Steganografie

Deze cursus is gearchiveerd · Deze cursus is nu afgesloten. Je kunt je niet meer registreren of nieuwe oplossingen indienen, maar je eerdere werk en resultaten blijven beschikbaar.

decToChar () {
  # converts single-byte decimal value to equivalent character
  # arg: decimal number from 0-255
  # out: one character
  echo -n -e "\x$(printf %02X "$1")"
}

readDecimal() {
  # Read the n'th decimal from a file. This function looks at the 2 least significant bits of 4 consecutive bytes in
  # the given file and pastes them together to decode the decimal.
  # arg1: decimal offset
  # arg2: filename from which the given decimal should be read and decoded.

  readcharDec () {
    # read one character from file & convert to equivalent decimal value
    # arg1: offset (# of bytes to skip before reading)
    # arg2: filename
    #  out: decimal value from 0-255
    VAL=$(xxd -p -l1 -s $1 $2)
    printf %d "0x$VAL"
  }

  readLSB2 () {
    # read the 2 LSB's from the byte at the given position in the given file.
    # arg1: byte offset
    # arg2: filename
    READ_BYTE=$(readcharDec $1 $2)
    echo $(( READ_BYTE & 3 ))
  }

  CURRENT_DECIMAL=0
  IDX=$1
  FILE=$2

  for ((j = 3; j > 0; j--)); do
    CURRENT_VAL=$(readLSB2 $(( IDX * 4 + j )) $FILE)
    CURRENT_DECIMAL=$(( (CURRENT_DECIMAL | CURRENT_VAL) << 2 ))
  done

  CURRENT_VAL=$(readLSB2 $(( IDX * 4 )) $FILE)
  CURRENT_DECIMAL=$(( (CURRENT_DECIMAL | CURRENT_VAL) ))
  echo "$CURRENT_DECIMAL"
}

Je kunt zo vaak indienen als je wenst. Er wordt enkel rekening gehouden met je laatst ingediende oplossing.