CS 101 - Introduction to Computing, Spring 2007

Spring 2007

Homework 3 - Encrypting and Decrypting a Sound File

Encrypting is just a fancy term for changing the format of data so other people can't read the data easily( in the best cases, they can't read it at all). So then decrypting is just restoring the changed data back to it original format.

X -->

Encrypting Program

--> X' -->

Decrypting Program

--> X

Where X is the original data and X' is the changed (or encrypted) data.

Depending on how good the algorithm used by the Encrypting/Decrypting program pair, to more (or less) secure the encrypted data is. We will use an algorithm that give us "OK" security but nothing you would want to use if you really wanted to make an unbreakable code.

To make things a little more easier (and a bit less secure), we will use an algorithm in which uses the same program for both the Encrypting program and the Decrypting program. Thus:

X -->

Encryption Program

--> X' -->

Encryption Program

--> X

We will do two changes to the sound data to encrypt it. We will

1. swap (or exchange) two blocks of data
2. reverse the swapped blocks of data

The blocks of data will consist of some number of consectutive samples from the sound object. This number will be called the block size This number will be specified by the user when encrypting or decrypting the sound object. So in order to properly decrypt the sound object, the block size used must be the same as the block size used when the sound was encrypted. A good value for the block size would be between 1000 to 15000. Since a sound object normally has 22,050 sample per second, a block size of 2205 would give us 10 blocks per second (or have each block be 0.1 second long). A block size of 11,025 would give us 2 blocks per second.

At some point we will need to translate block numbers into sample indexes. The following table should help us figure out how to do this translation. The table assumes that you are using the JES function of getSampleValueAt() or setSampleValueAt() when accessing the samples from a sound object.

	Block Size
	1000		2205		5000		11025		b
Block Number	Starting Sample index	Ending Sample Index	Starting Sample index	Ending Sample Index	Starting Sample index	Ending Sample Index	Starting Sample index	Ending Sample Index	Starting Sample index	Ending Sample Index
1	1	1000	1	2205	1	5000	1	11025	1	b
2	1001	2000	2206	4410	5001	10000	11026	22050	b+1	2*b
3	2001	3000	4411	6615	10001	15000	22051	33075	(2*b)+1	3*b
4	3001	4000	6615	8820	15001	20000	33076	44100	(3*b)+1	4*b
i									((i-1)*b)+1	i*b

How Will We Decide Which Blocks to Swap?

Fib(1)	=			1
Fib(2)	=			1
Fib(3)	=	Fib(1) + Fib(2)	=	2
Fib(4)	=	Fib(2) + Fib(3)	=	3
Fib(5)	=	Fib(3) + Fib(4)	=	5
Fib(6)	=	Fib(4) + Fib(5)	=	8
Fib(7)	=	Fib(5) + Fib(6)	=	13
Fib(8)	=	Fib(6) + Fib(7)	=	21
Fib(9)	=	Fib(7) + Fib(8)	=	44
Fib(10)	=	Fib(8) + Fib(9)	=	65
...
Fib(n)	=	Fib(n-2) + Fib(n-1)

For our encryption algorithm, we will swap the (Fib(n))-th block with the (2*(Fib(n))-th block. Thus we will swap the following blocks:

Swap 1:	Swap block 1 with block 2
Swap 2:	Swap block 1 with block 2
Swap 3:	Swap block 2 with block 4
Swap 4:	Swap block 3 with block 6
Swap 5:	Swap block 5 with block 10
Swap 6:	Swap block 8 with block 16
Swap 7:	Swap block 13 with block 26
Swap 8:	Swap block 21 with block 42
Swap n:	Swap block Fib(n) with block 2*Fib(n)

Needed Functions

• encrypt()

  This function called encrypt() that takes two parameters: a sound object and an integer block size. This function is to return the modified sound object.

• hw3()

  This function will be the driver that calls encrypt(). This function is to prompt the user for sound file, create a sound object from that file, call encrypt with some block size, play the returned sound object and store in a file the returned sound object.

Recommened Functions

• fib()

  This function is to take an integer n as its parameter and it will return the n-th value in the fibonacci sequence. If the parameter value given is zero or less, return the value of 1.

• swap()

  This function will swap two blocks from a sound object. This function is to take four parameters.

  1. The first parameter is to be the sound object which contains the blocks to be swapped.
  2. The second paramter is the index of the first block.
  3. The third parameter is the index of the second block.
  4. The fourth paramter is the block size (how many samples are in each block).

• reverse()

  This function will reverse the samples in a block of a sound object. This function will take three parameters.

  1. the first parameter is the sound object which contains the block to be reverse.
  2. The second parameter is the index of the first block.
  3. The third parameter is the block size (how many samples are in the block).

How do We swap Values?

Step	Code	Value in val1	Value in val2	Value in val3
0.1	val1 = 8	8
0.2	val1 = 5	8	5
1	val3 = val1	8	5	8
2	val1 = val2	5	5	8
3	val2 = val3	5	8	8

How Do We Create Code for the Fibonacci Sequence?

• We must use three variables. We will call these f, f1 and f2.
• We want to initialize f to 1, f1 to 0 and f2 to 0.
• Now, we loop n-1 times to produce the n-th value from the Fibonacci sequence (note that f already has Fib(1)).
• In the loop, we set f2 to f1, f1 to f and f to f1 + f2.
• After the loop we return the value in f.

Submitting Your Project

Your code must be commented and written in good programming style. You are to submit your final program to the digital drop box on the CS 101 Blackboard site.