Gostaria apenas de citar um pouco da Wikipedia aqui:
Even if a symmetric cipher is currently unbreakable by exploiting structural weaknesses in its algorithm, it is possible to run through the entire space of keys in what is known as a brute force attack. Since longer symmetric keys require exponentially more work to brute force search, a sufficiently long symmetric key makes this line of attack impractical.
With a key of length n bits, there are 2n possible keys. This number grows very rapidly as n increases. Moore's law suggests that computing power doubles roughly every 18 to 24 months, but even this doubling effect leaves the larger symmetric key lengths currently considered acceptably well out of reach. The large number of operations (2128) required to try all possible 128-bit keys is widely considered to be out of reach for conventional digital computing techniques for the foreseeable future. However, alternative forms of computing technology are anticipated which may have superior processing power than classical computers. If a suitably sized quantum computer capable of running Grover's algorithm reliably becomes available, it would reduce a 128-bit key down to 64-bit security, roughly a DES equivalent. This is one of the reasons why AES supports a 256-bit key length. See the discussion on the relationship between key lengths and quantum computing attacks at the bottom of this page for more information.
Assim, uma chave de 128 bits teria 340.282.366.920.938.463.463.374.607.431.768.211.456 permutações possíveis. Imagine passar por tudo isso. Até mesmo um poderoso computador de mesa só pode tentar alguns por segundo.
Assim, embora seja teoricamente possível forçar a descriptografia de um fluxo SSH, quando a chave for descriptografada pelo computador mais poderoso imaginável, duas coisas teriam acontecido:
- A chave teria sido alterada por SSH
- Todos nós teríamos morrido e o sol explodido e destruído a terra.