The secret that beats most ciphers
Imagine someone hands you a 500-letter ciphertext. You don’t know the key. Where do you even start?
You count the letters.
In English, letter frequencies are wildly uneven:
E is the most common letter — about 13% of all letters in English. Then T (9%), A (8%), O (7.5%), I (7%), N (6.7%), S (6.3%), R (6%). At the other end: J, Q, Z each show up less than 0.1% of the time.
Most simple ciphers don’t hide this. The cipher letter that took E’s place will still appear about 13% of the time.
So if you count letters in the ciphertext and find that H appears 13% of the time… it’s probably really an E underneath.
Try it
Paste any English text below. Watch the bar chart show the real letter frequencies — and compare to the typical English pattern (teal bars).
Now erase it all and paste the ciphertext below to see the same pattern shift:
WKH TXLFN EURZQ IRA MXPSV RYHU WKH ODCB GRJ. WKLV VHQWHQFH FRQWDLQV HYHUB OHWWHU RI WKH DOSKDEHW DW OHDVW RQFH.
(This is the same “quick brown fox” sentence, Caesar-shifted by 3.)
What to notice: The shape of the bar chart is identical — just shifted 3 bars to the right. The biggest bar is now at H, because E became H under the shift of 3.
The frequency fingerprint
Every language has a fingerprint. Here are the English top 8, roughly:
| Letter | E | T | A | O | I | N | S | R |
|---|---|---|---|---|---|---|---|---|
| % of text | 12.7 | 9.1 | 8.2 | 7.5 | 7.0 | 6.7 | 6.3 | 6.0 |
Memorize ETAOINSR — it’s the cheat code of cryptanalysis.
A few more tricks:
- Single letters as words in English are almost always
AorI. - Two-letter words:
OF,TO,IN,IS,IT,BE,AS,AT,SO,WE,HE,BY. - Three-letter words:
THEis by far the most common. If a 3-letter group appears over and over in ciphertext, guessTHE. - Double letters: most common doubles are
LL,EE,SS,OO,TT.
Why this beats Caesar instantly
For a Caesar cipher, you don’t even need to be clever. You just need to find the most common letter in the ciphertext and compute:
shift = position_of_cipher_E − position_of_E
If the most common letter is H (position 8), and E is position 5, then the shift is 3. Done. You just broke the cipher in one calculation.
Practice
Which letter is the most common in English?
E, at about 12.7%. Remember ETAOIN — the six most common letters in order.
In a long Caesar-ciphered message, the most common letter is R. What was the shift?
E is position 5, R is position 18. Shift = 18 − 5 = 13. The real letter E was pushed 13 forward to become R.
You see the 3-letter word GSI appear 14 times in a 600-word ciphertext. What is your best guess?
THE is the most common 3-letter word in English — about 5% of all written English is 'the'. Repetition = strong clue.
Which of these would make frequency analysis HARDER?
Frequency analysis needs lots of letters to 'average out'. In a single sentence, the most common letter could be anything by chance. You need at least a paragraph — ideally a page.