3.4-3.5, due on October 1

1. I didn't understand the proof of the Chinese Theorem, but I did understand the lemma contained in it. Also, I'd like to see an example of applying the Chinese Theorem.

2. It's nice that I took number theory this past summer because then I already had an idea what the book was saying and I just had to remember it. If it were my first time thinking about the Chinese Theorem, it would have taken more work. In fact, number theory has helped a couple times this semester with cryptography.

5.1-5.4, due on September 26

1. (Difficult) It's hard to understand how you get the extra columns [W(i)'s] while constructing the key schedule without an example. If it's really important, an example in class would be nice.

Also, in the construction of the S-Box, why does x --> x^-1 make the S-Box achieve non-linearity?

Lastly, why do we regroup the lines at the end deriving our decryption process. It seems backwards. Or I'm tired.

2. So is this an unbreakable system? There's got to be one and this seems really really really strong. Again, it's pretty straightforward but so long it's hard to wrap my head around it.

• How long have you spent on the homework assignments? Did lecture and the reading prepare you for them? I've spent 5-5 1/2 hours on the last two assignments and I think 3-4 hours on the group coding assignment. And yes, lecture helps a lot! By giving concrete example to better understand the reading and its concepts. Example are great!
• What has contributed most to your learning in this class thus far? Lecture and reading have helped a lot with the theory and a bit with the nitty gritty of how they work but honestly I think the homework has solidified both the concepts and the procedures the most.
• What do you think would help you learn more effectively or make the class better for you? Ok, I need to start the homework sooner. That's on me, everything else is great. I will do that starting this week. 

Thanks!

3.11, due on September 21

1. Difficult: Why when p=2 is every nonzero polynomial in GF(2^n) a generating polynomial? And I didn't follow the linear algebra/matrix part (which is most of the section) in the section about LFSR.

2. I feel like I'm in abstract algebra again. That was a while ago...so far it's making sense.

4.5-4.8, due on September 19

1. (Difficult) What's difficult is wrapping my head around all of this. The concepts of how to do all these different codes makes sense but all the different parts to it make it so complex I don't think I could code a message with one of these codes if I had to. Do we have to?

2. (Reflection) There is no such thing as an unbreakable code is there?!

4.1, 4.2, and 4.4, due on September 17

1. I understand how to encrypt the almost DES code, but not decrypt. That's still hazy. Also, in section 4.4 I understand what it means that the DES system is not a group, but I don't completely follow the proof. I get lost at the lemma because I don't understand why r=0. And about the proof of why DES isn't closed under composition, I'm not following all the notation I don't think so I'm not getting it. But I believe it. What's D?

2. Again, this seems like a really fast hard to break system that's not too complicated, and yet there are so many steps! How do you wrap your head around it? With a computer I guess.

2.9-2.11, due on September 14

1.The last section is the confusing one. So the period of a sequence is the length of the segment of that sequence that is repeated? or that generates the rest of the sequence? Or both? And I almost got the matrix thing which helps us break the recurrence code but I'm shaky. So I don't know the vocab. well or how to break it well but I do know how to create a string of bits using a recurrence relation. It's just a formula based on previous values.

2. So...once we have all these numbers what do we do with them? I see that we're doing lots of math to generate these numbers but do they still just map to letters like we've been doing? How do we go from mod 2 to mod 26? I guess my reflection is still misconceptions but they are reflection/connection questions rather than these sections' specific content questions.

Another reflection, wow I'm glad we have computers to do all this! I don't want to. The theory is more interesting. :-)

3.8 and 2.5-2.8, due on September 12

1. This reading was very straight forward. No one concept was difficult. The tricky thing was the codes have so many steps to them, it's hard to wrap my head around it. I guess that is the point though, isn't it.

2. The funnest part of this reading was the Sherlock Holmes story. But that aside, I simply think the introduced codes are very creative.

2.3, due on September 10

1. The difficult part about this section was understanding why the first method of finding the key works. I know how to follow the steps of computation and do it but I don't understand why when i=j the letters have been shifted the same amount...oh ok. I just looked at it again I think I got it. But this is still the hard part. The idea of why the first key finding method works is so big I'd like to be walked through it again to get my mind around it.

2. This is a really cool encryption process because letters don't keep mapping to the same things. That's what my group was trying to get around when we derived our code for the last hw assignment. We didn't do quite as good a job as a Vigenere cipher but we still made a good code. It's kind of fun breaking codes too. It would have taken me a long time to come up with a way to break the Vigenere cipher. It's pretty ingenious what other people have come up with. Cool.

And thanks with last lecture doing examples and letting us try an example. That clarifies a lot.

2.1-2.2 and 2.4, due on September 7

1. (Difficult): The most difficult part of this is decrypting the affine code. I understand encrypting it so I should  understand decrypting; maybe I just need another look at it and some more practice. How does the book know how many possible keys there are from the numbers? Not sure about that.

2. (Reflection): The most interesting part of these sections is the frequency table. I knew from common sense to look for special combinations when decrypting a substitution code but to have the probabilities and then consider the probabilities of commuted order of letters...wow lots of detail what an art! And a cool puzzle.

1. (Difficult) Most of the lecture was very easy to follow. The trickiest code was when she showed us the one dependent on a repeating word, like "milk" or "fifteen", mapped it's letters to a message and then mapped those letters again to other letters. I didn't completely follow all that but if I had more time to study it I'm sure I could get it. Clever idea.

2. (Reflection) I knew about the D&C name substitutions but didn't know about all of the encrypted letters sent from the Congress representatives to Utah. That's very clever some of the codes they sent. Especially when the decoded words were in a completely different letter. My favorite though was that they used the Hawaiian alphabet as a code and of course only Utah had someone who could read it because of missionary work. Yeah missionary work! I feel like BYU would have someone who could understand pretty much any foreign language thrown at us too. :-)