CSCI151 Assignment 2-Solved

•  e:  encrypt a file
Ask the user the name of the file to encrypt.  If the file doesn’t exist, or can’t be opened, print an error message, then go back to the menu.  Ask the user the name of the new file encrypted file.  If it can’t be opened, print an error message, and go back to the menu.

If both files are opened successfully, read the characters from the first, file, encrypt them, and write them into the second file.

• q:  quit
When the user quits the program, it should just print a message and exit.

•d:  decrypt a file
Ask the user the name of the file to decrypt.  If the file doesn’t exist, or can’t be opened, print an error message, then go back to the menu.  Ask the user the name of the new decrypted file.  If it can’t be opened, print an error message, and go back to the menu.

If both files are opened successfully, read the characters from the first, file, decrypt them, and write them into the second file.

• If a character other than e, q, or d is entered, the player should get an error message and the menu should be presented again.  The user is allowed to enter words that begin with the required letter; for example, quit or qqqq are both legal entries for q, and your program must handle this.  However, your program may assume that the user will never enter a single input longer than 256 characters (for a filename, password, or command).

Encryption mechanism 
The cipher I have developed for this homework is reversible, which means the decryption can be done by reversing the steps of the encryption.  It is slightly more complex than the Vigenere cipher, and much less complex than RC4.  It involves a key file and a password.   

•  The keyfile is a file that contains 255 unique characters (all the ascii characters) in some scrambled order.  You will be given a keyfile.

• The input file may be any length.

• The password may be any length up to 255.

The encryption process:

•  Take the first character from the plain text file

•  Add it to the first character from the password

•  Take modulo 255 of the sum to get a number between 0 and 254

•  Use that number as an index into the keyfile to find the encrypted character.

•  Keep going until the end of the plain text file (add second char from file to second char of password, and so on)

• If the end of the password is reached, start over at the first character of the password.

The decryption process is just the opposite:

•   Take the first character from the encrypted file.

•  Find its index in the keyfile

•  Take that index and subtract the first password character from it

•   Take modulo 255 of that result to get the plain text character

•   Keep going until the end of the encrypted file (using second char of password, and so on)

• If the end of the password is reached, start over at the first character of the password.

For example, suppose the original file contains “TEST TEXT” and your password is “pass”, and the keyfile contains the values as shown below:

Plain text 
T (84) 
E (69) 
S (83) 
T (84) 
     (32) 
T (84) 
E (69) 
X (88) 
T (84) 
Password (repeated as needed) 
p (112) 
a (97) 
s (115) 
s (115) 
p (112) 
a (97) 
s (115) 
s (115) 
p (112) 
(sum of 2 chars) % 255 
196 
166 
198 
199 
144 
181 
184 
203 
196 
Encrypted text (from keyfile) 
: 
X 
8 
7 
N 
I 
F 
3 
        144            166              181   182   183   184              196   197   198   199              203 

… 
N 
… 
X 
… 
I 
 
F 
: 
8 
7  
3 
Encrypted text
: 
X 
8 
7 
N 
I 
F 
3 
: 
Index from key file
196 
166 
198 
199 
144 
181 
184 
203 
196 
Password (repeated as needed)
p 
a 
s 
s 
p 
a 
s 
s 
p 
(index – pass char) % 255 -> original
T 
E 
S 
T 
 
T 
E 
X 
T 
 
Programming requirements 

You must write functions for doing the encryption and decryption, with the following signatures:

 void encrypt( FILE *fin, FILE *fout, char subarr[], char pass[] ); void decrypt( FILE *fin, FILE *fout, char subarr[], char pass[] ); 

The input and output files should be opened and checked for validity before calling encrypt or decrypt.  You must pass in the character substitution array (keyfile info) and password.   

Another function that must be written is a function for reading the keyfile, (hardcode the filename to “key.255”).  You may choose the details of that function yourself.   

Before showing the user the menu, you must ask the user for the password.  The same password is used to encrypt/decrypt all the files during one session.

 
Programming details 
This program, like many of the exercises you have done, involves you treating ascii characters as numbers.  But we use a keyfile of length 255, and half of those values are considered to be negative numbers (128-254), if not carefully cast.  Make sure you consistently use chars as the values you deal with, AND whenever the chars are used as their integer values, cast them as (unsigned char) so they are not seen as negative, for example:

 enc_key[(unsigned char) c] = i;       dec_key[(unsigned char) i] = c; It is not required for you to understand why this happens, but those of you who do want to know more, please see the notes at the end of this file.

The encryption and decryption algorithms require that characters from the keyfile must be looked up in 2 different ways:  

•       know the index – get the character (for encryption)

•       know the character – get the index (for decryption)

It is recommended (but not required) that you store the keyfile data in both ways, in 2 character arrays, each of length 255.  Since the keyfile contains 255 distinct values, which are 0 to 254, this is possible.  Suppose the keyfile was just 8 characters long, containing the values 0 to 7:  43756201, you would create 2 arrays, like this:

   0      1    2      3     4     5     6      7 

4 
3 
7 
5 
6 
2 
0 
1 
 
0      1    2      3     4     5     6      7 

6 
7 
5 
1 
0 
3 
4 
2 
 
Newline characters are even more troublesome for this program than the last one, and you must keep them in mind.  A filename cannot have a newline at the end; the password should not include a newline either.

Example User I/O 
The following exchange shows correct behavior for the program.  The red shows user input.  As in the previous homework, please include the word shown in bold caps in your program’s output.  Each line must end in a newline, including the last line, and do not include any extra lines.

WELCOME to the encryption service. ENTER your password 

abc123 
MENU: <e>ncode, <d>ecode, or <q>uit e 

ENTER a file to encrypt 

cats.c 
ENTER a filename for the encrypted file. cats.c.enc 

MENU: <e>ncode, <d>ecode, or <q>uit d 

ENTER a file to decrypt cats.c.enc 

ENTER a filename for the decrypted file. cats2.c 

MENU: <e>ncode, <d>ecode, or <q>uit v      

UNRECOGNIZED v 

MENU: <e>ncode, <d>ecode, or <q>uit eeeee 

ENTER a file to encrypt 

nofile.txt 
CANNOT open nofile.txt 

MENU: <e>ncode, <d>ecode, or <q>uit quit 

BYE! 

Extra (not required) information about negative numbers 
The char type is 1 byte (8 bits) long, so it can represent the numbers 0-255.   The simple ascii characters are in the range 0-127 (00000000-01111111).  We want to use the next set of values, 128-255, or all the values where the most significant bit is 1 (10000000-11111111).  Doing this gives a wider range of characters used in the encrypted file.

The problem is when a char, an 8-bit value, is interpreted as a number.  The most significant bit is used to indicate a negative number when it is 1.  You may try the following code:

char c = 129; 

printf( “%c %i %i”, c, c, (unsigned char) c ); 

You can see that even though you set the value to 129 (10000001), it gets printed as -127.  Only if you specifically say (unsigned char), can all 8 bits get included as part of the number.  Another question is, if a 1 in the first bit means negative, then why is 10000001 printed as -127 and not -1?  Computers use a representation called two’s complement for negative numbers.  You may read about this online.

So anywhere you want to use a char variable as a full 8-bit value, you must cast it to (unsigned char).