How do you get the IV + SALT + Message from a hexadecimal string?

Report · Sep 15, 2021

Hello everyone,

I am trying to exchange data, via a REST service I wrote with a 3rd party. We are using AES Symmetric encryption, which means I need to get an IV and a SALT from a large hexadecimal encoded string, and use that SALT (with a shared password) to generate the apprpriate key to decrypt the message part of the hexadecimal string. My problem is coming with converting the hexadecimal string into a readable SALT.

Here is my current code designed to read the hexadecimal string. You will notice that I am merely taking pieces of the string off (as the representative characters of the bytes represented) using Left and Mid. I then attempt to binaryEncode the strings as hex, and then I binaryDecode them as base64. While this produces a SALT that is of the same character lenght as the correct SALT, it does not produce the correct SALT.

What would be the correct way to decode a hexadecimal string to extract an IV + SALT + Message in a way that I can convert them to readable formats for decrypt() and generatePBKDFKey().

                pwd = "REDACTED FOR SECURITY";
                encryptionAlgorithm = "AES/GCM/NoPadding";
                PBKDFalgorithm = "PBKDF2WithSHA256";
                str_len = Len(data);
                iter = 65536;
                key_size = 128;

                str_nonce = Left(data, 24); //gets the NONCE  try at 23
                str_salt = Mid(data, 25, 40); //gets the SALT

                   

                    salt_dec = binaryDecode(str_salt, 'hex'); //converts SALT string to a binary
                    salt_conv = binaryEncode(salt_dec, 'base64');  //converts SALT binary to a base64 string
                    msg_locate = ((str_len - 32)-64); //figures out how long the message is.

                     str_message = Mid(data, 65, msg_locate); //gets the message characters.
                    
                    hex_dec = binaryDecode(str_message, 'hex'); //converts the message string to a binary    
                    
                    string_conv = binaryEncode(hex_dec, 'base64'); // converts the message binary to a base64 string

                    str_tag = Right(data, 32);

                derivedKey = GeneratePBKDFKey(PBKDFalgorithm ,pwd,salt_conv,iter,key_size); //generates a decryption key with the above parameters.

Report · Sep 19, 2021

To start with, debug by outputting each string. You can then see whether each is what you expect.