CS 344 Program 4 solved

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In this assignment, you will be creating five small programs that encrypt and decrypt
information using a one-time pad-like system. I believe that you will find the topic quite
fascinating: one of your challenges will be to pull yourself away from the stories of
real-world espionage and tradecraft that have used the techniques you will be

These programs serve as a capstone to what you have been learning in this course,
and will combine the multi-processing code you have been learning with socket-based
inter-process communication. Your programs will also accessible from the command
line using standard UNIX features like input/output redirection, and job control. Finally,
you will write a short compilation script.


All execution, compiling, and testing of this program should ONLY be done in the bash
prompt on our class server!
Use the following link as your primary reference on One-Time Pads (OTP): (Links to an external site.)Links to an external
The following definitions will be important:
Plaintext is the term for the information that you wish to encrypt and protect. It is human
Ciphertext is the term for the plaintext after it has been encrypted by your programs.
Ciphertext is not human-readable, and in fact cannot be cracked, if the OTP system is
used correctly.
A Key is the random sequence of characters that will be used to convert Plaintext to
Ciphertext, and back again. It must not be re-used, or else the encryption is in danger of
being broken.

The following excerpt from this Wikipedia article was captured on 2/21/2015:
“Suppose Alice wishes to send the message “HELLO” to Bob. Assume two pads of
paper containing identical random sequences of letters were somehow previously
produced and securely issued to both. Alice chooses the appropriate unused page from
the pad. The way to do this is normally arranged for in advance, as for instance ‘use the
12th sheet on 1 May’, or ‘use the next available sheet for the next message’.
The material on the selected sheet is the key for this message. Each letter from the pad
will be combined in a predetermined way with one letter of the message. (It is common,
but not required, to assign each letter a numerical value, e.g., “A” is 0, “B” is 1, and so

In this example, the technique is to combine the key and the message using modular
addition. The numerical values of corresponding message and key letters are added
together, modulo 26. So, if key material begins with “XMCKL” and the message is
“HELLO”, then the coding would be done as follows:
H E L L O message
7(H) 4(E) 11(L) 11(L) 14 (O) message
+ 23(X) 12(M) 2(C) 10(K) 11 (L) key
= 30 16 13 21 25 message + key
= 4(E) 16(Q) 13(N) 21(V) 25 (Z) message + key (mod 26)
E Q N V Z → ciphertext
If a number is larger than 26, then the remainder, after subtraction of 26, is taken [as the
result]. This simply means that if the computations “go past” Z, the sequence starts
again at A.

The ciphertext to be sent to Bob is thus “EQNVZ”. Bob uses the matching key page and
the same process, but in reverse, to obtain the plaintext. Here the key is subtracted
from the ciphertext, again using modular arithmetic:
E Q N V Z ciphertext
4(E) 16(Q) 13(N) 21(V) 25 (Z) ciphertext
-23(X) 12(M) 2(C) 10(K) 11 (L) key
= -19 4 11 11 14 ciphertext – key
= 7(H) 4(E) 11(L) 11(L) 14 (O) ciphertext – key (mod 26)
H E L L O → message

Similar to the above, if a number is negative then 26 is added to make the number zero
or higher.
Thus Bob recovers Alice’s plaintext, the message “HELLO”. Both Alice and Bob destroy
the key sheet immediately after use, thus preventing reuse and an attack against the
Your program will encrypt and decrypt plaintext into ciphertext, using a key, in exactly
the same fashion as above, except it will be using modulo 27 operations: your 27
characters are the 26 capital letters, and the space character ( ). All 27 characters will
be encrypted and decrypted as above.

To do this, you will be creating five small programs in C. Two of these will function like
“daemons” (but aren’t actually daemons), and will be accessed using network sockets.
Two will use the daemons to perform work, and the last is a standalone utility.
Your programs must use the network calls we’ve discussed in class (send(), recv(),
socket(), bind(), listen(), & accept()) to send and receive sequences of bytes
for the purposes of encryption and decryption by the appropriate daemons. The whole
point is to use the network, even though for testing purposes we’re using the same
machine: if you just open() the datafiles from the server without using the network
calls, you’ll receive 0 points on the assignment.

