Substitution cipher encode
Substitution cipher encode, classical cipher, a puzzle not a protection, computed on your device.
Runs on your device. The file is never uploaded.
Substitution cipher encode maps a to the first letter of your key, b to the second, and so on through all 26. The key defaults to zyxwvutsrqponmlkjihgfedcba, the reversed alphabet, which reproduces the Atbash tool exactly. Word lengths, case and punctuation are left standing, so counting letter frequencies reads the message without the key.
Questions
How do I give the alphabet?
As a 26-letter mapping, where the first letter is what A becomes, the second is what B becomes, and so on. Repeat a letter and the cipher stops being reversible, so keep all 26 distinct.
Is a substitution cipher secure?
No. It has about 4 times 10 to the 26 keys, which sounds large, but letter frequency analysis reads English through it without trying any of them. Any published solver breaks a paragraph in seconds.
What happens to case and punctuation?
Only letters are substituted. Spaces and punctuation pass through in place, which is exactly what makes the cipher easy to break: word lengths and patterns stay visible.
How do I reverse it?
Use Substitution cipher decode with the same 26-letter alphabet. It builds the inverse mapping for you.