I have read that it's a best practice to encrypt the byte array of a string rather than the string itself.
Crytographic algorithms generally work on byte arrays or byte stream, so yes. You don't encrypt objects (strings) directly, you encrypt their byte representations.
Also, I have read that certain encryption algorithms expect each encrypted packet to be a fixed length in size. If the last packet to be encrypted isn't the required size, then encryption would fail.
This is an implementation detail of the particular encryption algorithm you choose. It really depends on what the API interface is to the algorithm.
Generally speaking, yes, crytographic algorithms will break input into fixed-size blocks. If the last block isn't full then they may pad the end with arbitrary bytes to get a full chunk. To distinguish between padded data and data which just happens to have what-look-like-padding bytes at the end, they'll prepend or append the length of the plain text to the byte stream.
This is the kind of detail that should not be left up to the user, and a good encryption library will take care of these details for you. Ideally you just want to feed in your plain text bytes and get encrypted bytes out on the other side.
Therefore it seems like a good idea to encrypt data that has first been converted into a fixed length, such as hex.
Converting bytes to hex doesn't make it fixed length. It doubles the size, but that's not fixed. It makes it ASCII-safe so it can be embedded into text files and e-mails easily, but that's not relevant here. (And Base64 is a better binary→ASCII encoding than hex anyways.)
In the interest of identifying best practices for ensuring compatibility with encrypting and decrypting data across different languages and platforms, I would like a critique on the following steps as a process:
- plain text string
- convert plain text string to byte array
convert byte array to hex
- encrypt hex to encrypted string
- encrypted string
- plain text byte array to encrypted byte array
- decrypt encrypted string to hex
- convert hex to byte array
- encrypted byte array
- decrypt encrypted byte array to plain text byte array
- convert byte array to plain text string
- plain text string
To encrypt, convert the plain text string into its byte representation and then encrypt these bytes. The result will be an encrypted byte array.
Transfer the byte array to the other program in the manner of your choosing.
To decrypt, decrypt the encrypted byte array into a plain text byte array. Construct your string from this byte array. Done.