Encryption
| COS 126: Fall 1996 Encryption |
Due: Wed. 10/2, 11:59pm |
For this assignment, you'll write a program, scramble.c, that
encrypts or decrypts its input. Using a key,
scramble writes a scrambled rendition of the standard input to the standard
output. For example,
% lcc scramble.c % echo Hello world | a.out | od -c 0000000 c Z > 007 222 212 V @ p 266 % a 0000014
scrambles the input "Hello world\n". The output is
displayed with od, because it's gobblygook
it almost certainly contains nonprintable characters, i.e., bytes whose values
do not correspond to the ASCII codes for printable characters. As illustrated in
the output above, od ("octal dump") displays such
characters by printing their codes in octal. The 0000000 and 0000014 are the
byte offsets for the first characters in the output lines; these values are not
part of scramble's output. Do a "man od" for more
information about od.
If the input is already encrypted, running through scramble decrypts, or unscrambles, it:
% echo Hello world | a.out | a.out Hello world
Writing scamble.c involves using arrays, character
input/output, modular arithmetic, and bitwise operators. Also, as detailed
below, you'll write three functions that manipulate arrays.
First, here's how scramble scrambles its input. It uses a 256-element array, S, that holds a permutation of the integers 0 to 255. It reads the input a character at a time and uses an element of S to scramble each character. It also rearranges S as it reads the input. Here's pseudocode for this algorithm. (Pseudocode is explained in Deitel and Deitel, Sec. 3.1).
i
0, j
for each character c in the input:
The expressions involving "mod" denote modular arithmetic, e.g., (i+1)
mod 256 denotes the remainder of i+1 divided by 256, which is an
integer in the range 0 to 255 inclusive. The expression c xor Sk
denotes the "exclusive OR" operation; see Deitel and Deitel, Sec.
10.9. You must write a function that implements this algorithm; it should take
S as an argument, read characters from the standard input, and write
the scrambled characters to the standard output. Reading and writing characters
one at a time is easy: Use getchar and putchar (see
Deitel and Deitel, Sec. 8.5). Bug alert: getchar
returns the next character as an int, not a char, because the special value
EOF is not a character.
You must also write a function that takes S and two indices as arguments and exchanges those elements in S.
The following algorithm initializes the array S, given an array K, called the key.
for i
j
for i
S has 256 elements, but K may have fewer, so the elements of K are doled out repeatedly from the beginning as often as is necessary to initialize S. You must write a function that takes S and K as arguments and initializes S. This function should call the exchange function described above to rearrange S. Initialize the elements of K in your main function to 0x7A, 0x85, 0x0E, 0x9F, and 0x16.
Test your program on small inputs, like "Hello world\n",
as shown above, and on the file /u/cs126/data/secret. That is, run
the command
% a.out </u/cs126/data/secret
It will be obvious from the output whether or not your program is working properly!
Turn in your program and your documentation with the command
/u/cs126/bin/submit 3 readme scramble.c
Reminder: Your scramble.c must define and
use three functions in addition to main:
Peek ahead to Lecture 6, and make your
scramble.c take an optional, variable-length hexadecimal key from
the command line. Thus
% a.out 7A 85 0E 9F 16 </u/cs126/data/secret
uses the default key to reveal the message in /u/cs126/data/secret.
If there are no arguments, use the default key.
References
B. Schneier, Applied Cryptography:
Protocols, Algorithms, and Source Code in C, 2nd ed., Wiley, 1996, Chap.
17.