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#!/bin/bash

# Date: 2020-01-20T16:34Z
#
# Author: Steven Baltakatei Sandoval (baltakatei.com)
#
# License: This bash script, `bknpass`, is licensed under GPLv3 or
# later by Steven Baltakatei Sandoval:
#
#    `bknpass`, an alphanumeric password generator
#    Copyright (C) 2020  Steven Baltakatei Sandoval (baltakatei.com)
#
#    This program is free software: you can redistribute it and/or modify
#    it under the terms of the GNU General Public License as published by
#    the Free Software Foundation, either version 3 of the License, or
#    any later version.
#
#    This program is distributed in the hope that it will be useful,
#    but WITHOUT ANY WARRANTY; without even the implied warranty of
#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#    GNU General Public License for more details.
#
#    A copy of the GNU General Public License may be found at
#    <https://www.gnu.org/licenses/>.
#
# Description: This bash script generates alphanumeric passphrases
# with a char-count determined by a user-provided number of bits of
# entropy. The passphrase is then outputted to stdout with a trailing
# newline. It works as follows:
# 
#   - Prompt user for an integer. This integer is the number of bits
#     of entropy that the generated password should have.
#
#   - Check if user-provided string is an integer using `bash` regular
#     expression test.
#
#   - Calculate the minimum number of alphanumeric characters required
#     to encode the specified number of bits of entropy.
#
#       - This step uses `bc` to calculate a logarithm float string
#         and `awk` to convert the float into an integer, rounding up.
#
#   - Use `tr`, `/dev/urandom`, and `head` to generate a random
#     alphanumeric string with the length calculated in the previous
#     step.
#
#   - Use `echo` to display the passphrase in stdout with a trailing
#     newline.
#
# Dependencies: bash, echo, bc, awk, tr, head. See end of file
#
# Tested on:
#
#   - GNU/Linux Debian 10


#==Initialization==

let ALPHABET_SIZE="26+26+10" # number of unique chars in [:alnum:], argument fed to `tr -c` in 'Generate passphrase' step)
LOG_BASE=2 # Set logarithm base to 2

# Define `echoerr` function which outputs text to stderr
    # Note: function copied from https://stackoverflow.com/a/2990533
function echoerr {
    echo "$@" 1>&2;
}

# Define `rpass` function which generates an alphanumeric passphrase of length $1 (ex: `rpass 22` generates a 22-char string)
    # Note: function adapted from https://www.thegeekstuff.com/2010/04/unix-bash-function-examples/
function rpass {
    cat /dev/urandom | LC_ALL=C tr -cd "[:alnum:]" | head -c ${1:-22}
}


#==Main Program==

# Define $ENTROPY_BIT_COUNT1 as argument $1 or prompt user if $1 is not defined.
    # note: argument test adapted from https://stackoverflow.com/a/6482403
if [ -z "$1" ]
then
    echo "Entropy bit count argument (\$1) not supplied."
    # Get from user the number of bits of entropy.
    echoerr -n "Please specify the required strength of the password in bits of entropy (ex: 256):" # prompt via stderr
    read ENTROPY_BIT_COUNT1
else
    ENTROPY_BIT_COUNT1="$1"
fi

# Check if $ENTROPY_BIT_COUNT1 is an non-negative integer
    # Note: Regular expression test is adapted from https://stackoverflow.com/a/806923
RETEST1='^[0-9]+$'
if ! [[ $ENTROPY_BIT_COUNT1 =~ $RETEST1 ]] ; then
   echo "error: Not an integer." >&2; exit 1    
fi

# Calculate minimum count of chars needed to encode $ENTROPY_BIT_COUNT1 with alphabet size of $ALPHABET_SIZE as float
    # Solve ln(a^n)/ln(2)=b for n using `bc` where
      # a=$ALPHABET_SIZE
      # n=$CHAR_COUNT1_FLOAT
      # b=$ENTROPY_BIT_COUNT1
    # Note: `bc` logarithm usage adapted from http://phodd.net/gnu-bc/bcfaq.html#bashlog
CHAR_COUNT1_FLOAT=$(echo "$ENTROPY_BIT_COUNT1*l($LOG_BASE)/l($ALPHABET_SIZE)" | bc -l)
    # Note: Float will be of form "21.49744370650136860806". This particular float should be rounded to "22" later.

