Potential fix for unbound variable in MacOS (tests)

- Use `:+` bash parameter expansion to include pbkdf2 argument to
  openssl only if variable is set
- Simplify variable `pbkdf2_arg` from list to string, since the `[@]`
  referencing doesn't work in all cases for MacOS (at least not
  for the unit tests): an empty list errors with `unbound variable`

Author: jmurty

transcrypt

@@ -81,20 +81,20 @@ _openssl_encrypt() {
 		(
 			printf "Salted__" && printf "%s" "$final_salt" | xxd -r -p &&
 				# Encrypt file to binary ciphertext
-				ENC_PASS=$password "$openssl_path" enc -e "-${cipher}" -md "${digest}" -pass env:ENC_PASS -S "$final_salt" "${pbkdf2_args[@]}" -in "$tempfile"
+				ENC_PASS=$password "$openssl_path" enc -e "-${cipher}" -md "${digest}" -pass env:ENC_PASS -S "$final_salt" ${pbkdf2_arg:+"$pbkdf2_arg"} -in "$tempfile"
 		) |
 			base64
 	else
 		# Encrypt file to base64 ciphertext
-		ENC_PASS=$password "$openssl_path" enc -e -a "-${cipher}" -md "${digest}" -pass env:ENC_PASS -S "$final_salt" "${pbkdf2_args[@]}" -in "$tempfile"
+		ENC_PASS=$password "$openssl_path" enc -e -a "-${cipher}" -md "${digest}" -pass env:ENC_PASS -S "$final_salt" ${pbkdf2_arg:+"$pbkdf2_arg"} -in "$tempfile"
 	fi
 }
 
 _openssl_decrypt() {
 	# Expects that the following variables are set:
-	# password, openssl_path, cipher, digest, pbkdf2_args
+	# password, openssl_path, cipher, digest, pbkdf2_arg
 	# This works the same across openssl versions
-	ENC_PASS=$password "$openssl_path" enc -d "-${cipher}" -md "${digest}" -pass env:ENC_PASS -a "$@" "${pbkdf2_args[@]}"
+	ENC_PASS=$password "$openssl_path" enc -d "-${cipher}" -md "${digest}" -pass env:ENC_PASS -a ${pbkdf2_arg:+"$pbkdf2_arg"} "$@"
 }
 
 # compatible openssl list command
@@ -251,9 +251,7 @@ _load_vars_for_encryption() {
 	_load_transcrypt_config_vars
 
 	if [[ "$kdf" == "1" ]] || [[ "$kdf" == "pbkdf2" ]]; then
-		pbkdf2_args=('-pbkdf2')
-	else
-		pbkdf2_args=()
+		pbkdf2_arg='-pbkdf2'
 	fi
 
 	if [[ "$salt_method" == "password" ]]; then