Full transcript
0:06okay
0:08so let's look at some simple codes to
0:11begin with
0:14now the most basic type of code
0:16is known as the substitution cipher
0:20let me clarify a bit here
0:23in cryptographic terms
0:26a cipher is another word for a code
0:31and what we mean by a substitution
0:34cipher
0:35is when you create a code
0:37by replacing each original letter in a
0:41text
0:42with a different character
0:46an early example of this
0:48is the caesar cipher
0:51which replaced each letter of the
0:54alphabet
0:55with the letter three places further
0:58down the alphabet
1:00so a
1:01was replaced by d
1:04b was replaced by e
1:07and so on
1:10now
1:11every substitution cipher uses a key
1:16the key
1:18is the component which tells you what
1:20you need to do
1:22to create the code
1:24for example in the case of the caesar
1:27cipher
1:28it tells you how many spaces the letter
1:32has been moved along
1:35if you want to have a more secure code
1:38you can regularly change the key that is
1:40used
1:44over time
1:45substitution ciphers became more complex
1:50cryptographers started to use not just
1:52one key but many different keys in a
1:56message
1:57and this raises the question
2:00if the cryptographer uses many different
2:02keys in a single message
2:05how can anyone break such a code
2:09well
2:11one method is to use a crib
2:14a crib
2:15is a type of answer key
2:18this is usually part of the decoded text
2:22that someone has already found or has
2:24worked out
2:27when you have a crib
2:29you can use it to guess the meaning of
2:31the rest of the cipher text
2:34just to clarify
2:36a ciphertext
2:38means the coded message
2:41so if you guess for example
2:44that the characters e
2:45f
2:46b s in the letter represent d
2:50e
2:51a r the word dear
2:55then using those letters as a starting
2:57point
2:58you can work out what the key is
3:02so for the very simple example that i
3:05gave you before this lecture
3:07the answer is