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Assignment2 - A bit of crypto Solution
Your assignment is to download and build the mbed TLS package and to make a tiny modification to it's sample AES file encryption program as described below.
For class: scan through Arm's mbed TLS coding standards. It'd be a fine thing (but is not mandatory for marking) if students followed those standards too when working with this code.
IMPORTANT: Keep your work, including the build of mbed TLS, you'll need it again for other assignments! (You could repeat the work, but that'd be boring:-)
Setup a working environment
Let's say your working directory is going to be $HOME/code then you need to do the following (or equivalent) in a shell:
$ mkdir $HOME/code
$ cd $HOME/code
$ git clone https://github.com/sftcd/cs2014.git
...various output
$ cd assignments/assignment2
$ wget https://tls.mbed.org/download/mbedtls-2.6.0-apache.tgz
...various output
$ tar xzvf mbedtls-2.6.0-apache.tgz
...various output
$ cd mbedtls-2.6.0
$ make
... lots of output, takes a minute or two
$ cd ..
$ make
...various output
I'll explain those one by one:
First you make a working directory:
$ mkdir $HOME/code
Then go there...
$ cd $HOME/code
Then get a copy of the git repo for the course (you should have done that already btw!)
$ git clone https://github.com/sftcd/cs2014.git
Now go to the directory for this assignment:
$ cd assignments/assignment2
... and download a copy of the mbed TLS tarball:
$ wget https://tls.mbed.org/download/mbedtls-2.6.0-apache.tgz
... explode the tarball
$ tar xzvf mbedtls-2.6.0-apache.tgz
... cd into the mbed TLS directory
$ cd mbedtls-2.6.0
... build the mbed TLS code
$ make
... go back up one level to the assignment2 directory
$ cd ..
... and run the build for the assignment st
$ make
What's there?
The files in this assignment directory you should see now are:
• assignment2 - the excutable form of the base tool you'll modify in a bit
• assignment2.c - the base file encrypting tool you'll modify in a bit
◦ (Note: This is just a straight copy of what'll be in ./mbedtls-2.6.0/programs/aes/aescrypt2.c)
• assignment2.o - the object file for encrypting tool you'll modify in a bit
• Makefile - the Makefile to builld the above and link in the mbed TLS library
• mbedtls-2.6.0 - the directory with the mbed TLS stuff
• mbedtls-2.6.0-apache.tgz- the tarball you downloaded
• README.html - this HTML file
• README.md - the markdown source for this HTML file
Run the tool as-is:
$ ./assignment2
asssignment2 <mode> <input filename> <output filename> <key>
<mode>: 0 = encrypt, 1 = decrypt
example: aescrypt2 0 file file.aes hex:E76B2413958B00E193
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A simple modification...
This week, all you need to do is replace the clumsy "0" (for encryption) with any case-insensitive substring prefix of the word "encryption" and then simlarly replace the "1" for decryption, with any case-insensitive substring prefix of the word "decryption" and otherwise leave everything else as-is.
So, when you're done the following should work:
$ echo "hi" >plain.txt
$ ./asssignment2 eNcr plain.txt cipher.bin thisisnotagoodkey
$ ./asssignment2 De cipher.bin recoveredplain.txt thisisnotagoodkey
$ diff -s plain.txt recoveredplain.txt
Files plaintext and plaintext are identical
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And of course any variants of the start of the words "encryption" and "decryption" provided in the right place should work just as well, regardless of upper or lowercase.
A hint...
The function strcasestr() might be your friend here, check out it's man page - but if using that you'll need to add this to nearly the top of your source file:
#define _GNU_SOURCE /* See feature_test_macros(7) */
Otherwise, if you don't want to use strcasestr(), that's fine, you can write your own function to do the equivalent comparison.
Deadline
The deadline for submission of this assignment is 2017-10-16
Submission
For this assignment you should only submit your single file of source code, which can be called assignment2.c
