What is a substitution cipher?

A substitution cipher replaces each letter of plaintext with another letter. A 'simple' substitution uses a fixed permutation of the alphabet.

Can this tool break substitution ciphers automatically?

This tool uses frequency analysis plus a heuristic hill-climbing routine to propose likely keys; success depends on ciphertext length and language content.

Substitution Cipher Solver — Simple Substitution Decoder

Substitution Cipher Solver

Paste your ciphertext below and either edit the letter mapping manually or use Auto-suggest to try a best-fit solution. The solver provides frequency analysis, decode preview, and a basic hill-climbing optimizer that seeks a key maximizing common English word matches.

Ciphertext

Letter mapping (cipher → plaintext)

Click a target letter to change mapping; use Reset to restore identity mapping or Frequency to seed mapping.

Decoded preview

How it works

The substitution solver supports manual mapping and two automated helpers:

Frequency seed: maps the most frequent ciphertext letters to the most frequent English letters (ETAOIN SHRDLU...), a good starting point.
Hill-climb refine: performs many small random swaps in the candidate key and keeps swaps that increase a simple score (number of recognized common words) — this heuristic often converges to readable plaintext for medium/long ciphertexts.

Limitations

Short ciphertext or non-standard language reduces success rate. This tool is intended for learning and quick solving — for hardened cryptanalysis you may want statistical models and larger corpora.