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