Here are the specifications of the five programs:

otp_enc_d: This program will run in the background as a daemon. Upon execution,
otp_enc_d must output an error if it cannot be run due to a network error, such as the
ports being unavailable. Its function is to perform the actual encoding, as described
above in the Wikipedia quote. This program will listen on a particular port/socket,
assigned when it is first ran (see syntax below). When a connection is made,
otp_enc_dmust call accept() to generate the socket used for actual communication,
and then use a separate process to handle the rest of the transaction (see below),
which will occur on the newly accepted socket.

This child process of otp_enc_d must first check to make sure it is communicating with
otp_enc (see otp_enc, below). After verifying that the connection to otp_enc_d is
coming from otp_enc, then this child receives from otp_enc plaintext and a key via
the communication socket (not the original listen socket). The otp_enc_d child will
then write back the ciphertext to the otp_enc process that it is connected to via the
same communication socket. Note that the key passed in must be at least as big as the

Your version of otp_enc_d must support up to five concurrent socket connections
running at the same time; this is different than the number of processes that could
queue up on your listening socket (which is specified in the second parameter of the
listen() call). Again, only in the child process will the actual encryption take place,
and the ciphertext be written back: the original server daemon process continues
listening for new connections, not encrypting data.

In terms of creating that child process as described above, you may either create with
fork() a new process every time a connection is made, or set up a pool of five
processes at the beginning of the program, before connections are allowed, to handle
your encryption tasks. As above, your system must be able to do five separate
encryptions at once, using either method you choose.
Use this syntax for otp_enc_d:
otp_enc_d listening_port
listening_port is the port that otp_enc_d should listen on. You will always start
otp_enc_d in the background, as follows (the port 57171 is just an example; yours
should be able to use any port):
$ otp_enc_d 57171 &

In all error situations, this program must output errors to stderr as appropriate (see
grading script below for details), but should not crash or otherwise exit, unless the errors
happen when the program is starting up (i.e. are part of the networking start up
protocols like bind()). If given bad input, once running, otp_enc_d should recognize the
bad input, report an error to stderr, and continue to run. Generally speaking, though, this
daemon shouldn’t receive bad input, since that should be discovered and handled in the
client first. All error text must be output to stderr.
This program, and the other 3 network programs, should use “localhost” as the target IP
address/host. This makes them use the actual computer they all share as the target for
the networking connections.

otp_enc: This program connects to otp_enc_d, and asks it to perform a one-time
pad style encryption as detailed above. By itself, otp_enc doesn’t do the encryption –
otp_end_d does. The syntax of otp_encis as follows:
otp_enc plaintext key port
In this syntax, plaintext is the name of a file in the current directory that contains the
plaintext you wish to encrypt. Similarly, key contains the encryption key you wish to use
to encrypt the text. Finally, port is the port that otp_enc should attempt to connect to
otp_enc_d on.

When otp_enc receives the ciphertext back from otp_enc_d, it should output it to
stdout. Thus, otp_enc can be launched in any of the following methods, and should
send its output appropriately:
$ otp_enc myplaintext mykey 57171
$ otp_enc myplaintext mykey 57171 > myciphertext
$ otp_enc myplaintext mykey 57171 > myciphertext &
If otp_enc receives key or plaintext files with ANY bad characters in them, or the key
file is shorter than the plaintext, then it should terminate, send appropriate error text to
stderr, and set the exit value to 1.

