Diceware is a method for creating passphrases, passwords, and other cryptographic variables using an ordinary dice as a hardware random number generator. For each word in the passphrase, five rolls of the dice are required. The numbers from 1 to 6 that come up in the rolls are assembled as a five-digit number, e.g. 43146. That number is then used to look up a word in a word list. In the English list 43146 corresponds to munch. By generating several words in sequence, a lengthy passphrase can be constructed.
Lists have been compiled for several languages, including Chinese, Danish, Dutch, English, Esperanto, Finnish, French, German, Italian, Japanese, Maori, Norwegian, Polish, Romanian, Russian, Spanish, Swedish and Turkish. A Diceware word list is any list of unique words, preferably ones the user will find easy to spell and to remember. The contents of the word list do not have to be protected or concealed in any way, as the security of a Diceware passphrase is in the number of words selected, and the number of words each selected word could be taken from.
The level of unpredictability of a Diceware passphrase can be easily calculated: each word adds 12.9 bits of entropy to the passphrase (that is, bits). Originally, in 1995, Diceware creator Arnold Reinhold considered five words (64 bits) the minimal length needed by average users. However, starting in 2014, Reinhold recommends that at least six words (77 bits) should be used.
This level of unpredictability assumes that a potential attacker knows that Diceware has been used to generate the passphrase, knows the particular word list used, and knows exactly how many words make up the passphrase. If the attacker has less information, the entropy can be greater than 12.9 bits per word.
The above calculations of the Diceware algorithm's entropy assume that, as recommended by Diceware's author, each word is separated by a space. If, instead, words are simply concatenated, the calculated entropy is slightly reduced due to redundancy; for example, the three-word Diceware phrases "in put clammy" and "input clam my" become identical if the spaces are removed.
- Brute-force attack
- Key size discusses how many bits of key are considered "secure".
- The PGP biometric word list uses two lists of 256 words, each word representing 8 bits.
- S/KEY uses a list of 2,048 words to encode 64-bit numbers as six English words
- Password strength
- Random password generator
- Brodkin, Jon (2014-03-27). "Diceware passwords now need six random words to thwart hackers". Ars Technica.
- Internet Secrets, 2nd Edition, John R. Levine, Editor, Chapter 37, IDG Books, 2000, ISBN 0-7645-3239-1
- English diceware page has the complete description and word lists in several languages.
- Diceware for Android
- A client-side diceware multi-wordlist password generator with complete source code
- Web-based diceware app that uses the cryptographically secure getRandomValues() function
- Diceware method for generating easy-to-remember and cryptographically strong passwords and passphrases