Lesson 2: Vigenere Ciphers Part 1

[Amanda Blaine]

Okay class, it is time for lesson 2. We will be learning about Vigenere Ciphers. If you are still having difficulty with Ceasar Ciphers, I suggest that you study them a bit more before doing your homework for this lesson.

Vigenere Ciphers differ from Ceasar ciphers in that you are doing different shift ciphers for each letter. For example, if you coded "Poodle" into a Ceasar cipher, you could immediately narrow down the word because it has 2 identical letters together, the two "O"s. Vigenere Ciphers eliminate this problem by using a key phrase to cipher and decipher text. This code can be two letters, or an entire book.

Okay class, open your textbooks to page 342, and read the DESCRIPTION section.

I will be avaliable to help in the tutoring thread and will post some helpful (but somewhat cheating) links.

Homework, code the following into a Vigenere Cipher with the key "KEY."

Text to code:

Once upon the time there was a student at Spymaster. She went to the library after hours and the librarian threw her out. The end.

[Harem Del Villar]

Harem sits down and opens her book. She brielfy glances at the page and begins to write down. As soon as she is done, she stands up and leaves the paper at your desk.

"Yrao ynyr rri rsqc dlcbi ukw y cxsnild er Ctwweqdip. Clc gild xm dlc vmzbepi eddip rssbw yxh rri jsfpkvgkr rrvcg lcb ssd. Xfo iln." the paper says.

As Harem walks into the next class a slight smirk is shown on her face, but is quickly hidden by her hair.

Too easy...

[Nathan Dark]

Nathan read the textbook carefully, and was soon fascinated enough to continue with reading up on the history and different ways to decode the cipher. Only at the last minute did he realise the class was over, and carefully copied the homework to take with him.

Once back in his room, Nathan studied the text carefully, and then opened his textbook again. Now, this couldn't be that hard, could it? After all, he was just supposed to encrypt this text, not the other way around. After a little more thought, he made a copy of the Vigenère square shown in their textbook and marked the lines K, E and Y, so he would be able to find them more quickly.

After that, it was just a question of working. First the K-column... O became a Y, then E: N-R and Y: C-A... and so on. Before too long, he had the encoded text before him, and having done that, decided to go to bed and turn in the homework tomorrow.

"Yrao ynyr rri rsqc dlcbi ukw y cxsnild er Ctwweqdip. Clc gild xm dlc vmzbepi eddip rssbw yxh rri jsfpkvgkr rrvcg lcb ssd. Xfo iln."

The next morning, when he woke up, he reread the homework, and then realised that if he had to, he could decode it. He made a note of it before heading off to class.

This could be decoded if someone noticed the sequence "rri" appearing at two places in the text, and assumed that they represented the word "the", as they actually do.

[MeiMei]

She sat there fumbling to write things down, she was already doing poorly in this class. "Oii!" She blinked, noticeing her little outburst had drawn attention to herself from a few people.

She quickly glanced back down into her book and began scribbling on her notebook, she'd atleast try harder on this lesson than the last one.

[In the notebook]

' Yrao ynyr rri rsqc dlcbi ukw y cxsnild er Ctwweqdip. Clc gild xm dlc vmzbepi eddip rssbw yxh rri jsfpkvgkr rrvcg lcb ssd. Xfo iln. '

She cringed slightly doubting she'd even gotten one thing correct , but none the less, she'd turn in her work.

[Fredrick Nova]

Fredrick started working on the cipher as soon as he got to the box with all the alphabets in it. It was tedious work, and he amused himself by thinking of ways to make it harder to figure out. By the time the bell rang he was only halfway through, and he hadn't finished the whole chapter, so he piled things into his bag and went off to sit in the library. He liked working in there and hoped that by being a regular he could somehow make up for defacing a book, if they ever figured out it was he who had done it.

He finished the translation and then, because he was bored, he finished reading the chapter. He remembered that Nathan had been studying this cipher before and wondered what had facinated him so much. On the one hand, it was interesting in a purely useful way, but the theory behind it all seemed very much like a piece of music that had too many key changes; annoying.

Fredrick liked things that were simple in general. He thought most things probably had a simple solution if you really thought about it. Simple was elegant, it was clean, it didn't make you change your fingering on the violin more than three times per line of music.

He glanced back over the letters he'd transcribed. Well, there was no use trying to make some things simple. If he had to work for his grade, he figured the teacher should, too. He smiled to himself and started copying things out again, this time in all capitals and with breaks in the text that were different from the original text and no punctuation.

He wound up with:

YR AOYNYRRR IRSQ CDLCBIUK WYC XS NILDERCTW WEQDIPCLCGI LDX M DL CVMZBE PIEDDI PRSS BWYXHRRIJSF PKUGKRRRV CGLC BS SDXFOILN

He smiled, happy with the fact that, while technically correct, his answer was at least a bit misleading, then he put his crypography things away and started on his next assignment.

[Ai]

Ai looked thoughtfully at the homework and tried to work it out. She scribbled some notes along her notebook. She scribbled and scribbled until her whole notebook was covered in elegible letters that only she could understand. It was a mess but it was a success. She had it.

Yrao ynyr rri rsqc dlcbi ukw y cxsnild er Ctwweqdip. Clc gild xm dlc vmzbepi eddip rssbw yxh rri jsfpkvgkr rrvcg lcb ssd. Xfo iln.

Sighing, happy that she got the work done. She packed her stuff and took her homework to the teacher to grade.