# Round $CHAR_COUNT1_FLOAT1 up to next highest integer for use as argument in later bash functions.
    # Note: awk expression from https://bits.mdminhazulhaque.io/linux/round-number-in-bash-script.html
CHAR_COUNT1=$(echo "$CHAR_COUNT1_FLOAT" | awk '{print ($0-int($0)>0)?int($0)+1:int($0)}') 

# Generate passphrase
PASS1=$(rpass "$CHAR_COUNT1")
echo -e "$PASS1"


#==References==
#
# - How to echo a string as stderr instead of stdout.
#   https://stackoverflow.com/a/2990533
#   Author: James Roth
#   Date: 2010-06-07T14:52Z
#   Date Accessed: 2020-01-20
#
# - How to check if script argument exists or not.
#   https://stackoverflow.com/a/6482403
#   Author: phoxix
#   Date: 2011-06-26T05:55Z
#   Date Accessed: 2020-01-20
#
# - How to check that a string is an integer using regular expression test.
#   https://stackoverflow.com/a/806923
#   Author: Charles Duffy
#   Date: 2009-04-30T13:32Z
#   Date Accessed: 2020-01-20
#
# - How to use `bc` to calculate logarithms in Bash
#   http://phodd.net/gnu-bc/bcfaq.html#bashlog
#   Author: unknown
#   Date Accessed: 2020-01-20
#
# - How to use `awk` to convert and round up a float to an integer.
#   https://bits.mdminhazulhaque.io/linux/round-number-in-bash-script.html
#   Author: Md. Minhazul Haque
#   Date: 2015-01-09
#   Date Accessed: 2020-01-20
#
# - How to use `/dev/urandom`, `tr`, and `head` to generate a random password in Bash.
#   https://www.thegeekstuff.com/2010/04/unix-bash-function-examples/
#   Author: SASIKALA, Ramesh Natarajan
#   Date: 2010-04-21
#   Date Accessed: 2020-01-20
#
# - Dependencies: bash, echo, bc, awk, tr, head.
#
#     - GNU bash, version 5.0.3(1)-release (x86_64-pc-linux-gnu)
#       Copyright (C) 2019 Free Software Foundation, Inc.
#       License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
#       This is free software; you are free to change and redistribute it.
#       There is NO WARRANTY, to the extent permitted by law.
#    
#     - echo (GNU coreutils) 8.30
#       Copyright (C) 2018 Free Software Foundation, Inc.
#       License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
#       This is free software: you are free to change and redistribute it.
#       There is NO WARRANTY, to the extent permitted by law.
#
#       Written by Brian Fox and Chet Ramey.
#    
#     - bc 1.07.1
#       Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006, 2008, 2012-2017 Free Software Foundation, Inc.
#    
#     - GNU Awk 4.2.1, API: 2.0 (GNU MPFR 4.0.2, GNU MP 6.1.2)
#       Copyright (C) 1989, 1991-2018 Free Software Foundation.
#    
#       This program is free software; you can redistribute it and/or modify
#       it under the terms of the GNU General Public License as published by
#       the Free Software Foundation; either version 3 of the License, or
#       (at your option) any later version.
#    
#       This program is distributed in the hope that it will be useful,
#       but WITHOUT ANY WARRANTY; without even the implied warranty of
#       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#       GNU General Public License for more details.
#    
#       You should have received a copy of the GNU General Public License
#       along with this program. If not, see http://www.gnu.org/licenses/.
#    
#     - tr (GNU coreutils) 8.30
#       Copyright (C) 2018 Free Software Foundation, Inc.
#       License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
#       This is free software: you are free to change and redistribute it.
#       There is NO WARRANTY, to the extent permitted by law.
#    
#       Written by Jim Meyering.
#    
#     - head (GNU coreutils) 8.30
#       Copyright (C) 2018 Free Software Foundation, Inc.
#       License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
#       This is free software: you are free to change and redistribute it.
#       There is NO WARRANTY, to the extent permitted by law.
#    
#       Written by David MacKenzie and Jim Meyering.