Breaking a cipher, also known as CRYPTANALYSIS, without having the key is quite difficult and very time consuming. And, depending on the type of cipher system used to create the secret message, there may be more than one key that cracks the code!
To break a message, it helps to know something about the message.
>> Who sent the message? >> What language is the message written in?
>> Who was it sent to? >> Is there a key word you can look for?
After answering these questions, try counting the number of times each letter is used in the message - this is called a FREQUENCY COUNT. Knowing how often letters are used can help you guess which cipher letter equals which plaintext letter. From there you can make other guesses based on language rules.
Today, high level encryption is done with complex mathematics and computers, and the same is needed to break them.