CS 344 Program 4 – OTP


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Program 4 – CS 344
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 implementing.
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
Use the following link as your primary reference on One-Time Pads (OTP):
http://en.wikipedia.org/wiki/One-time_pad (http://en.wikipedia.org/wiki/One-time_pad)
The following definitions will be important:
Plaintext is the term for the information that you wish to encrypt and protect. It is human readable.
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 on.)
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
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 cipher.”
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_d must 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
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 plaintext.
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_enc_d does. The syntax of otp_enc is
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 files).
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 for that portion of the
homework grade (review the “What to Turn In” section). 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
$ keygen 10 > mykey
$ cat mykey
$ 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
and decrypting. However, all of the plaintext input files end with a newline character, and all text files you
generate must end in a newline character.
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, you
must 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:
…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 programs.
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.
As our Syllabus says, please be aware that neither the Instructor nor the TA(s) are alerted to comments
added to the text boxes in Canvas that are alongside your assignment submissions, and they may not be
seen. No notifications (email or otherwise) are sent out when these comments are added, so we aren’t
aware that you have added content! If you need to make a meta-comment about this assignment, please
include it in a README file in your .zip file, or email the person directly who will be grading it (see
the Home page for grading responsibilities).
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 script.
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.
The TAs will use this exact set of instructions: Program4 Grading.pdf to grade your submission.