Chapter 25 // Exercise 13 - Principles & Practice Using C++

In this exercise I am using Visual Studio 2019 and a modified version of the std_lib_facilities header found here.

Chapter 25 // Exercise 13

Use TEA (section 25.5.6) to communicate "securely" between two computers. Email is minimally acceptable.

Github: N/A

Using the program I created from here:

https://github.com/l-paz91/principles-practice/blob/master/Chapter%2025/Exercise%201/Try%20This%20pg973

I created this "secure" message:

https://github.com/l-paz91/principles-practice/blob/master/Chapter%2025/Exercise%2013%20-%20Secure%20Message%20for%20you%20to%20decipher

The key is "plumbus".

If you crack it, paste the message in the comments.

(If you're using my code, you'll have to edit the decipher function to read the correct text file).

Chapter 25 // Exercise 12 - Principles & Practice Using C++

In this exercise I am using Visual Studio 2019 and a modified version of the std_lib_facilities header found here.

Chapter 25 // Exercise 12

Write out the clear text of the example from section 25.5.6.

Github: N/A

Deciphering the text was part of Exercise 1. You can find the code here:

https://github.com/l-paz91/principles-practice/blob/master/Chapter%2025/Exercise%201/Try%20This%20pg973

The deciphered text reads:

The Tiny Encryption Algorithm is one of the fastest and most efficient cryptographic algorithms in existence.

It was developed by David Wheeler and Roger Needham at the Computer Laboratory of Cambridge University.

It is a Feistel cipher which uses operations from mixed (orthogonal) algebraic groups - XOR, ADD and SHIFT in this case.

This is a very clever way of providing Shannon's twin properties of diffusion and confusion which are

necessary for a secure block cipher, without the explicit need for P-boxes and S-boxes respectively.

It encrypts 64 data bits at a time using a 128-bit key. It seems highly resistant to differential

cryptanalysis, and achieves complete diffusion (where a one bit difference in the plaintext will

cause approximately 32 bit differences in the ciphertext) after only six rounds. Performance on a

modern desktop computer or workstation is very impressive.

Chapter 25 // Exercise 11 - Principles & Practice Using C++

In this exercise I am using Visual Studio 2019 and a modified version of the std_lib_facilities header found here.

Chapter 25 // Exercise 11

Repeat the previous exercise, but keep the bits in a bitset<32>.

Github: https://github.com/l-paz91/principles-practice/blob/master/Chapter%2025/Exercise%2011

This was extremely easy and just involved converting the bitset to unsigned long longs in places. I like that you can subscript a bitset however, it's weird that you can't do other container-like things like set a range of the bitset.

Chapter 25 // Exercise 10 - Principles & Practice Using C++

In this exercise I am using Visual Studio 2019 and a modified version of the std_lib_facilities header found here.

Chapter 25 // Exercise 10

Look at the bitfield example from section 25.5.5. Write an example that initialises a PPN, then reads and prints each field value, then changes each field value (by assigning to the field) and prints the result. Repeat this exercise, but store the PPN information in a 32-bit unsigned integer and use bit-manipulation operators (section 25.5.4) to access the bits in the word.

Github: https://github.com/l-paz91/principles-practice/blob/master/Chapter%2025/Exercise%2010

When I first read this, my immediate thought was "wtf?" Then I read it again, looked at the example and reminded myself it's just a simple bitfield exercise. Then I got to the second half where you have to read and write to an integer and that took me an hour to wrap my head around.

It's safe to say though, I now understand how shifting and masking works. It's actually much simpler that google makes it out to be. I hope my comments on the code show that. I found drawing it out on paper really helped visualise which bits to mask/shift.

Chapter 25 // Exercise 9 - Principles & Practice Using C++

In this exercise I am using Visual Studio 2019 and a modified version of the std_lib_facilities header found here.

Chapter 25 // Exercise 9

Without using any standard headers (such as <limits>) or documentation, compute the number of bits in

an int and determine whether char is signed or unsigned on your implementation.

Github: https://github.com/l-paz91/principles-practice/blob/master/Chapter%2025/Exercise%209

So to print the number of bits in an int we know that sizeof() will return us the number of bytes in a type so you can just multiply that by 8. I don't think this is cheating as sizeof is a keyword in C++ and doesn't require any standard headers to be used. 

For the second part, I used the example from page 964. It shows that char is default signed on my machine.