where four-square fits in lessons, puzzles, and test fixtures

On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split — free interactive exploration and a priced, identical API — matches the rest of the KIT tier-A catalog and lets you move from a classroom demo to an automated pipeline without changing the transformed result.