pwget

pwget is a stateless password manager, a tool that generates all your passwords from a single master password. Its main difference from similar tools is that it maintains a list of revoked passwords. You can revoke a password if a service that you use has had their password database leaked, or if you just like to change your passwords regularly.

Of course, one can argue that the revocation list makes pwget not-stateless-anymore, but I still consider it mostly stateless because the revocation list can always be recovered by starting from an empty list, generating passwords, and revoking those that don't work until you get to the first non-revoked password.

Security considerations

WARNING: I'm not a professional cryptographer. I deny all responsibility if you use this code and then lose your passwords or identity.

Having said that, I consider the main attack vector to be identity theft through sniffing the master password, e.g. via a keylogger. That attack vector exists for all password managers that use a master password.

But for a stateless password manager, a master password cannot easily be changed, unless you are willing to change all passwords derived from it. I don't see a practical difference, though: If your master password is compromised, you should always assume all derived passwords compromised as well, since the adversary has likely already obtained a copy of your password store (if your password manager maintains one).

In the end, the same advice as always holds true: Know your threat model, and carefully weigh security against utility.

Installation

# option 1
make
sudo make install

# option 2
make
ln -s $PWD/build/pwget ~/bin # or wherever you put your tools

Usage

To generate a password,

pwget $DOMAIN

where $DOMAIN identifies the service that you're trying to log on. It's your job to come up with a consistent scheme for these $DOMAIN values. pwget does not enforce anything. The following three, for example, will generate completely different passwords:

pwget example.com
pwget www.example.com
pwget johndoe@example.com

To revoke a password,

pwget --revoke $DOMAIN

Then generate the next password with pwget $DOMAIN. The list of revoked passwords is stored at $HOME/.pwget-revocation, which contains the SHA256 hash of every revoked password. pwget will always show a list of all revoked passwords that it encounters, so you can recover from an undesired revocation easily.

Maximum length restriction

Some stupid services enforce a maximum length of passwords. You should stop using these services, but if you absolutely cannot, you can pass the maximum password length as a second argument to truncate the generated password:

# stupidbank.com rejects passwords longer than 20 chars
pwget stupidbank.com 20

Algorithm

In pseudo-code:

func getPassword(masterPassword, domain) {
    for iteration in 0..infinity {
        salt = iteration + ":" + domain
        pw = scrypt(password, salt, N = 2^16, r = 8, p = 16, keylength = 32 bytes)
        if pw is not revoked {
            return z85_encode(pw)
        }
    }
}

Where scrypt() is the SCrypt key derivation function, and z85_encode() encodes the bytearray obtained from scrypt() to text using the Z85 encoding, a variant of Base85.

Encoding passwords in Z85 ensures that the overwhelming majority of passwords include lowercase letters, uppercase letters, numbers and symbols, like a lot of stupid websites require.