Other substitution ciphers improve on the Caesar cipher by not having all the letters in order, and some older written ciphers use different symbols for each symbol. If the same letter occurs more than once in the plaintext then it appears the same at each occurrence in the ciphertext.įor example the phrase "HELLO THERE" has multiple H's, E's, and L's.Īll the H's in the plaintext might change to "C" in the ciphertext for example.Ĭaesar cipher is an example of a substitution cipher. You will probably want to refer back to it later while working through the remainder of the sections on Caesar cipher.Ī substitution cipher simply means that each letter in the plaintext is substituted with another letter to form the ciphertext. Try experimenting with the following interactive for Caesar cipher. Of course, there may be ways to reduce the amount of work required – for example, if you know that the person who locked it never has a correct digit showing, then you only have 9 digits to guess for each place, rather than 10, which would take less than three quarters of the time! If it's a three-digit lock, you'll only have 1000 values to try out, which might not take too long.Ī four-digit lock has 10 times as many values to try out, so is way more secure. The combination number is the key for the box. We'll assume that the only way to open the box is to work out the combination number. In the physical world, a combination lock is completely analagous to a cipher (in fact, you could send a secret message in a box locked with a combination lock). Having a huge number of different possible keys is important, because it would take a computer less than a second to try all 25 Caesar cipher keys. While Caesar cipher only has 25 possible keys, real encryption systems have an incomprehensibly large number of possible keys, and preferably use keys which contains hundreds or even thousands of binary digits. More generally though, a key is simply a value that is required to do the math for the encryption and decryption. In the examples above, we used keys of "8" and "10". In a Caesar cipher, the key represents how many places the alphabet should be rotated. If instead we used a key of 8, the conversion table would be as follows. If you were unable to break the Caesar cipher in the previous section, go back to it now and decode it using the table.įor this example, we say the key is 10 because keys in Caesar cipher are a number between 1 and 25 (think carefully about why we wouldn't want a key of 26!), which specify how far the alphabet should be rotated. It is okay if your conversion table mapped the opposite way, i.e. Here's the table for the letter correspondences, where the letter "K" translates to an "A". The conversion table you drew should have highlighted this. When you looked at the Caesar cipher in the previous section and (hopefully) broke it and figured out what it said, you probably noticed that there was a pattern in how letters from the original message corresponded to letters in the decoded one.Įach letter in the original message decoded to the letter that was 10 places before it in the alphabet.
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