I understand there are already few question here which are similar but mine is a bit different in that I want to split AES 256 bit into two 128 bit key and then use a different AES key of 128bit to encrypt the two 128 bit key for transport of the key between two processor. is this secure to do?
It only affords 128-bit security, because an attacker only needs to break the 128-bit transport key. Which is secure, but you don't gain any security from using a 256-bit key.
If you use two separate 128-bit keys, it actually gets a little bit tricky. If the attacker has some way of independently verifying that they correctly decrypted each half of the 256-bit key—for example, if the key encryption algorithm is authenticated—then they can decrypt by
- Finding the first 128-bit key by brute force ($2^{128}$ steps);
- Finding the second 128-bit key by brute force ($2^{128}$ steps);
which is $2^{128} + 2^{128} = 2 \times 2^{128} = 2^{129}$ steps, and thus you only get 129-bit security.
However, if the key encryption algorithm offers no way of verifying correct decryptions, and the 256-bit key is random, that attack doesn't work because there is no way of verifying at step #1 that you've got the correct key for the first half. The brute force algorithm then becomes:
- For each possible value (out of $2^{128})$ of the first key:
- For each possible value (out of $2^{128}$) of the second key:
- Try decrypting each transported key half with that combination, and then decrypting the message with the 256-bit key you get.
And this is $2^{128} \times 2^{128} = 2^{128 + 128} = 2^{256}$ steps, and no better than just attacking the 256-bit key by brute force.
The encryption algorithms normally recommended these days are called "AEADs" (authenticated encryption with associated data), which would fall into the first category and thus you'd only get 129-bit strength. You'd have to use older, non-AEAD algorithms to transport the key halves.
