compute the checksum word knowing the first 11 words of a 12 word-mnemonic

bip39_mnemonic_computation.rb

require 'digest'

lang = "english"
mnemonic = "claim snack company sea faith vacant blame card come large add" # 11-word or 23-word mnemonic phrase to be completed with checksum word
wordlist = File.readlines(File.join(Dir.pwd, "#{lang}.txt"), chomp: true)

a = mnemonic.split(" ")
entropy = ""
 
a.each do |word|
  n = 0
  digits = ""
  while n < 2048 and word != wordlist[n]
    n += 1
  end
  if n == 2048
    puts "#{word} is not in bip39 #{lang} wordlist"
  else
    puts "#{word} is word #{n+1}"
  end
  digits = n.to_s(2).rjust(11, '0')
  entropy += digits
end

i = 0 
size = (a.count + 1)/3 # checksum size is 8 bits for 256-bit entropy and 4 bits for 128-bit entropy
m = 11 - size
puts "Valid checksum words:"
while i < 2 ** m # number of valid checksum words to complete a given 11-word or 23-word sentence
  s = i.to_s(2).rjust(m, '0')
  i += 1
  e = entropy + s
  sha256 = Digest::SHA256.digest([e].pack("B*"))
  checksum = sha256.unpack("B*").join[0..size-1]
  full = e + checksum
  r = full.scan(/.{11}/).last.to_i(2)
  word = wordlist[r]  # get the corresponding checksum word from wordlist
  puts "#{word} is word #{r+1}"
end