otp_enc should NOT be able to connect to otp_dec_d, even if it tries to connect on
the correct port – you’ll need to have the programs reject each other. If this happens,
otp_enc should report the rejection to stderr and then terminate itself. In more detail: if
otp_enc cannot connect to the otp_enc_d server, for any reason (including that it has
accidentally tried to connect to the otp_dec_d server), it should report this error to
stderr with the attempted port, and set the exit value to 2. Otherwise, upon successfully
running and terminating, otp_enc should set the exit value to 0.
Again, any and all error text must be output to stderr (not into the plaintext or ciphertext

otp_dec_d: This program performs exactly like otp_enc_d, in syntax and usage. In
this case, however, otp_dec_d will decrypt ciphertext it is given, using the passed-in
ciphertext and key. Thus, it returns plaintext again to otp_dec.
otp_dec: Similarly, this program will connect to otp_dec_d and will ask it to decrypt
ciphertext using a passed-in ciphertext and key, and otherwise performs exactly like
otp_enc, and must be runnable in the same three ways. otp_dec should NOT be able
to connect to otp_enc_d, even if it tries to connect on the correct port – you’ll need to
have the programs reject each other, as described in otp_enc.

keygen: This program creates a key file of specified length. The characters in the file
generated will be any of the 27 allowed characters, generated using the standard UNIX
randomization methods. Do not create spaces every five characters, as has been
historically done. Note that you specifically do not have to do any fancy random number
generation: we’re not looking for cryptographically secure random number generation!
rand() is just fine. The last character keygen outputs should be a newline. All error
text must be output to stderr, if any.

The syntax for keygen is as follows:

keygen keylength
Where keylength is the length of the key file in characters. keygen outputs to stdout.
Here is an example run, which redirects stdout to a key file of 256 characters called
“mykey” (note that mykey is 257 characters long because of the newline):
$ keygen 256 > mykey

Files and Scripts

You are provided with 5 plaintext files to use (one, two, three, four, five). The grading
will use these specific files; do not feel like you have to create others.
You are also provided with a grading script (“p4gradingscript”) that you can run to test
your software. If it passes the tests in the script, and has sufficient commenting, it will
receive full points (see below). EVERY TIME you run this script, change the port
numbers you use! Otherwise, because UNIX may not let go of your ports immediately,
your successive runs may fail!
Finally, you will be required to write a compilation script (see below) that compiles all
five of your programs, allowing you to use whatever C code and methods you desire.
This will ease grading. Note that only C will be allowed, no C++ or any other language
(Python, Perl, awk, etc.).


Here is an example of usage, if you were testing your code from the command line:
$ cat plaintext1
$ otp_enc_d 57171 &
$ otp_dec_d 57172 &
$ keygen 10 > myshortkey
$ otp_enc plaintext1 myshortkey 57171 > ciphertext1
Error: key ‘myshortkey’ is too short
$ echo $?
$ keygen 1024 > mykey
$ otp_enc plaintext1 mykey 57171 > ciphertext1
$ cat ciphertext1
$ keygen 1024 > mykey2
$ otp_dec ciphertext1 mykey 57172 > plaintext1_a
$ otp_dec ciphertext1 mykey2 57172 > plaintext1_b
$ cat plaintext1_a
$ cat plaintext1_b
$ cmp plaintext1 plaintext1_a
$ echo $?

$ cmp plaintext1 plaintext1_b
plaintext1 plaintext1_b differ: byte 1, line 1
$ echo $?
$ otp_enc plaintext5 mykey 57171
otp_enc error: input contains bad characters
$ echo $?
$ otp_enc plaintext3 mykey 57172
Error: could not contact otp_enc_d on port 57172
$ echo $?

Compilation Script

You must also write a short bash shell script called “compileall” that merely compiles
your five programs. For example, the first two lines might be:
gcc -o otp_enc_d otp_enc_d.c

This script will be used to compile your software, and must successfully run on our class
server. The compilation must create all five programs, in the same directory as
“compileall”, for immediate use by the grading script, which is named “p4gradingscript”.

