ROT1, Caesar shift, and Morse code are all of the same type: mono alphabetic substitution, meaning that each letter of the alphabet is replaced according to the key with another letter or symbol. Without knowing the key, these are actually easy to decipher. The most common letter in English is well-known to be E. Therefore, in any mono alphabetic cipher, the most common letter or symbol will also be E. The second most common English letter is T, and the third most common is A, and so these two letters can also be determined. From this point, a person deciphering a message can continue using the frequencies of English letters or they can look for nearly-complete words, such as “T_E” which is most likely “THE.” Unfortunately, this only works for long messages, and not on ones with only a few words, since they do not have enough letters to show which are the most frequent. Mary Queen of Scots famously used a mono alphabetic cipher with several variations that was incredibly difficult, however when it was finally broken, the messages therein gave the evidence needed by her enemies to sentence her to death.

Ptbndcb ymdptmq bnw yew, bnwzw raw rkbcriie wrze bd owktxnwa.