Breaking Letter Substitution codes

An example of a letter substitution cipher can be seen below, and we are going to solve it!

The encrypted message in white capitals on black: MKF WMKFY CZG E UZP PZM UEMK LG UERF ZVC PKF ZPTFC LF MW NZPP KFY MKF SENPMEAT ZVC E ZAAECFVMZSSG NZPPFC KFY Z XSHF PMEAT

Now there are 2 facts about the English language which will help considerably when solving a Cipher.

1 – The most common letter in the English language is E

2 – The most common 3 letter words are AND & THE

Using the first fact have a look at the cipher above and see what the most common letter here is used?

The same cipher text with every instance of the three-letter word MKF underlined

We can see that F is the most common Cipher letter, let us initially presume that F equals e (This isn’t always the case, so you may need to try other common letters if you find it doesn’t work)…

The cipher with M, K and F replaced by T, H and E, revealing the word THE in red

Then let’s use this information on the second fact (The most common 3 letter words in the English language are both AND & THE)

I’ve highlighted below where 3 letters are repeated (MKe is repeated twice and so is ZVC)

The cipher text with the repeated word MKF and the letter pattern ZVC marked up

We can make a guess that the MKe one is ‘the‘ and we could also try replacing ZVC with the word ‘and‘.

More letters solved — the words 'The', 'and' and 'her' now readable in red among the remaining cipher letters

and we can now fill in the newly discovered letters and replace all the Z V and C‘s with a, n & d‘s (As per our guess that ZVC is actually the word ‘and‘), and replace all the M & K‘s with t‘s and h‘s (As per our guess that MKe is actually the word ‘the’)

Your Cipher should now look like the below image.

Further letters filled in, with 'The other day' and 'she' emerging from the cipher

Now I would say that the assumptions we have made earlier, have come to be true, as this is starting to look like English.

We still have a while to go but we are on the right track, so lets continue!

Now here comes the fun part. We have to study the Cipher and make assumptions.

The easiest one to guess is the Cipher letter that is on its own ‘E‘

The part-solved cipher with an E circled in yellow at the end of two words

Letters that are usually words on their own are A‘s and I‘s as in ‘I am going to the shops for A drink…’

We can rule out A on the basis that it has been used within the word AND. So we can guess E is actually I…

Also we can see there are two 3 letter words ending in ‘he‘ – these are most likely either ‘the‘ or ‘she‘ and seen as we have used ‘the‘ earlier, we can assume it’s ‘she‘

also we have a 2 letter word beginning with ‘t‘ which is definitely going to be ‘to‘

The part-solved message with the repeated letter pattern circled in green and blue

So, lets start replacing those highlighted letters…

The message with 'she' and 'and i' now solved, leaving fewer cipher letters

And then we can go through the Cipher replacing the letters with the newly found ones, as below…

The message reading 'The other day I was sat with LG UIRe and she asTed Le to Nass heY the SINstiAT'

Now we can study and make assumptions, focusing on certain letters which look easy to solve…

The remaining unsolved words circled in green, blue and orange for guessing

A list of the remaining letter clues: a five-letter word starting 'sti', a four-letter word ending 'ith', a five-letter word starting 'othe', a three-letter word starting 'da', and a twelve-letter word

The message with more guesses filled in: 'The other day I was sat with Ly wiRe and she asTed Le to Nass her the liNsticT'

Focus and assume

The last few cipher letters circled as the sentence becomes readable

The message almost complete: 'The other day I was sat with my wiRe and she asked me to pass her the lipstick'

Now we can guess the sentence based on the context…

The fully solved message: 'The other day I was sat with my wife and she asked me to pass her the lipstick and I accidentally passed her a glue stick. She still isnt talking to me'

Congratulations, you have solved your first Cipher! It wasn’t that difficult was it?

The finished decoding key — the plain alphabet above and the cipher letters that replaced it below

Della

Breaking Letter Substitution codes

More from the casebook.

Knock Code

Column Ciphers

There's a cipher like this
in a box with your name on it.

Knock Code

Column Ciphers

There's a cipher like this in a box with your name on it.

There's a cipher like this
in a box with your name on it.