Starting as governor of a newly formed province in the Roman Empire your goal is simple: advance up the empire's political ladder to become the next Caesar. As governor, you will have total control over your province: including city planning, finances, and defenses.

EncryptionEncryption using the Shift Cipher is very easy. First we must create the ciphertext alphabet, which as discussed above is simply found by 'shifting' the alphabet to the left by the number of places given by the key. Thus a shift of 1 moves 'A' to the end of the ciphertext alphabet, and 'B' to the left one place into the first position. As the key gets bigger, the letters shift further along, until we get to a shift of 26, when 'A' has found it's way back to the front. We have already seen a shift of 3 in the table above, and below we have a shift of 15. Once we have created the table, the encryption process is easy, as we just replace each occurence within the plaintext of a letter with the corresponding ciphertext letter as given by the ciphertext alphabet. Hence, if we wanted to encrypt the plaintext 'julius caesar' with the key he himself used, namely 3, we look along the plaintext alphabet row in the first table to find 'j', and note that this encrypts to 'M'.

We then look for 'u', and take the ciphertext letter 'X'. Continuing in this way, we finally encrypt to 'MXOLXV FDHVDU'.

If it is known that a Shift Cipher has been used, but the key is unknown, then it is fairly simple to break the code by a simple brute force attack. This simply means using a trial and error approach to attack the cipher. The main weakness of the Shift cipher is the fact that there are only 26 keys, and hence ciphertext alphabets, one of which is the identity mapping that leaves the plaintext unaltered. For this reason, the Brute Force method of attack is very effective on the Shift cipher. In it's most bare form, this entails going through each key, and working out what the plaintext would be if that key had been used. That is, given the intercepted ciphertext 'RFWHZX HWFXXZX', where we do not know what key has been used, but we do know that a Shift Cipher has been implemented, we must first try a key of 1, then a key of 2, then a key of 3 and so on, until a plaintext that makes sense is returned. For this ciphertext we would get:.

a key of 1 gives the plaintext 'qevgyw gvewwyw'. a key of 2 gives the plaintext 'pdufxv fudvvxv'. a key of 3 gives the plaintext 'octewu etcuuwu'. a key of 4 gives the plaintext 'nbsdvt dsbttvt'. a key of 5 gives the plaintext 'marcus crassus'.

The second problem with the above method is that the message intercepted could be longer than two words, and hence performing each decryption could take a significant amount of time. The way around this is fairly simple, and that is to just look at the first two or three words of the intercept, and perform the calculations on these. You will still find the key, by finding the key that deciphers those words into a meaningful phrase, and can then use the key to decrypt the rest of the message as you would if you had known the key to start with.