maghoff/pgen

Passphrase Generator

pgen(1) – Passphrase Generator

Generate passphrases using the wordlists for random passphrases made by the EFF.

The EFF wordlists consist of words that are easy to type and easy to remember.

By default, passphrases generated by pgen consist of twelve words randomly selected from the autocomplete-optimized wordlist. Be sure to read the article to learn about the difference between the different wordlists provided by the EFF.

Table of Contents

• Usage
  • Options
• How many bits of entropy does your passphrase need?
• Is a CSPRNG really needed here?
• Installation

Usage

pgen [--dice] [-l | -s] [-e] [-k <k>] [-n <n>]
pgen -h | --help
pgen -V | --version

Options

-l Use long wordlist instead of autocomplete-optimized short wordlist. Recommended for the creation of memorable passphrases since the increased number of words as well as the greater effective word length allows for good entropy with a lower amount of words compared to the autocomplete-optimized short wordlist. Mutually exclusive with option -s.

-s Use non-optimized short wordlist instead of autocomplete-optimized short wordlist. Mutually exclusive with option -l.

-e Calculate and print the entropy for the passphrase(s) that would be generated with the given settings. What is password entropy? Entropy is a measure of what the password could have been, so it relates to the selection process.

-k Specify the number of passphrases to generate k. Default value: 1.

-n Specify the number of words to use n. Default value:

• Twelve (12) words if either of the short wordlists are being used (meaning that the -l option was not specified).
• Ten (10) words if the large wordlist is being used (meaning that the -l option was specified.)

--dice Use physical six-sided dice instead of letting the computer pick words. Useful in case you distrust the ability or willingness of your computer to generate "sufficiently random" numbers. Even though pgen will do the right thing and use /dev/urandom by default on Unix platforms [1], what if the hardware source(s) for the entropy that the /dev/urandom CSPRNG is collecting is/are rigged? With the --dice option you need not worry about that at least. (But have you considered the risk of undetectable malware? [2], [3])

-h, --help Show help and exit.

-V, --version Print version information and exit.

How many bits of entropy does your passphrase need?

How many bits of entropy should your passphrase consist of?

Looking at the article about password strength on Wikipedia, you will find that the following is said:

The minimum number of bits of entropy needed for a password depends on the threat model for the given application. If key stretching is not used, passwords with more entropy are needed. RFC 4086, "Randomness Requirements for Security", presents some example threat models and how to calculate the entropy desired for each one. Their answers vary between 29 bits of entropy needed if only online attacks are expected, and up to 128 bits of entropy needed for important cryptographic keys used in applications like encryption where the password or key needs to be secure for a long period of time and stretching isn't applicable.

In the case of web services such as webmail, social networks, etc., given that historically we have seen password databases leaked, where weak hashing algorithms such as MD5 were used, it is the opinion of the author that the neighbourhood of 128 bits of entropy is in fact an appropriate default for such use.

When calculating the entropy of a password or a passphrase, one must assume that the password generation procedure is known to the attacker. Hence with 12 words from either of the short wordlists, each of which consist of 1296 words, we get a password entropy of log2(1296^12) ~= 124.08 bits. Similarily, with 10 words from the long wordlist (7776 words), we get a password entropy of log2(7776^10) ~= 129.25 bits.

Is a CSPRNG really needed here?

Using a CSPRNG ensures uniform distribution of probability. This in turn ensures that the password entropy calculations are correct. Hence it makes sense to use a CSPRNG.

Installation

1. Install Rust.
2. Run cargo install pgen