00:01
In this problem, our job is to use the viginaire cipher to encode the given message.
00:07
So the plain text message we are given is this is a test, and the key we are to use is secret.
00:16
So with vigionaire, we take the plain text, and we copy that down.
00:25
So we'll spread this out, so we have room to do our encoding.
00:34
What we want to do after we write the message down is to copy the key below it, and we will copy the key.
00:44
As many times as we need to encipher each of the letters in our message.
00:51
So underneath we will write secret over and over again until we reach the end.
00:58
So we write secret once, and then we start writing it again until we have beneath each plain text letter, we have a letter in our key.
01:16
And now in each case, we want to take the letter in the plain text.
01:22
That tells us which column to use in the table, and the letter in the key that tells us which row to use.
01:30
So for our first pair, to take the column with t and the row with s, the column with t and the row with s, we see that they intersect in the letter l.
01:45
So our ciphertext starts out with the letter l.
01:51
Using s, we advance t in the alphabet until we hit the letter l, and that's what we're doing with the chart there.
01:59
And we're going to do this all the way across.
02:02
So just to help us remember, the plain text tells us which column to use...