I didn't know how to do this before watching that video, but here goes my attempt .....
ONE QUESTION TO GO =
15 - 14 - 5 - 0 - 17 - 21 - 5 - 19 - 20 - 9 - 15- 14- 0 - 20 - 15 - 0 - 7 - 15
Since the encoding matrix is a 2 x 2........and our "encoded" matrix will have 18 entries...... and we must have a 9 x 2 "encoded" matrix because the columns of the encoded matrix MUST equal the rows of our encoding matrix ... [ this isn't specifically pointed out in the video !!!]......so, setting the character string up into the "encoded" matrix, we have...
[ 15 14
5 0
17 21
5 19 [ 1 - 2
20 9 -3 7 ]
15 14
0 20
15 0
7 15 ]
Now, multiply the above "encoded" matrix and "encoding" matrix together......It will probably be easiest to employ some modern technology here...I used this website for all of the calculations : http://matrix.reshish.com/multiplication.php
.....and we get
[ - 27 68
5 -10
-46 113
-52 123
-7 23
-27 68
-60 140
15 -30
-38 91]
This is the matrix that the "receiver" would see from the "sender"....!!!
However......without knowing what the encoding matrix is, it would be useless.....so, now....
We need to find the invese of the encoding matrix.....this is given by
[7 2
3 1]
Now, Multiply the matrix found in the first multiplication of matrices by the inverse of the encoding matrix [ again, using the website for the calculations ]
[ - 27 68
5 -10
-46 113
-52 123
-7 23 [ 7 2
-27 68 3 1]
-60 140
15 -30
-38 91]
And we have
[ 15 14
5 0
17 21
5 19
20 9
15 14
0 20
15 0
7 15 ]
So.....the string of resulting digits is :
15-14-5-0-17-21-5-19-20-915-14-0-20-15-0-7-15
Matching this to the original character key, the "receiver" would be able to read the original message :
ONE QUESTION TO GO .......which is correct !!!!
Note.......without the possession of the character key and encoding matrix, this message would be "unbreakable" to outside eyes.....!!!!
