encrypt an ASCII message with an RSA public key; decrypt with the private key

decr-bash.sh

#!/bin/bash

if [ "$#" -lt 0 -o "$#" -gt 3 ];then
        echo -e "\nUsage:  decr  [<input file>]  [<RSA's d>]  [<RSA's n>]\n"
        echo -e 'input file is "ciphermessage" by default'
        echo -e 'private key is 5783-and-7031 by default\n'
        exit 1
fi

# default private key is 5783-and-7031
PRIVATE=$1                              #a.k.a. "d"
: "${PRIVATE:=5783}"
PRIMESPRODUCT=$2                        #a.k.a. "n"
: "${PRIMESPRODUCT:=7031}"

INFILE=$3
: "${INFILE:=ciphermessage}"

decrypt() {
        local c="$1" d="$2" n="$3" m=1 i
        for (( i=0; i<d; i++ )); do
                m=$(( m * c % n ))
        done
        # given its ascii value, return a letter
        printf "\\$(printf '%03o' $m)"
}

while read -r INCODE
do
#       asc=$( echo "d=$PRIVATE;n=$PRIMESPRODUCT;c=$INCODE;c^d%n" | bc )
#                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
#                                                ^^^^^^^^^
#                            private key         cipertext

        decrypt "$INCODE" "$PRIVATE" "$PRIMESPRODUCT"

done < "${INFILE}"
echo

encr-bash.sh

#!/bin/bash

if [ "$#" -lt 1 -o "$#" -gt 4 ];then
        echo -e "\nUsage:  encr <input file>  [<RSA's e>]  [<RSA's n>]  [<output file>]\n"
        echo -e 'output file is "ciphermessage" by default'
        echo -e 'output file receives one character, encrypted, per line\n'
        echo -e 'public key is 1943-and-7031 by default\n'
        exit 1
fi

INFILE=$( cat "$1" )

# default public key is 1943-and-7031
SOME_RELATIVE_PRIME=$2                  #a.k.a. "e"
: "${SOME_RELATIVE_PRIME:=1943}"
PRIMESPRODUCT=$3                        #a.k.a. "n"
: "${PRIMESPRODUCT:=7031}"

OUTFILE=$4
: "${OUTFILE:=ciphermessage}"

len=${#INFILE}
> "${OUTFILE}"

encrypt() {
        local m="$1" e="$2" n="$3" c=1 i
        for (( i=0; i<e; i++ )); do
                c=$(( c * m % n ))
        done
        echo $c
}

i=0
while [ "$i" -lt "$len" ]
do
        letter=${INFILE:i:1}

        # given a letter, returns its ascii value
        #  this obscure systax of printf relies on the single quote to work
        asc=$(printf "%d" "'${letter}")

        #echo "$letter becomes $asc"

        #echo "e=$SOME_RELATIVE_PRIME;n=$PRIMESPRODUCT;m=$asc;m^e%n" | bc >>  ${OUTFILE}
#             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
#                   together, the public key          ^^^^^^
#                                                    cleartext
        encrypt "$asc" "$SOME_RELATIVE_PRIME" "$PRIMESPRODUCT" >> "$OUTFILE"

        let i=i+1
done

[cooper@sputnik ~]$ time ./decr-bash 216835 278441 /home/common/ciphermessage-cooper
VAP

real 0m22.524s
user 0m20.746s
sys 0m1.543s

[cooper@sputnik ~]$ time ./decr 216835 278441 /home/common/ciphermessage-cooper
VAP

real 3m44.986s
user 3m42.710s
sys 0m0.111s

[cooper@sputnik ~]$ time ./encr-bash tempin 15811 278441 tempout

real 0m1.605s
user 0m1.432s
sys 0m0.155s

[cooper@sputnik ~]$ time ./encr tempin 15811 278441 tempout

real 0m0.668s
user 0m0.631s
sys 0m0.022s