Substitution Cipher: Weakness

To attack a substitution cipher, you would note that the letter E is the most common written English letter. You will look into the cipher text for the most common letter, which you will undoubtedly know should be replaced by E.

Next, you'll notice that T is the next most common written English letter, so you'll look into the 2nd most common letter in the cipher text, and replace it with T.

This procedure of gradually replacing cipher text with plain text letters is called frequency analysis, which renders the substitution cipher weak and, therefore, insecure.

Continuing this way, you will replace many cipher text letters with plain text and will start revealing the plain text!

To make this word even easier, you may also want to keep in mind the frequency of English words, such as "I", "the", "a", "an", etc., common letter pairs, such as "th", "an", "in", "qu", and common patterns like "tion" (as in "nation") and "ing" (as in "doing").