AES-256 Encryption Best Practices

Brian Milby brian at
Tue Jul 3 00:57:17 EDT 2018

I would suggest using "randombytes" instead of "random" on desktop/server
(according to dictionary is isn't available in mobile, but I have not
actually verified).  That uses the openssl library to generate random
numbers.  The problem with using an IV based on a pseudorandom number
generator seeded from something derived from the time means that it is
potentially predictable.

I was playing around with a function to generate an IV that is guaranteed
to not repeat.  The middle 4 bytes are the seconds, so it reduces the
randomness by 4 bytes.  I'm not sure how much of an issue that would be.
It does avoid the birthday problem (which should not really be an issue
with a good random number generator I would guess).  Maintaining your own
counter would be another option.  Ensuring uniqueness and unpredictability
is the goal.

One other thing that I was reading is that we should also include a (H)MAC
after the encryption to ensure that the payload is not tampered with.  We
would then only decrypt if the message had not been changed (and the IV
would be included in the MAC calculation).

Below is the code that I was experimenting with:

function generateIV pLength
   local tSeconds, tBytes

   put randomBytes(6) into tBytes
   put the seconds into tSeconds
   repeat until tSeconds < 256
      put numToByte(tSeconds mod 256) after tBytes
      put tSeconds div 256 into tSeconds
   end repeat
   put numToByte(tSeconds) after tBytes

   if pLength is empty then put 16 into pLength
   subtract length(tBytes) from pLength
   if pLength < 0 then
      delete byte 1 to (- pLength) of tBytes
      put randomBytes(pLength) after tBytes
   end if
   return tBytes
end generateIV

On Mon, Jul 2, 2018 at 10:37 PM, William Prothero via use-livecode <
use-livecode at> wrote:

> Folks:
> I’ve been working on a sample stack to demonstrate encryption, best
> practices (as far as I can determine).
> The online lessons are not adequate for a robust solution to this vital
> security issue. I’ve posted a demo stack at:
> <http://
>>  This stack has
> benefited from feedback and ideas from folks on this list. Feedback is
> welcome.
> This stack generates a random iv vector and uses AES-256 encryption to
> encode an array containing commands for interaction with a mySQL server.
> The server side php script that decodes the data and encodes the returned
> response is included.
> On thing I am still unsure about is the best way to generate a random
> string of characters that I use for the random IV (initialization vector)
> that is used for the encryption. I’ve included some code below, which is
> used to encrypt and decrypt the data sent and returned from the server. The
> encode and decode scripts are put into the launcher, or stack that is
> created when a standalone or mobile version is built.
> Here are the handlers. The encryption key will be more secure if it is
> obfuscated by putting it in as a property of a control or hidden in some
> way. I am wondering if the generation of the random seed is optimum.
> Feedback welcome.
> local theRandomSeed
> function randomChrs n
>    if theRandomSeed = "" then
>       setRandomSeed
>    end if
>    put "" into tChars
>    repeat with i=1 to n
>       put random(256) into nChar
>       put numToNativeChar(nChar) after tChars
>    end repeat
>    return tChars
> end randomChrs
> on setRandomSeed
>    put (the milliseconds) into tMS
>    put trunc(tMs/10000000) into tDiv
>    put tMS mod tDiv into theRandomSeed
>    set the randomseed to theRandomSeed
> end setRandomSeed
> function theRandomIV
>    if theRandomSeed = "" then
>       setRandomSeed
>    end if
>    put randomChrs(16) into tIVBytes
>    return tIVBytes
> end theRandomIV
> --This handler encodes the data. First it generates a random
> --initialization vector (iv), then encrypts the data and puts
> --adds iv to the encoded data.
> --tArray is an array that controls the action of the php script.
> function theEncoded tArray
>    put  theRandomIV() into tIV
>    put base64Encode(tIV) into tB64IV
>    put ArrayToJSON(tArray,"string”,”") into tJson
>    put "AES-256-CTR" into tCipher
>    encrypt tJson using tCipher with key tEncryptionKey and iV tIV
>    put base64encode(it) into tDataToSend
>    --comment out next statement if iv not included in data
>    put tB64IV&tDataToSend into tDataToSend
>    return tDataToSend
> end theEncoded
> --This decodes the data that is returned by the php on the
> --remote server.
> --The iv is expected as the first 24 bytes of the returned data.
> function theDecoded tData
>    put byte 1 to 24 of tData into tIVB64
>    put base64decode(tIVB64) into tIV
>    put the number of bytes in tData into n
>    put byte 25 to n of tData into tRetB64Data
>    put base64decode(tRetB64Data) into tRetData
>    put "AES-256-CTR" into tCipher
>    decrypt tRetData using tCipher with key tEncryptionKey and iV tIV
>    put it into tReturn
>    return tReturn
> end theDecoded
> -- End of handlers that should be in the main stack
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