Encrypting long text

Mark Waddingham mark at livecode.com
Thu Dec 16 11:45:38 EST 2021

On 2021-12-16 16:23, Sean Cole via use-livecode wrote:
> Hi all,
> I'm trying to use RSA to encrypt data from a text field like an address 
> or
> notes. When I try to use the encrypt command I get a result 'message 
> too
> long'. What is the method for encrypting long or large data?
> My current line of code:
>                encrypt tData using rsa with public key tMyKey and
> passphrase tMyPass

RSA encryption can only encrypt data up to a certain length (I can't 
remember off the top of my head the exact relation, but it is related to 
the size of the key) so it isn't designed to be used on arbitrary length 

Encrypting arbitrary length messages is the domain of symmetric 
encryption functions - like AES and friends.

The solution, therefore, is to combine the two:

   1) Generate a random (using randomBytes()) fixed length encryption key 

   2) Encrypt the actual data using a symmetric algorithm with FixedKey 
as password

   2) Use RSA to encrypt the (fixed length!) key FixedKey

   4) Make you message the RSA-encrypted FixedKey followed by the 
encrypted data

The RSA encrypted FixedKey will be a constant length, and thus you can 
just split that off of the combined data, decrypt it using RSA and then 
use the result to decrypt the payload.

Hope this helps!


Mark Waddingham ~ mark at livecode.com ~ http://www.livecode.com/
LiveCode: Everyone can create apps

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