Linux Crypto: Passwords

It’s now becoming more widely known that using guessable passwords or using the same password for more than one account is a serious security risk, because an attacker able to control one account (such as an email account) can do a lot of damage. If an attacker gets the hash of your password from some web service, you want to be assured that the hash will be very difficult to reverse, and even if it can be reversed, that it’s unique and won’t give them access to any of your other accounts.

This growing awareness has contributed to the popularity of password managers, tools designed to securely generate, store, and retrieve passwords, encrypted with a master password or passphrase. In some cases these are locally stored, such as KeePass, and in others they are stored on a web service, such as LastPass. Both are good tools, and work well with Linux. I personally have some reservations about LastPass as I don’t want my passwords stored on a third party service, and I don’t trust JavaScript encryption.

Interestingly, because we now have a tidy GnuPG setup to handle the encryption ourselves, another option is the pass(1) tool, billing itself as “the standard UNIX password manager”. It’s little more than a shell script and some bash(1) completions wrapped around existing tools like git(1), gpg2(1), pwgen(1), tree(1), and xclip(1), and your choice of $EDITOR. If you’re not already invested in an existing password management method, you might find this a good first application of your new cryptography setup, and a great minimal approach to secure password storage accessible from the command line (and therefore SSH).

On Debian-derived systems, it’s available as part of the pass package:

# apt-get install pass

This includes a manual:

$ man pass

Instructions for installing on other operating systems are also available on the site. Releases are also available for download, and a link to the development repository. If you use this, make sure you have the required tools outlined above installed as well, although xclip(1) is only needed if you run the X Windows system.

Setup

We can get an overview of what pass(1) can do by invoking it with no arguments:

$ pass

To start, we’ll initialize our password store. For your own passwords, you will want to do this as your own user rather than root. Because pass(1) uses GnuPG for its encryption, we also need to tell it the ID of the appropriate key to use. Remember, you can find this eight-digit hex code by typing gpg --list-secret-keys. A unique string identifying your private key such as your name or email address may also work.

$ pass init 0x77BB8872
mkdir: created directory ‘/home/tom/.password-store’
Password store initialized for 0x77BB8872.

Indeed, we note the directory ~/.password-store has been created, although it’s presently empty except for the .gpg-id file recording our key ID:

$ find .password-store
.password-store
.password-store/.gpg-id

Inserting

We’ll insert an existing password of ours with pass insert, giving it a descriptive hierarchical name:

$ pass insert google.com/gmail/example@gmail.com
mkdir: created directory ‘/home/tom/.password-store/google.com’
mkdir: created directory ‘/home/tom/.password-store/google.com/gmail’
Enter password for google.com/gmail/example@gmail.com:
Retype password for google.com/gmail/example@gmail.com:

The password is read from the command line, encrypted, and placed in ~/.password-store:

$ find .password-store
.password-store
.password-store/google.com
.password-store/google.com/gmail
.password-store/google.com/gmail/example@gmail.com.gpg
.password-store/.gpg-id

Notice that pass(1) creates a directory structure for us automatically. We can get a nice view of the password store with pass with no arguments:

$ pass
Password Store
└── google.com
    └── gmail
            └── example@gmail.com

Generating

If you’d like it to generate a new secure random password for you, you can use generate instead, including a password length as the last argument:

$ pass generate google.com/gmail/example@gmail.com 16
The generated password to google.com/gmail/example@gmail.com is:
!Q%i$$&q1+JJi-|X

If you have some service that doesn’t cooperate with symbols in passwords, you can add the -n option to this call:

$ pass generate -n google.com/gmail/example@gmail.com 16
The generated password to google.com/gmail/example@gmail.com is:
pJeF18CrZEZzI59D

pass(1) uses pwgen(1) for this password generation. In each case, the password is automatically inserted into the password store for you.

If we need to change an existing password, we can either overwrite it with insert again, or use the edit operation to invoke our choice of $EDITOR:

$ pass edit google.com/gmail/example@gmail.com

If you do this, you may like to be careful that your editor is not configured to keep backups or swap files in plain text of documents it edits in temporary directories or memory filesystems. If you’re using Vim, I wrote some configuration in an attempt to solve this problem.

Note that adding or overwriting passwords does not require your passphrase; only retrieval and editing does, consistent with how GnuPG normally works.

Retrieval

This password can now be retrieved and echoed onto the command line given the appropriate passphrase:

$ pass google.com/gmail/example@gmail.com
(...gpg-agent pinentry prompt...)
Tr0ub4dor&3

If you’re using X windows and have xclip(1) installed, you can put the password on the clipboard temporarily to paste into web forms:

$ pass -c google.com/gmail/example@gmail.com
Copied google.com/gmail/example@gmail.com to clipboard. Will clear in 45 seconds.

In each case, note that if you have the bash completion installed and working, you should be able to complete the full path to the passwords with Tab, just as if you were directly browsing a directory hierarchy.

Deletion

If we no longer need the password, we can remove it with pass rm:

$ pass rm google.com/gmail/example@gmail.com
Are you sure you would like to delete google.com/gmail/example@gmail.com? [y/N] y
removed ‘/home/tom/.password-store/google.com/gmail/example@gmail.com.gpg’

We can delete whole directories of passwords with pass rm -r:

$ pass rm -r google.com
Are you sure you would like to delete google.com? [y/N] y
removed ‘/home/tom/.password-store/google.com/gmail/example@gmail.com.gpg’
removed directory: ‘/home/tom/.password-store/google.com/gmail’
removed directory: ‘/home/tom/.password-store/google.com’

Version control

To keep historical passwords, including deleted ones if we find we do need them again one day, we can set up some automatic version control on the directory with pass git init:

$ pass git init
Initialized empty Git repository in /home/tom/.password-store/.git/
[master (root-commit) 0ebb933] Added current contents of password store.
 1 file changed, 1 insertion(+)
 create mode 100644 .gpg-id

This will update the repository every time the password store is changed, meaning we can be confident we’ll be able to retrieve old passwords we’ve replaced or deleted:

$ pass insert google.com/gmail/newexample@gmail.com
mkdir: created directory ‘/home/tom/.password-store/google.com’
mkdir: created directory ‘/home/tom/.password-store/google.com/gmail’
Enter password for google.com/gmail/newexample@gmail.com:
Retype password for google.com/gmail/newexample@gmail.com:
[master 00971b6] Added given password for google.com/gmail/newexample@gmail.com to store.
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 google.com/gmail/newexample@gmail.com.gpg

Backups

Because the password files are all encrypted only to your GnuPG key, you can relatively safely back up the store on remote and third-party sites simply by copying the ~/.password-store directory. If the filenames themselves contain sensitive information, such as private usernames or sites, you might like to back up an encrypted tarball of the store instead:

$ tar -cz .password-store \
    | gpg --sign --encrypt -r 0x77BB8872 \
    > password-store-backup.tar.gz.gpg

This directory can be restored in a similar way:

$ gpg --decrypt \
    < password-store-backup.tar.gz.gpg \
    | tar -xz 
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This entry is part 6 of 10 in the series Linux Crypto.