This homework assignment is an exercise in input sanitization and
encryption.

The program will read a series of files whose names are specified on
the command line.  If no file names are given, it will read stdin.
Each file contains a series of commands to encrypt or decrypt files.
Conceptually, what is happening here is that these files are being
run by a privileged program -- it could be the "server" side from
the first homework assignment, but in the interests of avoiding
interdependency and burdening people who did poorly on the first
assignment I am not doing that.  That said, input files MUST be
considered untrusted; all errors, oddball parameters, must be detected
and handled properly.  Do not abort an input file on an error on
one line; keep going.

Each line in the file consist of a command and some arguments.  The
following commands exist (values shown in <...> are variables; DO NOT
actually use < and > in commans).

	encrypt <inputfile> <outputfile>
	decrypt <inputfile> <outputfile>
	password <password> <keyfile>
	keyfile <keyfile>
	cd <directory>
	mkdir <directory>

ALL arguments are either strings of non-whitespace characters or
are strings of ANY characters except NUL (a byte equal to zero)
enclosed in either single or double quotes.  A string that does not
begin with a quote mark may contain quote marks within the string.
If a quote mark occurs inside a quoted string argument, precede it
with a \; precede a \ with a \.  Do not do shell-like \ escapes in
unquoted strings.  Thus, the following arguments are legal:

	AbcDef.d"f+=32\
	"abc def"
	'123\'456'
	xy\z
	"abc'def"

but these are illegal:

	"abc
	pqr stuv
	'jklm"

To encrypt a file, use OpenSSL.  In particular, invoke the 'enc'
subcommand as follows:

	openssl enc -aes-128-cbc -e -in <inputfile> -out <outputfile> \
	    -pass file:<keyfile>

You MUST invoke the external command; you MUST NOT do the encryption
yourself.  (See https://www.openssl.org/docs/man1.0.1/apps/enc.html
for documentation or see the man page for 'enc' on your VM.)

An encrypt or decrypt command uses the argument of the preceding
keyfile command.  The password command takes a passphrase and
converts it to a key stored in a key file; use the PBKDF2 function
described in https://tools.ietf.org/html/rfc2898.  In other words,
to encrypt a file with the passphrase 'Secret', the input would be

	password Secret s.keyf
	keyfile s.keyf
	encrypt <inputfile> <outputfile>

but it is legal to have use a pre-existing key file.  In other
words, a keyfile command need not be preceded by a password command.

After converting the passphrase to a key, write it out as a series
of hex bytes to the key file.

The purpose of the mkdir and cd commands is to aid in construction
of test scripts.  Your test script should create and populate a
directory with suitable input files before running you command
(which MUST be called encdec).  To make life easier for the TAs
when testing, ALL absolute filenames must either refer to existing
system files or some file under /tmp.  This is NOT something your
code should check for; rather, it's a restriction on the test inputs
you supply.

You MUST NOT use argument-parsing libraries; you MUST parse the
strings by hand.

In both your code and the test inputs, pay a lot of attention to
input sanitization; that's a major purpose of this assignment.