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Programming and Scripting => Scripting Languages => Topic started by: str0be on June 21, 2013, 09:54:10 AM

Title: [bash] Base64
Post by: str0be on June 21, 2013, 09:54:10 AM

Ok, so Base64 encoding/decoding in POSIX sh has been rolling around in my head for a bit and I've decided that its impossible! I started working with bash and I think I've made some progress:

Code: (bash) [Select]


#!/bin/bash


# array of standard base64 characters
CHARS=($(echo -n "A B C D E F G H I J K L M N O P Q R S T U V W X Y Z a b c \
       d e f g h i j k l m n o p q r s t u v w x y z 0 1 2 3 4 5 6 7 8 9 + /"))


# standard character used for padding
# XXX needs implementation
PAD="="


# return input as a string of space-separated decimal numbers
function to_dec {
    echo -n "$1" | hexdump -ve '/1 "%d "'
}


function encode {
    typeset -a -r bytes=($(to_dec "$1")) # Read-only array of input as numbers.
    typeset -i byte                      # For translating to a CHARS index.
    typeset -i bytei                     # Index to current byte of input.
    typeset -i chari                     # Index to char to translate to.
    typeset -i left=0                    # Bits from previous translation.
    typeset -i a                         # tmp
    typeset -i b                         # tmp
    typeset encoded=""                   # Output buffer.


    for ((bytei=0; bytei < ${#bytes[*]}; bytei++)); do
        if [ $left -eq 0 ]; then
            byte=${bytes[$bytei]}
            chari=$(( $byte >> 2 ))
            encoded=${encoded}${CHARS[$chari]}
            left=2
        elif [ $left -eq 6 ]; then
            byte=${bytes[$(( $bytei - 1 ))]}
            chari=$(( $byte & 63 ))
            encoded=${encoded}${CHARS[$chari]}
            byte=${bytes[$bytei]}
            chari=$(( $byte >> 2 ))
            encoded=${encoded}${CHARS[$chari]}
            left=2
        else
            a=$(( (${bytes[$(( $bytei - 1 ))]}) & (2 ** $left - 1) ))
            b=$(( (${bytes[$bytei]}) >> ($left + 2)  ))
            chari=$(( ($a << (6 - $left)) | $b ))
            encoded=${encoded}${CHARS[$chari]}
            left=$(( $left + 2 ))
        fi
    done


    if [ $left -ne 0 ];then
        byte=${bytes[$(( ${#bytes[*]} - 1 ))]}
        chari=$(( ($byte & (2 ** $left - 1)) << (6 - $left) ))
        encoded=${encoded}${CHARS[$chari]}
    fi


    echo -n "$encoded"
}


function decode {
    echo "Not Implemented"
}


if [ $# -eq 0 ]; then
    F=$(cat -)
    echo $(encode "$F")
else
    echo "Wrong args..."
fi

This doesn't seem to be compatible with GNU core utils Base64 but I can't figure out why...

I realize this doesn't have much practical value but I'm wondering if someone can come up with a POSIX sh way of doing things? Or show me where I'm going wrong with bash...

Title: Re: [bash] Base64
Post by: Deque on June 21, 2013, 09:04:33 PM

You don't take the newline into account. Your script translates "hello\n" the same way as it does with "hello".

The other thing that's wrong is the padding. Have another look into that.

Code: [Select]

deque@desolate:~/scripts$ base64
hello
aGVsbG8K
deque@desolate:~/scripts$ base64
helloaGVsbG8=
deque@desolate:~/scripts$ ./base.sh
hello
aGVsbG8
deque@desolate:~/scripts$ ./base.sh
helloaGVsbG8


Title: Re: [bash] Base64
Post by: str0be on June 26, 2013, 04:32:26 AM

Thanks for a pointer in the right direction. I hadn't even considered that 'encode' wasn't getting a pristine copy of the input. Turns out that process substitution silently truncates. After I discovered this I made a simple demonstration:

pitcher

Code: (bash) [Select]

#!/bin/bash

printf "This is output that needs to be

preserved exactly



"

catcher

Code: (bash) [Select]

#!/bin/bash

CATCH1=$(./pitcher)
echo CATCH1
echo -n $CATCH1

echo
echo
echo "---------------------------"
echo

CATCH2=$(./pitcher)
echo CATCH2
echo -n "$CATCH2"

echo
echo
echo "---------------------------"
echo

TMP=$(./pitcher; echo XXX)   # Add our own 'end of input' sentinel.
CATCH3="${TMP%XXX}"          # Del sentinel == exact copy of subprocess output.
echo CATCH3
echo -n "$CATCH3"

Output from executing catcher:

Code: [Select]

CATCH1
This is output that needs to be preserved exactly

---------------------------

CATCH2
This is output that needs to be

preserved exactly

---------------------------

CATCH3
This is output that needs to be

preserved exactly





Here's the updated script:

Code: (bash) [Select]

#!/bin/bash

# array of standard base64 characters
CHARS=($(echo -n "A B C D E F G H I J K L M N O P Q R S T U V W X Y Z a b c \
       d e f g h i j k l m n o p q r s t u v w x y z 0 1 2 3 4 5 6 7 8 9 + /"))

IFS=' '

# standard character used for padding
# XXX needs implementation
PAD="="

# return input as a string of space-separated decimal numbers
function to_dec {
    echo -n "$1" | hexdump -ve '/1 "%d "'
}

function encode {
    TMP=$(to_dec $1; echo XXX)
    typeset -a -r bytes=(${TMP%XXX})     # Read-only array of input as numbers.
    typeset -i byte                      # For translating to a CHARS index.
    typeset -i bytei                     # Index to current byte of input.
    typeset -i chari                     # Index to char to translate to.
    typeset -i left=0                    # Bits from previous translation.
    typeset -i a                         # tmp
    typeset -i b                         # tmp
    typeset encoded=""                   # Output buffer.

    for ((bytei=0; bytei < ${#bytes[*]}; bytei++)); do
        if [ $left -eq 0 ]; then
            byte=${bytes[$bytei]}
            chari=$(( $byte >> 2 ))
            encoded=${encoded}${CHARS[$chari]}
            left=2
        elif [ $left -eq 6 ]; then
            byte=${bytes[$(( $bytei - 1 ))]}
            chari=$(( $byte & 63 ))
            encoded=${encoded}${CHARS[$chari]}
            byte=${bytes[$bytei]}
            chari=$(( $byte >> 2 ))
            encoded=${encoded}${CHARS[$chari]}
            left=2
        else
            a=$(( (${bytes[$(( $bytei - 1 ))]}) & (2 ** $left - 1) ))
            b=$(( (${bytes[$bytei]}) >> ($left + 2)  ))
            chari=$(( ($a << (6 - $left)) | $b ))
            encoded=${encoded}${CHARS[$chari]}
            left=$(( $left + 2 ))
        fi
    done

    if [ $left -ne 0 ];then
        byte=${bytes[$(( ${#bytes[*]} - 1 ))]}
        chari=$(( ($byte & (2 ** $left - 1)) << (6 - $left) ))
        encoded=${encoded}${CHARS[$chari]}
    fi

    echo -n "$encoded"
}

function decode {
    echo "Not Implemented"
}

if [ $# -eq 0 ]; then
    TMP=$(cat -; echo -n XXX)
    F=${TMP%XXX}
    echo $(encode $F)
else
    echo "Wrong args..."
fi


Code: [Select]

$ echo hello | ./base64.sh | /usr/bin/base64 -d
hello

btw, I'm just ignoring proper padding until I get the core algorithms working. Thanks again.  ;D