create_passphrase.sh

#!/usr/bin/env bash

word_max=9 # Exclude words longer than this
nphrases=3 # Generate this many passphrases
defwords=4 # Default number of words/phrase

pls() {
  local -i n=${#1} r=$2
  local s
  printf -v s -- '  %s%*s%s\n'                                   \
         "$1"  2 '' '-h | --help   print this help text'         \
         "$1"  2 '' "[NUM [DICT]]  create $nphrases passphrases" \
         '  ' $n '' ' │    │'                                    \
         '  ' $n '' ' │    └────── dictionary file'              \
         '  ' $n '' " └─────────── words/phrase ($defwords)"
  printf 'Usage:\n%s' "$s"
  exit $r
}

if (( "$#" )); then
  case "$1" in
  '-h'|'--help') pls "${0##*/}" ;;
  *) (( "$1" > 0 )) || pls "${0##*/}" 1 ;;
  esac
fi

num_words="$1"
orig_dict="$2"
dict_file=''
ret_value=0

(( num_words )) || (( num_words = defwords ))

[[ "$orig_dict" ]] || {
  for sfx in '-small' '' '-large' '-huge' '-insane'; do
    orig_dict="/usr/share/dict/american-english$sfx"
    [[ ! -e "$orig_dict" ]] || break
    orig_dict="/usr/share/dict/british-english$sfx"
    [[ ! -e "$orig_dict" ]] || break
  done
}

[[ -e "$orig_dict" ]] || {
  echo 'No dictionary found' >&2
  exit 1
}

dict_file="$(mktemp -p /tmp tmp-dict.XXXXXXXX)"
LANG=C egrep "^[[:alpha:]]{1,$((word_max))}$" "$orig_dict" > "$dict_file"

dict_size=$(wc -l "$dict_file" 2>/dev/null | cut -d' ' -f1)
if (( dict_size < 1 )); then
  echo 'Empty dictionary' >&2
  ret_value=1
else
  if [[ -t 2 ]]; then
    B='\e[1;32m'; L='\e[38;5;158m'; E='\e[0;33m'; R='\e[38;5;177m'; N='\e[0m'
  else
    B=''; L=''; E=''; R=''; N=''
  fi

  entropy=$(bc -l <<< "scale=3; l($dict_size^$num_words)/l(2)")

  printf "$B[$N ${L}%s$E:$N $R%7s$N %s $B]$N\n" \
         'phrase size' "$num_words" 'words'     \
         ' dictionary' "$dict_size" 'words'     \
         '    entropy' "$entropy"   'bits '     >&2

  rand_mask='0xffffffff'

  while (( rand_mask >= dict_size )); do
    (( m = rand_mask >> 1 ))
    (( m >= dict_size )) || break
    rand_mask=$(printf '0x%x\n' "$m")
  done

  for (( i = 0; i < nphrases; i++ )); do
    (( n = num_words ))
    while (( n > 0 )); do
      R=$(od -N4 -tu4 -An -w4 /dev/urandom | tr -d ' ')
      R=$((R & rand_mask))
      (( R < dict_size )) || continue
      W=$(tail -n+$((R+1)) "$dict_file" | head -1)
      W="$(tr '[:lower:]' '[:upper:]' <<< ${W:0:1})${W:1}"
      echo -n "$W"
      (( n-- ))
    done
    echo
  done
fi

rm -f "$dict_file"

exit $ret_value