I am have one block of data to encrypt that is 32-bytes long. Is it better to us
ID: 652601 • Letter: I
Question
I am have one block of data to encrypt that is 32-bytes long. Is it better to use the AES-256 primitive for this single block (not a stream) or AES-256-CTR, and also would there be any cons or pros to using one and not the other such as to security or something else I am not thinking about.
Use AES-256 Or AES-CTR-256 For One Block?
In relation I was wondering if I should just use the AES-256 to encrypt the sub-key with the master key instead of using the nonce and XOR operation, or maybe it would be better to use AES-256-CTR and use a nonce for the master key prefixed to the file.
Explanation / Answer
The main issue with ECB mode (i.e. using AES directly on 128-bit blocks) is that you leak whether two blocks are equal. When encrypting perfectly random data, that means there's a 2?128 chance the two parts of the key are equal, and the attacker knows that. The probability you see a collision between halves of any two keys becomes significant once you reach about 263 keys, but that probably doesn't help the attacker much unless he knows one of the keys already.
In contrast, with CTR mode the chance that you get two equal ciphertext blocks is the same, but that only leaks the fact that the two halves are not equal, which helps the attacker a minuscule amount
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