A cipher with a key immediately makes me think of a Vigenere cipher, which is probably the most common cipher employing a key.
The way a Vigenere cipher works is much like a typical Caesar cipher (which just shifts all letters a certain number of places in the alphabet), but it also employs a key, which allows each letter to be shifted a different number of places in the alphabet.
For example, if your key is
SECRET and you want to encrypt the message
THE MISSION STARTS AT NOON, you would assign a number to each letter of the key (
B=1, etc.). Then you would shift each letter of your message that number of characters.
THE MISSION STARTS AT NOON
SEC RETSECR ETSECR ET SECR
T is 19 and
S is 18, so we add $19 + 18 = 37$.
37 is more than 25 (=
Z), so we need to "wrap around", which is the same as subtracting 26 from our result. $37-26 = 11$ =
L, so the first letter of our encryption is
By following the same method, the second letter would be encrypted as $7 + 4 = 11$ =
L, and the third as $4 + 2 = 6$ =
You can see that even though the first three letters of the plaintext are different (
THE), two of them encrypt to the same letter (
L). This makes a Vigenere cipher much harder to crack than a Caesar cipher, because a given character in the ciphertext doesn't represent the same character in the plaintext each time it appears. This also means that frequency analysis is useless, since the most frequent letter in the ciphertext might represent six different letters in the plaintext, none of which are particularly frequent.
There are still weaknesses, though. As the text gets longer, it's more likely that common words (
a, etc.) will end up being encrypted by the same section of the key, and will thus end up encrypted as the same sequence of letters. By analyzing the ciphertext, we can pick out certain letter sequences that repeat multiple times. By looking at the spacing between them, we can guess at how long the key is. For example, if a certain sequence appears at positions 65, 89, 105, and 121, we might guess that the key is of length 8, because all of the distances between those numbers are divisible by 8 (24, 16, 16).
Knowing the length of the key can be very helpful, because it lets us know which parts of the message were encrypted using the same letter of the key. In a long-enough message, we can look at those letters and actually do a frequency analysis of just the letters encrypted by a single character of the key. Since they were all encrypted the same way, the more-frequent letters will probably map to the more-frequent English letters, and from that point we can make some intelligent guesses about what some words might be.
There are many online tools that can help with this kind of thing. Some try to offer a one-stop solution, which determines key length, finds the key, and decrypts the ciphertext without any input from you. These tend to be mediochre at best. Other tools will help you with one aspect of the problem. I used this tool (the
FIND KEY LENGTH button) to help me determine the key length. It gave me a very high likelihood that the key was 6 characters long.
I then used this tool to do a frequency analysis based on the key length. Basically, it took the characters at position 1, 7, 13, 19, etc. (6 apart), and looked at which letters were most common. It then took the most common English letter, and determined the difference between them. It assumed that was the first letter of the key. Then it did the same with characters at positions 2, 8, 14, 20, 26, etc., and so on.
I anticipated that it would not find the exact solution, but hoped it would be able to fill in enough that I could guess what was missing.
Looking at the second suggestion it spit out, I got a key of
IOXNAR and a plaintext that included the phrase
NOXHONG CEN NARM QE. I thought this looked a lot like English, and guessed that it might be
NOTHING CAN HARM ME. Looking at the incorrect letters, they were all at position 1 or 3 of the key, so it looked like my key was correct except for the first and third characters: 'xOxNAR`.
I did the math to figure out what letter would be required to get a plaintext of
NOTHING CAN HARM ME, and determined that the first character of the key was
M and the third was
D, making the key
MODNAR. Looks nonsensical at first, until you realize that it is simply the word RANDOM backwards!
Using that key to decipher the whole message, I got
I MUST ADMIT THAT ALTHOUGH NOTHING CAN HARM ME, I'VE HAD MOMENTS OF PARANOIA AND EVEN YOU CAN NOW SENSE THAT.
Afterward, I looked a little further through the tool's results, and realized that it had actually found the solution, at attempt #42.