Question

I found a “lua aes” solution on the web a while ago. And have some concern about its safety.

It states that:

— Do not use for real encryption, because the password is easily viewable while encrypting.

It says this at its “file encryption test” script.

My questions are:

Why is that, how is it any different from encrypting a string and writing it to a file?

How could it be viewable while encryption? Is it viewable after encryption too?

Basically, Is it safe to use or not?

Is there anyone who can confirm this who has used it? I mailed the original developer but the email address was invalid.

Should I be using it at all?

Answer 1

I assume there are two reasons why that recommendation was made:

1. Strings are immutable in Lua, so there is no way to overwrite a string with different data
   once it’s created.
2. In Lua, objects are garbage collected. The garbage collector runs only at certain points in
   the program, and the application has no way of telling when the garbage collector will run after there are no more references to the object. Until then, the password string will remain in memory by point 1.

See Java’s case, which is similar to Lua:

Why is char[] preferred over String for passwords?

As you can see there, using char arrays instead of strings is a better way to store passwords, since arrays are mutable and can be reinitialized to zero when done.

The closest Lua equivalent to a char array is a table filled with numbers. Here the password is stored as a table, rather than a string, where each element in the table consists of the integer representation of each character. For example, "pass" becomes {0x70,0x61,0x73,0x73}. After the table containing the password is used to encrypt or decrypt, it is filled with zeros before it’s unreachable by the program and eventually gets garbage collected.

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According to your comment, I may have misunderstood. Maybe the “file encryption test” stores the password in plain text along with the encrypted file, allowing anyone with access to the file, even attackers, the ability to trivially decrypt it. The points above still apply, though. This is still only a guess, however; I can’t know exactly what you mean unless you provide a link to the encryption library you mention.

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I’ve taken a look at the AES library and the concern about the password being “easily viewable” occurs because the user types the password in plain text, through the command line or terminal, in order to start the Lua program, even though the output of the program contains only cipher text. A slightly more secure way of providing the password would be not to show the input (as is done in sudo) or to mask the input with dots or stars (as is done in many Web pages).

Either that or the points given above are perhaps the only logical explanation.