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#!/bin/bash
# gronsfeld.bash
# License: GPL3
# Reldate 06/23/11
# This is an implementation of the Gronsfeld Cipher.
# It's essentially a stripped-down variant of the
#+ polyalphabetic Vigenère Tableau, but with only 10 alphabets.
# The classic Gronsfeld has a numeric sequence as the key word,
#+ but here we substitute a letter string, for ease of use.
# Allegedly, this cipher was invented by the eponymous Count Gronsfeld
#+ in the 17th Century. It was at one time considered to be unbreakable.
# Note that this is ###not### a secure cipher by modern standards.
# Global Variables #
Enc_suffix="29379" # Encrypted text output with this 5-digit suffix.
# This functions as a decryption flag,
#+ and when used to generate passwords adds security.
Default_key="gronsfeldk"
# The script uses this if key not entered below
# (at "Keychain").
# Change the above two values frequently
#+ for added security.
GROUPLEN=5 # Output in groups of 5 letters, per tradition.
alpha1=( abcdefghijklmnopqrstuvwxyz )
alpha2=( {A..Z} ) # Output in all caps, per tradition.
# Use alpha2=( {a..z} ) for password generator.
wraplen=26 # Wrap around if past end of alphabet.
dflag= # Decrypt flag (set if $Enc_suffix present).
E_NOARGS=76 # Missing command-line args?
DEBUG=77 # Debugging flag.
declare -a offsets # This array holds the numeric shift values for
#+ encryption/decryption.
########Keychain#########
key= ### Put key here!!!
# 10 characters!
#########################
# Function
: ()
{ # Encrypt or decrypt, depending on whether $dflag is set.
# Why ": ()" as a function name? Just to prove that it can be done.
local idx keydx mlen off1 shft
local plaintext="$1"
local mlen=${#plaintext}
for (( idx=0; idx<$mlen; idx++ ))
do
let "keydx = $idx % $keylen"
shft=${offsets[keydx]}
if [ -n "$dflag" ]
then # Decrypt!
let "off1 = $(expr index "${alpha1[*]}" ${plaintext:idx:1}) - $shft"
# Shift backward to decrypt.
else # Encrypt!
let "off1 = $(expr index "${alpha1[*]}" ${plaintext:idx:1}) + $shft"
# Shift forward to encrypt.
test $(( $idx % $GROUPLEN)) = 0 && echo -n " " # Groups of 5 letters.
# Comment out above line for output as a string without whitespace,
#+ for example, if using the script as a password generator.
fi
((off1--)) # Normalize. Why is this necessary?
if [ $off1 -lt 0 ]
then # Catch negative indices.
let "off1 += $wraplen"
fi
((off1 %= $wraplen)) # Wrap around if past end of alphabet.
echo -n "${alpha2[off1]}"
done
if [ -z "$dflag" ]
then
echo " $Enc_suffix"
# echo "$Enc_suffix" # For password generator.
else
echo
fi
} # End encrypt/decrypt function.
# int main () {
# Check for command-line args.
if [ -z "$1" ]
then
echo "Usage: $0 TEXT TO ENCODE/DECODE"
exit $E_NOARGS
fi
if [ ${!#} == "$Enc_suffix" ]
# ^^^^^ Final command-line arg.
then
dflag=ON
echo -n "+" # Flag decrypted text with a "+" for easy ID.
fi
if [ -z "$key" ]
then
key="$Default_key" # "gronsfeldk" per above.
fi
keylen=${#key}
for (( idx=0; idx<$keylen; idx++ ))
do # Calculate shift values for encryption/decryption.
offsets[idx]=$(expr index "${alpha1[*]}" ${key:idx:1}) # Normalize.
((offsets[idx]--)) # Necessary because "expr index" starts at 1,
#+ whereas array count starts at 0.
# Generate array of numerical offsets corresponding to the key.
# There are simpler ways to accomplish this.
done
args=$(echo "$*" | sed -e 's/ //g' | tr A-Z a-z | sed -e 's/[0-9]//g')
# Remove whitespace and digits from command-line args.
# Can modify to also remove punctuation characters, if desired.
# Debug:
# echo "$args"; exit $DEBUG
: "$args" # Call the function named ":".
# : is a null operator, except . . . when it's a function name!
exit $? # } End-of-script
# ************************************************************** #
# This script can function as a password generator,
#+ with several minor mods, see above.
# That would allow an easy-to-remember password, even the word
#+ "password" itself, which encrypts to vrgfotvo29379
#+ a fairly secure password not susceptible to a dictionary attack.
# Or, you could use your own name (surely that's easy to remember!).
# For example, Bozo Bozeman encrypts to hfnbttdppkt29379.
# ************************************************************** #
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