base64_encode.sh

#!/bin/bash

# file            base64_encode.sh
# author          mknod @ freenode
# license         none
# bash version    3.2+
# description     a beautifully slow implementation of base64 encoding in pure Bash
# synopsis        ./base64_encode.sh
# stdin           raw data
# stdout          base64 encoded data
# exit status     0

# variables

base64_table=({A..Z} {a..z} {0..9} "+" "/")
base64_padding_char="="

# routines

base64_encode() {

      local d="" byte_rem="3" word="0" \
            LC_CTYPE="C" IFS=""

      while read -r -d "" -n 1; do

            ((byte_rem--))

            printf -v d "%d" "'$REPLY"
            ((word |= (d < 0 ? 256+d : d) << byte_rem*8))

            if ((byte_rem == 0)); then

                  printf '%s' "${base64_table[word >> 18]}"        \
                              "${base64_table[word >> 12 & 0x3F]}" \
                              "${base64_table[word >> 6 & 0x3F]}"  \
                              "${base64_table[word & 0x3F]}"

                  word="0" byte_rem="3"
            fi
      done

      if ((byte_rem == 2)); then
            printf '%s'  "${base64_table[word >> 18]}"        \
                         "${base64_table[word >> 12 & 0x3F]}" \
                         "$base64_padding_char"                      \
                         "$base64_padding_char"
      elif ((byte_rem == 1)); then
            printf '%s' "${base64_table[word >> 18]}"        \
                        "${base64_table[word >> 12 & 0x3F]}" \
                        "${base64_table[word >> 6 & 0x3F]}"  \
                        "$base64_padding_char"
      fi
}

# main

base64_encode