Where to Start

First, write keygen – it’s simple and fun! Then, use our sample network programs
client.c and server.c (you don’t have to cite your use of them) to implement
otp_enc and otp_enc_d. Once they are functional, copy them and begin work on
otp_dec and otp_dec_d.
If you have questions about what your programs needs to be able to do, just examine
the grading script. Your programs have to deal with exactly what’s in there: no more, no
less. 🙂

Sending Data

Recall that when sending data, not all of the data may be written with just one call to
send() or write(). This occurs because of network interruptions, server load, and other
factors. You’ll need to carefully watch the number of characters read and/or written, as
appropriate. If the number returned is less than what you intended, you’ll need to restart
the process from where it stopped. This means you’ll need to wrap a loop around the
send/receive routines to ensure they finish their job before continuing.
If you try to send too much data at once, the server will likely break the transmission, as
in the previous paragraph. Consider setting a maximum send size, breaking the
transmission yourself every 1000 characters, say.

There are a few ways to handle knowing how much data you need to send in a given
transmission. One way is to send an integer from client to server (or vice versa) first,
informing the other side how much is coming. This relatively small integer is unlikely to
be split and interrupted. Another way is to have the listening side looking for a
termination character that it recognizes as the end of the transmission string. It could
loop, for example, until it has seen that termination character.

Concurrency Implications

Remember that only one socket can be bound to a port at a time. Multiple incoming
connections all queue up on the socket that has had listen() called on it for that port.
After each accept() call is made, a new socket file descriptor is returned which is your
server’s handle to that TCP connection. The server can accept multiple incoming
streams, and communicate with all of them, by continuing to call accept(), generating
a new socket file descriptor each time.

About Newlines

You are only supposed to accept the 26 letters of alphabet and the “space” character as
valid for encrypting/decrypting. However, all of the plaintext input files end with a
newline character. Text files need to end in a newline character for various reasons.
When one of your programs reads in an input file, strip off the newline. Then encrypt
and decrypt the text string, again with no newline character. When you send the result
to stdout, or save results into a file, tack a newline to the end, or your length will be off in
the grading script. Note that the newline character affects the length of files as reported
by the wc command! Try it!

About Reusing Sockets

In the p4gradingscript, you can select which ports to use: I recommend ports in the
50000+ range. However, UNIX doesn’t immediately let go of the ports you use after
your program finishes! I highly recommend that you frequently change and randomize
the sockets you’re using, to make sure you’re not using sockets that someone else is
playing with. In addition, to allow your program to continue to use the same port (your
mileage may vary), read this: (Links to
an external site.)Links to an external site.
…and then play around with this command:
setsockopt(sock_fd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int));

Where to Develop

I HIGHLY recommend that you develop this program directly on our class server! Doing
so will prevent you from having problems transferring the program back and forth, which
can cause compatibility issues. Do not use any other non-class server to develop these
If you do see ^M characters all over your files, which come from copying a
Windows-saved file onto a UNIX file system, try this command:
$ dos2unix bustedFile

What to Turn In and When

Please submit a single zip file of your program code, which may be in as many different
files as you want. Inside that zip file, include the following files:
1. All of your program code
2. The compilation script named “compileall”
3. All five plaintext# files, numbered 1 through 5
4. A copy of the grading script named “p4gradingscript”
Failing to submit one of the required pieces results in an 8-point deduction, while we
attempt to contact you to submit what’s missing. Your submission date & time is
whenever you send in the missing piece.
The due date given below is the last minute that this can be turned in for full credit. The
“available until” date is NOT the due date, but instead closes off submissions for this
assignment automatically once 48 hours past the due date has been reached, in
accordance with our Syllabus Grading policies.


In a bash prompt, on our class server, the graders will run the “compileall” script, and
will then run the “p4gradingscript”. They will make a reasonable effort to make your
code compile, but if it doesn’t compile, you’ll receive a zero on this assignment.
If it compiles, it will have the “p4gradingscript” script ran against it for final grading, in
this manner, in a bash prompt on our class server, where you fill in numbers for PORT1
and PORT2:

$ ./p4gradingscript PORT1 PORT2 > mytestresults 2>&1
The graders will change the ports around each time they run the grading script, to make
sure the ports used aren’t in-use. Points will be assigned according to this grading

150 points are available in the grading script, while the final 10 points will be based on
your style, readability, and commenting. Comment well, often, and verbosely (at least
every five lines, say): we want to see that you are telling us WHY you are doing things,
in addition to telling us WHAT you are doing.