Substitution Cipher Decryption Tool

This tool analyses the letter count and letter 'pair' count in cipher text, creating an ordered list with recommendations for possible matches (guesses only based on the frequency of letter/pair occurances in the English language.


Paste your cipher text in to the text box and click Analyze. You will be provided with two tabs showing the frequencies of your cipher text, use the dropdowns or the buttons to quickly 'try out' different substitutions. Putting your mouse over a button in the 'Likely' column will give you more information about it.


If your cipher text has spaces, look for single letters first as these are generally one of two letter: A or I

Next move on to letter pairs as there are less and the more common ones are much more common

Move on to single letters and use the buttons to try and make common words such as and, the and so on - for convenience the most common two, three and four letter words are on a tab above.

Further Reading

For anyone wanting to learn more about the basics and history of cryptography, I highly recommend "The Code Book" by Simon Singh.

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Online Substitution Cipher Decoding Utility

Common Words

Most Frequent Two-Letter Words

of, to, in, it, is, be, as, at, so, we, he, by, or, on, do, if, me, my, up, an, go, no, us, am

Most Frequent Three-Letter Words

the, and, for, are, but, not, you, all, any, can, had, her, was, one, our, out, day, get, has, him, his, how, man, new, now, old, see, two, way, who, boy, did, its, let, put, say, she, too, use

Most Frequent Four-Letter Words

that, with, have, this, will, your, from, they, know, want, been, good, much, some, time