Substitution Cipher Decoder
A substitution cipher decoder inverts the letter permutation. With the key, use Cipher Portal; without it, study patterns in the Frequency Lab and substitution guide.
Decode with a known key
Paste the 26-letter tableau, ciphertext, choose Decrypt, and run. One wrong symbol scrambles the mapping—verify the key string has no duplicates and includes every letter.
Decode without the key
Attackers guess high-frequency ciphertext letters map to E, T, A, then refine using digrams and word patterns. Cipher Portal’s frequency lab loads substitution samples for practice. Step-by-step: substitution cipher cracker and frequency analysis decoder.
Automated hill-climbers exist; this site focuses on education, not industrial breaking.
Special cases: Caesar and Atbash
Caesar decoders try 26 shifts (Caesar decoder). Atbash is the fixed reverse alphabet (Atbash decoder).
Check decoding quality
After mapping, scan for high-frequency English bigrams in the partially decoded text. If you see repeated TH and HE patterns, the mapping is likely on track; if not, revisit the tallest ciphertext bars.
Portal key recovery may propose a full 26-letter key—compare it to your manual map letter by letter to learn where statistics differ from human intuition on short quotes.
- Plaintext
- HELLO
- Key (A→Z map)
- QWERTYUIOPASDFGHJKLZXCVBNM
- Ciphertext
- ITSSG
Frequently asked questions
How do I decode substitution cipher online?
Cipher Portal substitution mode with the 26-letter key and decrypt.
What is a monoalphabetic decoder?
A tool that reverses a fixed letter-for-letter substitution.
Can I solve cryptograms automatically?
Use frequency tools here; external solvers handle contest-grade puzzles.
Where are substitution examples?
/substitution-examples.html and /examples.html.