Quick-and-dirty symmetric encryption

#!/bin/sh

# $Id: encrypt,v 1.3 2011/01/28 04:38:22 anomie $

# NOTE -
# Save this script in your users' PATH as
#       "encrypt"
# and create a symlink (or hard link) to it called
#       "decrypt"
#
# The script behaves according to the name it is invoked as. 


PATH=/bin:/usr/bin

# -------------------------------------------------------------------- #
# Variable assignments
# -------------------------------------------------------------------- #

_cipher=aes256


# -------------------------------------------------------------------- #
# Functions
# -------------------------------------------------------------------- #

infile_check() {

  if [ -z "${_infile}" ] ; then

    echo "Usage: ${0} input-file"
    exit 1

  fi

  if [ ! -f "${_infile}" ] || [ ! -r "${_infile}" ] ; then

    echo "To perform this operation, the input file:"
    echo "  1) must exist"
    echo "  2) must be a regular file;"
    echo "  3) and - you must have read access to it"
    exit 1

  fi

}

encrypt_file() {

  infile_check

  _outfile="$(basename ${_infile}).enc"

  if [ -e "${_outfile}" ] ; then

    echo "Error: ${_outfile} already exists"
    exit 1

  fi

  echo "Ready to encrypt ${_infile} using ${_cipher} cipher..." 

  openssl enc -${_cipher} -salt -in "${_infile}" \
      -out "${_outfile}" -e -a

  if [ ${?} -ne 0 ] ; then

    echo "Error: openssl reported a problem"
    exit 1

  fi

  echo "Encrypted output file is ${_outfile}"

}

decrypt_file() {

  infile_check 

  _outfile=$( basename "${_infile}" | sed 's/\.enc$//' )

  if [ "${_infile}" = "${_outfile}" ] ; then

    echo "Error: I only decrypt files with .enc extension"
    exit 1

  fi

  if [ -e "${_outfile}" ] ; then

    echo "Error: ${_outfile} already exists"
    exit 1

  fi

  echo "Ready to decrypt ${_infile} using ${_cipher} cipher..." 

  openssl enc -${_cipher} -in "${_infile}" \
      -out "${_outfile}" -d -a

  if [ ${?} -ne 0 ] ; then

    echo "Error: openssl reported a problem"
    exit 1

  fi

  echo "Decrypted output file is ${_outfile}"

}


# -------------------------------------------------------------------- #
# Main logic
# -------------------------------------------------------------------- #

_infile="${1}"

case "$(basename ${0})" in

  "encrypt" ) encrypt_file 
              ;;
  "decrypt" ) decrypt_file 
              ;;
  *         ) echo "Hmm, you invoked me as ${0}"
              echo "I expect to be invoked as 'encrypt' or 'decrypt'"
              exit 1
              ;; 
esac

exit 0