Question 1

what is a simple substitution cipher

Accepted Answer

A cipher where every letter of the alphabet is always replaced by the same other letter. The key is a shuffled alphabet: under “a” stands, say, “q”, under “b” — “w”, and so on for all 26 letters. Spaces, punctuation and digits stay where they are. There are about 4 × 10²⁶ possible keys, but nobody tries them all: letter frequencies give the cipher away.

Question 2

how do i crack a substitution cipher with frequency analysis

Accepted Answer

Count which ciphertext letters occur most often: they most likely stand for e, t, a, o, i and n. Put those in and look for short words — a one-letter word is usually “a” or “I”, a three-letter one is often “the” or “and”. From there the text helps you: a half-revealed word is easy to finish. Here the chart is built at once, letters are set with a click, and the solver needs a second or two.

Question 3

what are the letter frequencies in english

Accepted Answer

In Robert Lewand’s table the most common letter is “e” at 12.7 % of all letters, then “t” at 9.1 %, “a” at 8.2 %, “o” at 7.5 %, “i” at 7.0 % and “n” at 6.7 % — the familiar “etaoin”. The rarest are “j”, “x”, “q” and “z”, each well under 0.2 %. In a short text the order can differ noticeably from this table.

Question 4

how much text do i need to crack a substitution cipher

Accepted Answer

The solver needs at least 150 letters: on a passage that long it usually restores almost the whole text, and from 300 letters it does so reliably. The shorter the ciphertext, the further its frequencies drift from the language average and the more wrong keys produce text that looks plausible. Crack a short one by hand: start with the most frequent letters and short words such as “the” and “and”.

Question 5

how do i make a substitution key from a keyword

Accepted Answer

Type a keyword: its letters without repeats go to the front of the key alphabet, and the remaining letters follow in order. The word “key” gives the key “keyabcdfghijlmnopqrstuvwxz”: “a” becomes “k”, “b” becomes “e”, and the word “cipher” is enciphered as “ygofbq”. A key like this is easy to remember and pass on by voice, but it is also easier to guess than a random one.

Question 6

how are atbash and caesar related to substitution

Accepted Answer

Atbash and the Caesar cipher are special cases of simple substitution with a particular key. For Caesar the key is the alphabet shifted by a few letters: with shift 3, “a” becomes “d”. For Atbash it is the alphabet reversed: “a” becomes “z”. That is why both fall to the same frequency analysis — and they do not even need it: Caesar has 25 possible keys and Atbash only one.

Ciphers · substitutionSubstitution cipher a→q

Cipher

Frequently asked questions

what is a simple substitution cipher

how do i crack a substitution cipher with frequency analysis

what are the letter frequencies in english

how much text do i need to crack a substitution cipher

how do i make a substitution key from a keyword

how are atbash and caesar related to substitution

Ciphers · substitutionSubstitution cipher a→q

Cipher

Frequently asked questions

what is a simple substitution cipher

how do i crack a substitution cipher with frequency analysis

what are the letter frequencies in english

how much text do i need to crack a substitution cipher

how do i make a substitution key from a keyword

how are atbash and caesar related to substitution

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