Security of bash script involving gpg symmetric encryption

Question

Notice: the very same vulnerability has been discussed in this question, but the different setting of the problem (in my case I don't need to store the passphrase) allows for a different solution (i.e. using file descriptors instead of saving the passphrase in a file, see ilkkachu's answer).

Suppose I have a symmetrically encrypted file my_file (with gpg 1.x), in which I store some confidential data, and I want to edit it using the following script:

read -e -s -p "Enter passphrase: " my_passphrase
gpg --passphrase $my_passphrase --decrypt $my_file | stream_editing_command | gpg --yes --output $my_file --passphrase $my_passphrase --symmetric
unset my_passphrase

Where stream_editing_command substitutes/appends something to the stream.

My question: is this safe? Will the variable $my_passphrase and/or the decrypted output be visible/accessible in some way? If it isn't safe, how should I modify the script?

Answer 1

gpg --passphrase $my_passphrase 

My question: is this safe? Will the variable $my_passphrase and/or the decrypted output be visible/accessible in some way?

No, that's not really considered safe. The passphrase will be visible in the output of ps, just like all other running processes' command lines. The data itself will not be visible, the pipe is not accessible to other users.

The man page for gpg has this to say about --passphrase:

--passphrase string    

Use string as the passphrase. This can only be used if only one passphrase is supplied. Obviously, this is of very questionable security on a multi-user system. Don't use this option if you can avoid it.

Of course, if you have no other users on the system and trust none of your services have been compromised there should be no-one looking at the process list.

But in any case, you could instead use --passphrase-fd and have the shell redirect the passphrase to the program. Using here-strings:

#!/bin/bash
gpg --passphrase-fd 3 3<<< "$my_passphrase" --decrypt "$my_file" |
    stream_editing_command |
    gpg --yes --output "$my_file" --passphrase-fd 3 3<<< "$my_passphrase" --symmetric

Note that that only works if the second gpg doesn't truncate the output file before getting the full input. Otherwise the first gpg might not get to read the file before it's truncated.

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To avoid using the command line, you could also store the passphrase in a file, and then use --passphrase-file. But you'd then need to be careful about setting up the access permissions of the file, to remove it afterwards, and to choose a proper location for it so that the passphrase doesn't get actually stored on persistent storage.