Tutorial: setting up derivepassphrase vault for three accounts, with a master passphrase

The scenario

In this tutorial, we will setup derivepassphrase for three services, using a master passphrase and the standard vault passphrase derivation scheme. We will assume the following three services with the following passphrase policies:

• email account

  ---

  • between 12 and 20 characters
  • no spaces
  • 1 upper case letter, 1 lower case letter, 1 digit
  • no character may appear 3 times (or more) in a row

• bank account

  ---

  • only digits
  • exactly 5 digits
  • an additional one-time password via a hardware token (“two-factor authentication”)

• work account

  ---

  • exactly 8 characters
  • no spaces
  • 1 special character, 1 letter, 1 digit
  • must be changed every quarter (January, April, July and October) to a different value (“passphrase rotation” or “rollover”)
  • must actually be different from the previous two passphrases

Installing derivepassphrase

You will need Python 3, and a package installer such as pip (bundled with Python), pipx or similar.

Info: Current supported Python versions

---

pippipx

With pip, using a “virtual enviroment” at ~/.venv to avoid clobbering our system configuration:

$ python3 -m venv ~/.venv
$ . ~/.venv/bin/activate
$ pip install derivepassphrase

$ pipx install derivepassphrase

---

Check that the installation was successful.

$ devirepassphrase --version
derivepassphrase, version 0.3.0

(…or similar output.)

Info: Current derivepassphrase version

Choosing a master passphrase

derivepassphrase uses a master passphrase MP, and derives all other passphrases P from MP. We shall choose the master passphrase: I am an insecure master passphrase, but easy to type.

Setting up the email account

In derivepassphrase, each passphrase configuration contains a service name, which is how derivepassphrase distinguishes between configurations. This service name can be chosen freely, but the resulting passphrase depends on the chosen service name. For our email account, we choose the straightforward service name email.

We need to translate the passphrase policy into options for derivepassphrase:

• A policy “(at least) n lower case letters” translates to the option --lower n, for any n > 0. Upper case letters (--upper), digits (--number), symbols (--symbol), spaces (--space) and dashes (--dash) work similarly.
• A policy “spaces forbidden” translates to the option --space 0. Again, other character classes behave similarly.
• A policy “no character may appear n times (or more) in a row” translates to the option --repeat (n − 1), for any n > 1. In particular, --repeat 1 means no character may be immediately repeated. (See the mnemonic below.)
• A policy “between n and m characters long” translates to --length k, for any choice of k which satisfies n ≤ k ≤ m. (derivepassphrase does not explicitly choose k for you.)

Mnemonic: the --repeat option

The --repeat option denotes the total number of consecutive occurrences of the same character. Or alternatively: if you request --repeat n, then derivepassphrase will avoid deriving any passphrase that repeats a character another n times.

Examples:

option	valid examples	invalid examples
--repeat 1	abc, aba, abcabc	aa, abba, ababb
--repeat 4	122333111123, 4444	55555, 67788888999996
--repeat 11	01234567899999999999	$$$$$$$$$$$$$$$$$$$$$$$

For the email service, we choose passphrase length 12. This leads to the command-line options --length 12 --space 0 --upper 1 --lower 1 --number 1 --repeat 3. Because we are using a master passphrase, we also need the -p option.

Note: interactive input

In code listings, sections enclosed in [[...]] signify input to the program, for you to type or paste in.

Also, it is normal for passphrase prompts to not “echo” the text you type in.

$ derivepassphrase vault --length 12 --space 0 --upper 1 --lower 1 \
>                        --number 1 --repeat 3 -p email
Passphrase: [[I am an insecure master passphrase, but easy to type.]]
kEFwoD=C?@+7

By design, we can re-generate the same passphrase using the same input to derivepassphrase:

$ derivepassphrase vault --length 12 --space 0 --upper 1 --lower 1 \
>                        --number 1 --repeat 3 -p email
Passphrase: [[I am an insecure master passphrase, but easy to type.]]
kEFwoD=C?@+7

We can then visit our email provider and change the passphrase to kEFwoD=C?@+7.

Storing the settings to disk

Because it is tedious to memorize and type in the correct settings to re-generate this passphrase, derivepassphrase can optionally store these settings, using the --config option.

$ derivepassphrase vault --config --length 12 --space 0 --upper 1 --lower 1 \
>                        --number 1 --repeat 3 email

Warning: -p and --config

Do not use the -p and the --config options together to store the master passphrase! The configuration is assumed to not contain sensitive contents and is not encrypted, so your master passphrase is then visible to anyone with appropriate privileges!

