Frequency Analysis Revisited
Although random substitution cipher is invulnerable to brute force attack, it can be attacked by frequency analysis. Given a long enough piece of ciphertext, we can count the frequency of each letter and map it to the corresponding letter in the usual frequency of letters in English texts. You can copy the ciphertext at the bottom of this page to the CrypTool Frequency Analysis page. You should find that the most frequent letter in the ciphertext is T, which is likely E in plain English text. The second most frequent letter in the ciphertext is D, which is likely T in plain English. The third most frequent letter in the ciphertext is H, which is likely A in plain English. Now, we already have parts of the key as below.
You can use the CrypTool Substitution Cipher page to quickly decipher a text using a given key. On this page, you need to paste the ciphertext into the Input box, select "Decipher", click "Alphabet", and click "Define own alphabet". On the line below Plaintext alphabet, only keep the capital letters. On the line above Ciphertext alphabet, write the key. Below is an example.
If you complete the above key and use it to decrypt the below ciphertext, you will find that some letters are mistranslated. In particular, the H in ciphertext should not be mapped to A. The next most frequent letter in the ciphertext, P, should be mapped to A. This error is caused by the length of the text being too short, so our guesses are sometimes imprecise. You will have to do some trials and errors to figure out the correct key.
