Is it safe to pass a key to the openssl command via the command line parameters in Linux? I know it nulls out the actual parameter, so it can't be viewed via /proc, but, even with that, is there some way to exploit that?
I have a python app that I want to use OpenSSL to do the encryption/description through stdin/stdout streaming in a subprocess, but I want to know my keys are safe.
Passing the credentials on the command line is not safe. It will result in your password being visible in the system's process listing - even if openssl erases it from the process listing as soon as it can, it'll be there for an instant.
openssl gives you a few ways to pass credentials in - the man page has a section called "PASS PHRASE ARGUMENTS", which documents all the ways you can pass credentials into openssl. I'll explain the relevant ones:
env:var
Lets you pass the credentials in an environment variable. This is better than using the process listing, because on Linux your process's environment isn't readable by other users by default - but this isn't necessarily true on other platforms.
The downside is that other processes running as the same user, or as root, will be able to easily view the password via /proc.
It's pretty easy to use with python's subprocess:
new_env=copy.deepcopy(os.environ)
new_env["MY_PASSWORD_VAR"] = "my key data"
p = subprocess.Popen(["openssl",..., "-passin", "env:MY_PASSWORD_VAR"], env=new_env)
fd:number
This lets you tell openssl to read the credentials from a file descriptor, which it will assume is already open for reading. By using this you can write the key data directly from your process to openssl, with something like this:
r, w = os.pipe()
p = subprocess.Popen(["openssl", ..., "-passin", "fd:%i" % r], preexec_fn=lambda:os.close(w))
os.write(w, "my key data\n")
os.close(w)
This will keep your password secure from other users on the same system, assuming that they are logged in with a different account.
With the code above, you may run into issues with the os.write call blocking. This can happen if openssl waits for something else to happen before reading the key in. This can be addressed with asynchronous i/o (e.g. a select loop) or an extra thread to do the write()&close().
One drawback of this is that it doesn't work if you pass closeFds=true to subprocess.Popen. Subprocess has no way to say "don't close one specific fd", so if you need to use closeFds=true, then I'd suggest using the file: syntax (below) with a named pipe.
file:pathname
Don't use this with an actual file to store passwords! That should be avoided for many reasons, e.g. your program may be killed before it can erase the file, and with most journalling file systems it's almost impossible to truly erase the data from a disk.
However, if used with a named pipe with restrictive permissions, this can be as good as using the fd option above. The code to do this will be similar to the previous snippet, except that you'll need to create a fifo instead of using os.pipe():
pathToFifo = my_function_that_securely_makes_a_fifo()
p = subprocess.Popen(["openssl", ..., "-passin", "file:%s" % pathToFifo])
fifo = open(pathToFifo, 'w')
print >> fifo, "my key data"
fifo.close()
The print here can have the same blocking i/o problems as the os.write call above, the resolutions are also the same.