Check that the settings are stored correctly:

$ derivepassphrase vault --export -
{"services": {"email": {"length": 12, "repeat": 3, "lower": 1, "upper": 1, "number": 1, "space": 0}}}

Once the settings are stored, only the service name and the master passphrase option are necessary:

$ derivepassphrase vault -p email
Passphrase: [[I am an insecure master passphrase, but easy to type.]]
kEFwoD=C?@+7

Setting up the bank account

We choose the straightforward service name bank. The passphrase policy leads to the command-line options --length 5 --lower 0 --upper 0 --number 5 --space 0 --dash 0 --symbol 0.

The additional one-time password is generated by the hardware token, and therefore out of the scope for derivepassphrase.

The rest is similar to the email account: we configure our stored settings, generate the passphrase, and request the bank change the account passphrase to match the generated passphrase.

$ derivepassphrase vault --config --length 5 --lower 0 --upper 0 --number 5 \
>                        --space 0 --dash 0 --symbol 0 bank
$ derivepassphrase vault -p bank
Passphrase: [[I am an insecure master passphrase, but easy to type.]]
98517

Setting up the work account

We first take care of the first two constraints (passphrase length and permitted/required characters), then deal with the passphrase change/reuse aspects afterwards. Again, we start with the straightforward service name work, we choose “upper case letters” to fulfill the “1 letter” requirement, and add the options --length 8 --space 0 --symbol 1 --upper 1 --number 1.

$ derivepassphrase vault --length 8 --space 0 --symbol 1 --upper 1 --number 1 \
>                        -p work
Passphrase: [[I am an insecure master passphrase, but easy to type.]]
r?9\XQR&

Then we attempt to set the work passphrase to r?9\XQR&… but our employer’s identity management system returns an error: illegal character: &. What happened?

Complication 1: What is a (permitted) “special character”?

derivepassphrase considers the characters !"#$%&'()*+,./:;<=>?@[\]^{|}~-_' to be permitted special characters. Other service providers may permit other characters (quite rare) or fewer characters (quite common). (Service providers may also not explicitly say which special characters they permit, except through trial and error.)

Further reading

→ How to deal with “supported” and “unsupported” special characters (TODO)

For this case specifically, we restrict ourselves to the dashes as the only permitted special characters, and hope that this passes their passphrase policy.

$ derivepassphrase vault --length 8 --space 0 --symbol 0 --dash 1 \
>                        --upper 1 --number 1 -p work
Passphrase: [[I am an insecure master passphrase, but easy to type.]]
it90-HPO

This works. For now.

Complication 2: How to implement passphrase rotation?

derivepassphrase can only ever derive one passphrase per configuration, so passphrase rotation cannot be accomplished by reusing the same configuration. So some part of the configuration—generally the service name—needs to change upon each rotation.

Further reading

→ How to deal with regular passphrase rotation (TODO)

We choose to append a very coarse timestamp to the “base” service name work: the 4-digit year, a Q, and the “quarter” number (1, 2, 3 or 4). As of October 2024, this leads to the final service name work-2024Q4.

$ derivepassphrase vault --config --length 8 --space 0 --symbol 0 --dash 1 \
>                        --upper 1 --number 1 work-2024Q4
$ derivepassphrase vault -p work-2024Q4
Passphrase: [[I am an insecure master passphrase, but easy to type.]]
-P268G0A

Summary

We have installed derivepassphrase and set up three accounts for use with the vault passphrase derivation scheme, and the master passphrase I am an insecure master passphrase, but easy to type.. Our configuration should look like this:

$ derivepassphrase vault --export -
{"services": {"email": {"length": 12, "repeat": 3, "lower": 1, "upper": 1, "number": 1, "space": 0}, "bank": {"length": 5, "lower": 0, "upper": 0, "number": 5, "space": 0, "dash": 0, "symbol": 0}, "work-2024Q4": {"length": 8, "upper": 1, "number": 1, "space": 0, "dash": 1, "symbol": 0}}}

We should also get the following output when asking for those passphrases again:

$ derivepassphrase vault -p email
Passphrase: [[I am an insecure master passphrase, but easy to type.]]
kEFwoD=C?@+7
$ derivepassphrase vault -p bank
Passphrase: [[I am an insecure master passphrase, but easy to type.]]
98517
$ derivepassphrase vault -p work-2024Q4
Passphrase: [[I am an insecure master passphrase, but easy to type.]]
-P268G0A

This completes the tutorial.