We had a quiz in class today where we had to break the ciphertext with the key given, but not the algorithm. Suffice to say that I wasn't able to decrypt it within the alloted time of 12 mins and will probably get a 0 on the quiz. So, I was just wondering if there are some kind of standard techniques that are followed when decrypting a simple substitution ciphertext.
When trying to break an unknown cipher, one first needs to figure out what kind of cipher one it is. Generally, a good starting point would be to start with the most common and well known classical ciphers, eliminate those that obviously don't fit, and try the remaining ones to see if any of them might work.
An obvious first step is to look at the ciphertext alphabet: does the ciphertext consist of letters (and if so, in what alphabet), numbers, abstract symbols or some combination of those? If it's letters, does it include spaces, punctuation or case distinctions — and, if it does, do they look like they're also scrambled somehow, or are they perhaps just left as they are in the plaintext?
Compiling a letter (or symbol) frequency table of the ciphertext, and comparing it to the corresponding table of plain English text, can often yield information about the general type of cipher one is dealing with:
Knowing whether the cipher has a key or not, and what form the key takes (word, number, sequence of numbers, etc.) can also help reduce the range of possibilities. For example, let's say that the ciphertext is uppercase letters with no spaces or punctuation, and that we know it has a key which is a word or a short phrase. That narrows down the likely choices quite a bit:
Since you already know what the key is supposed to be, testing each cipher should be pretty straightforward: just try to decrypt the message with the key and see if the output makes sense.
Note that, in some cases, effort can be shared between ciphers. For example, the Vigenère and autokey ciphers are identical for the beginning of the message; they only start to behave differently when the end of the keyword is reached. It may also be a good idea to try simple variants of these ciphers, such as switching the encryption and decryption rules around; some of them work equally well in both directions, and may have been used so.
If it is a simple substitution cipher, there are a few standard techniques:
A substitution cipher consist of a mapping from letters in the alphabet to letters in the alphabet (not necessarily the same alphabet, but probably is in this case). There are many forms that a key can take on. Ones I've seen in practice are:
You then substitute by finding the letter in the first row and going down the the second row.
Since you were given the key, chances are you had to use one of these methods (or possibly another) to come up with the mapping.