# Function to quickly rotate ASCII characters by 90 places
# emulates charXor in a lazy way but is infinitely faster
charRotate() {
  # shellcheck disable=SC2020
  tr ' !"#$%&()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~' 'z{|}~ !"#$%&()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxy'
}

# xor function derived from http://zurlinux.com/?p=712
charXor() {
  while read -r line; do
    # Start by removing whitespace to reduce character bias
    # pipe to 'od' which converts each character to one octal per line
    for i in $(printf "%s" "${line}" | tr -d " " | od -A n -t o1 -w1 -v); do
      # for each octal, run an xor and then print it
      # shellcheck disable=SC2059
      printf \\"$(printf '%03o' "$(( i ^ 90 ))" )"
    done
  done
}

# Redacted stuff

  # Check we have the required binaries
  ErrCount=0
  for Prog in od tr tail head; do
    if ! command -v "${Prog}" >/dev/null 2>&1; then
      printf "%s\n" "[ERROR]: '${Prog}' is required but was not found."
      ErrCount=$(( ErrCount + 1 ))
    fi
  done

  # If any packages are missing, then ErrCount's going
  # to be greater than 0, in which case, exit.
  if [ "${ErrCount}" -gt 0 ]; then
    exit 1
  fi

  # Let's try and find a hashing method, we use this sparingly for now
  for cryptopt in sha256sum sha1sum shasum md5sum digest csum cksum; do
    if command -v "${cryptopt}" > /dev/null 2>&1; then
      crypt="${cryptopt}"
      if [ "${crypt}" = digest ]; then
        crypt="digest -v -a md5"
      fi
      break
    else
      printf "%s\n" "[ERROR]: no hash method is available"
      exit 1
    fi
  done

  # We need some "entropy", so let's first check for an existing entropy file from
  # a previous invocation.  If found, we want to rotate its characters
  if [ -f /tmp/entropy ]; then
    charRotate < /tmp/entropy > /tmp/entropy2
    mv /tmp/entropy2 /tmp/entropy

  # Otherwise, let's generate some "entropy" by simply running through this list of commands
  # And trying to remove as much whitespace as possible, then do a fast 'xor'
  # Some of these may fail, so we redirect stderr into the stream anyway.  Throw it all at the wall, see what sticks.
  else
    {
      ps aux
      printf "%s" "$$"
      top -n 1
      date '+%H%M%S'
      df -k
      printf "%s" "$$"
      netstat -a
      date '+%H%M%S'
      vmstat -i
      printf "%s" "$$"
      vmstat -s
      date '+%H%M%S'
      iostat
      printf "%s" "$$"
      pstat -afipSsT
    } 2>&1 | tr -d " " | tr -d "\n" | charRotate > /tmp/entropy
  fi

# Redacted stuff

    # Now we have to churn the entropy file.  We remove the leading 32 Bytes
    # of the entropy file, and temporarily store the result
    tail -c +33 /tmp/entropy | tr -d "\n" > /tmp/entropy2

    {
      # Seed some more data into the temporary entropy file, here we hash the PID
      printf "%s" "$$" | charXor | tr -d " " | tr -d "\n" | "${crypt}"
      # Rotate the entropy, get the first 64 chars, xor it to the bottom of the temp file
      printf "%s" "$(fold -b -w 64 /tmp/entropy | head -n 1 | charXor)"
      date '+%H%M%S' | charXor | tr -d " " | tr -d "\n"
    } >> /tmp/entropy2

    # Move the temporary file into place
    mv /tmp/entropy2 /tmp/entropy