Contents


- Description
- Assets
- Notes
- Recipe
- Examples






Description


This recipe describes a method of using dice to generate entropy bytes.






Assets


Asset: A script that converts dice roll results into bytes.
convert_dice_rolls_to_hex_bytes_3.py [paywalled]




Notes


Using this recipe to generate 32 bytes of entropy (enough for a Bitcoin private key) should take about 10 minutes.

Suggestions:
- Roll the dice on a flat tray with raised edges to stop them from scattering out of reach. Add a dust cloth on top of the tray to muffle the sound of the dice rolls.
- After a roll, gather the dice into a line to make it easier to read the results and type them into a text file.
- Listen to music to make the dice rolling a more tolerable experience.

5 dice is just a convenient number of dice to roll repeatedly. This recipe will still work with a different number of dice.

Some dice roll result values will be discarded so that the value domain becomes base4, which can be then be converted to base16 (i.e. hex bytes). About 66% of the entropy will be preserved.

Previous version of this recipe:
Recipe for generating entropy bytes using dice

This recipe was originally published in the article Generating entropy with dice. Please read the linked article for more information about the development of the recipe, the algorithm behind it, its disadvantages, and suggestions for further work.

The code used in this recipe has been run in Python 2.7.5 on CentOS 7.6. It should run successfully in Python 2.7.x.








Recipe


1) Download the script convert_dice_rolls_to_hex_bytes_3.py. It is linked in the Assets section of this article.


2) Obtain 5 dice.


3) Choose a desired number of bytes of entropy. Multiply this number by 8 to convert to bits. Divide the result by the expected-bit-rate-per-dice-roll value (1.3333) to find the expected number of dice rolls that should generate this number of entropy bits. Multiply this result by 1.1 to add a 10% margin, then round up to the nearest integer.

Example: To get 32 bytes of entropy, you should do 212 dice rolls.

Calculations:



4) Perform this number of dice rolls and record the results in a text file. Save this file as "dice_rolls.txt" in the same directory as the script convert_dice_rolls_to_hex_bytes_3.py.

An individual dice roll must be recorded as one individual character from the list "123456". Whitespace (newline, tab, space) can be used to separate groups of dice roll values.

Easiest approach: Roll 5 dice at a time, record 5 dice rolls per line, and separate groups of 5 lines with an extra newline. Each group will then consist of 25 dice rolls, making it straightforward to count the total number of dice rolls.

Example group of 25 dice rolls in a text file:



5) In the script convert_dice_rolls_to_hex_bytes_3.py, scroll to the section of text that lies between

and

- Set the variable to be the desired number of bytes of entropy. By default it is 32.
- Set the variable to be the path to the text file containing the dice roll results. By default it is "dice_rolls.txt".


6) Open a terminal. Change directory to the directory containing this script. Run the script using the following command:



7) The output of the script should contain the generated entropy as hex bytes.


8) (optional) If more dice rolls are needed in order to reach the desired number of bytes of entropy, the script will calculate a suggested number of new dice rolls. Perform the new dice rolls, add the results to the dice roll results text file, and run the script again.








Examples


Here are 212 dice rolls, stored in a file.




Next, we run the script:




The desired result (32 bytes of entropy) is:





If we delete the last 5 groups of rolls, and keep only the first 4 (i.e. 100 rolls in total), and run the script again, we see: