BIP: 39 Layer: Applications Title: Mnemonic code for generating deterministic keys Author: Marek Palatinus
Pavol Rusnak Aaron Voisine Sean Bowe Comments-Summary: Unanimously Discourage for implementation Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0039 Status: Proposed Type: Standards Track Created: 2013-09-10
It consists of two parts: generating the mnemonic, and converting it into a binary seed. This seed can be later used to generate deterministic wallets using BIP-0032 or similar methods.
This guide is meant to be a way to transport computer-generated randomness with a human readable transcription. It's not a way to process user-created sentences (also known as brainwallets) into a wallet seed.
First, an initial entropy of ENT bits is generated. A checksum is generated by taking the first
ENT / 32bits of its SHA256 hash. This checksum is appended to the end of the initial entropy. Next, these concatenated bits are split into groups of 11 bits, each encoding a number from 0-2047, serving as an index into a wordlist. Finally, we convert these numbers into words and use the joined words as a mnemonic sentence.
The following table describes the relation between the initial entropy length (ENT), the checksum length (CS) and the length of the generated mnemonic sentence (MS) in words.
CS = ENT / 32 MS = (ENT CS) / 11
| ENT | CS | ENT CS | MS |------- ---- -------- ------| 128 | 4 | 132 | 12 | | 160 | 5 | 165 | 15 | | 192 | 6 | 198 | 18 | | 224 | 7 | 231 | 21 | | 256 | 8 | 264 | 24 |
a) smart selection of words
- the wordlist is created in such way that it's enough to type the first four letters to unambiguously identify the word
b) similar words avoided
- word pairs like "build" and "built", "woman" and "women", or "quick" and "quickly" not only make remembering the sentence difficult, but are also more error prone and more difficult to guess
c) sorted wordlists
- the wordlist is sorted which allows for more efficient lookup of the code words (i.e. implementations can use binary search instead of linear search) - this also allows trie (a prefix tree) to be used, e.g. for better compression
The wordlist can contain native characters, but they must be encoded in UTF-8 using Normalization Form Compatibility Decomposition (NFKD).
To create a binary seed from the mnemonic, we use the PBKDF2 function with a mnemonic sentence (in UTF-8 NFKD) used as the password and the string "mnemonic" passphrase (again in UTF-8 NFKD) used as the salt. The iteration count is set to 2048 and HMAC-SHA512 is used as the pseudo-random function. The length of the derived key is 512 bits (= 64 bytes).
This seed can be later used to generate deterministic wallets using BIP-0032 or similar methods.
The conversion of the mnemonic sentence to a binary seed is completely independent from generating the sentence. This results in rather simple code; there are no constraints on sentence structure and clients are free to implement their own wordlists or even whole sentence generators, allowing for flexibility in wordlists for typo detection or other purposes.
Although using a mnemonic not generated by the algorithm described in "Generating the mnemonic" section is possible, this is not advised and software must compute a checksum for the mnemonic sentence using a wordlist and issue a warning if it is invalid.
The described method also provides plausible deniability, because every passphrase generates a valid seed (and thus a deterministic wallet) but only the correct one will make the desired wallet available.
Also see https://github.com/bip32JP/bip32JP.github.io/blob/master/test_JP_BIP39.json
(Japanese wordlist test with heavily normalized symbols as passphrase)
.NET C# (PCL):
.NET C# (PCL):