Post Quantum Symmetric Cryptography So I am looking for the most secure method of symmetric key cryptography for long term messaging use between two users. I have heard that most symmetric key algorithms are not absolutely compromised by quantum computers, but rather weakened. 

I have read that AES 256 is in fact the weakest, followed by AES 128, and AES 192 being the strongest.

Let us imagine that today quantum computers exist with "good" capacity to break cryptography.

Is AES still the best symmetric key solution as of right now? If not which is? If so then which AES key space is the strongest?

These are many questions which remain unanswered or lack good discussion as there isn't much in regards to post quantum security concerns.
AES-256 is still considered the strongest (and is considered secure) as related key attacks are not particular to analysis with quantum computers.

Related key attacks could happen when AES is used within a construction such as a hash function, where the output of one round is used as a key for the next round.



As far as we know now, quantum computing won't have as much as an impact on most symmetric algorithms as it does on asymmetric cryptography (that usually relies on specific mathematical problems that can be solved using quantum computers).

As mentioned, the best generic attack on symmetric ciphers, Grover's attack, about halves the key strength. But you need a lot of qbits to create the attack. Of course, there may be new attacks found that are particular to a specific block cipher, but as far as we can see now, most constructs seems pretty secure against quantum computers. That means that a 256 bit key will still deliver 128 bits of security against analysis using quantum crypt-analysis.

That means that there seems little reason to double the 256 bit key strength, as 128 bits of security are considered plenty against any attack that require such kind of order of operations.



If you require 256 bits security with quantum computers (why?) you could consider Threefish-512 - an algorithm actually designed to deliver that kind of security.