# Cracking a Caesar Cipher with CrypTool 2

## What you need

• A Windows machine, real or virtual.

## Purpose

To crack a Caesar cipher with frequency analysis, which is a weakness of all simple substitution ciphers.

## Getting CrypTool 2

On your Windows machine, in a Web browser, go to

On the right side, in the "FREE DOWNLOADS" section, click "CrypTool 2", as shown below.

Download and install the latest stable version. When I did it, that version was Stable Build 6222.1.

## Using the Wizard to Crack a Classical Caesar Cipher

Open CrypTool 2, as shown below.

On the left side, in the "Main Functions" section, click the Wand icon labelled "Use the wizard...".

In the "TASK SELECTION" screen, on the left side, click Cryptanalysis, as shown below.

Then click Next.

In the "AGE SELECTION" screen, accept the default selection of "Classic Encryption" and click Next.

In the "ALGORITHM SELECTION" screen, accept the default selection of Caesar and click Next.

In the "CAESAR ANALYSIS" screen, some example Ciphertext appears, as shown below.

Leave the "Language" set to German and click Next.

The next screen shows the "Deciphered Ciphertext" in the lower left pane, and the Key of 5 in the lower right pane, as shown below.

To understand how this was done, scroll down to see the "Character (unigram) frequency", as shown below.

Frequency analysis works because some letters are far more common than others. As you can see, J is the most common letter in the ciphertext, followed by S.

In German, the most common letter is E, followed by N.

E shifted forward by 5 letters is J
N shifted forward by 5 letters is S

The Caesar shift is easily found by adjusting the key until the observed frequency of letters most closely matches the average usage of letters in the target language.

## Challenge 1: A Simple English Paragraph

Decrypt this:
``` "ITQZ YQJUOA EQZPE UFE BQABXQ, FTQK'DQ ZAF EQZPUZS FTQ NQEF. FTQK'DQ ZAF EQZPUZS KAG, FTQK'DQ EQZPUZS BQABXQ FTMF TMHQ XAFE AR BDANXQYE MZP FTQK'DQ NDUZSUZS FTAEQ BDANXQYE IUFT GE. FTQK'DQ NDUZSUZS PDGSE. FTQK'DQ NDUZS ODUYQ. FTQK'DQ DMBUEFE... MZP EAYQ, U MEEGYQ, MDQ SAAP BQABXQ.” -- PAZMXP FDGYB ```
Use the form below to put your name on the WINNERS PAGE.
 Your Name (without spaces): The author of that quote, like this: MICKEY MOUSE

## Challenge 2: A Simple English Sentence

Decrypt this:
``` ESP BFTNV MCZHY QZI UFXAD ZGPC ESP WLKJ OZR. ```

## Hint

The text is so short that automated frequency analysis fails. Examine the "Character (unigram) frequency" chart to choose likely shift values, and then test them one-by-one using the "Encryption" task instead of the "Cryptanalysis" section.
Use the form below to put your name on the WINNERS PAGE.
 Your Name (without spaces): The complete decrypted sentence:

## Challenge 3: An English Song Lyric

Decrypt this:
``` AXLT VN KJKH, AXLT VN JUU WRPQC UXWP ```

## Hint

Frequency analysis is essentially worthless, because the text is short and uses an unusual pattern of words.

You can just try keys one by one in CrypTool till you find the key: there are only 25 possible keys. This problem is easier to solve with a custom Python program than with CrypTool.

Use the form below to put your name on the WINNERS PAGE.
 Your Name (without spaces): The complete decrypted text:

## Source

Frequency of Occurrence of Letters in German

Posted 5-13-16 by Sam Bowne
Modified 8-1-16
Quotes fixed and Winners forms added 8-7-16
http links changed to https 9-1-17