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Suppose that someone seized my seed with a phrase of 12 words. I deliberately changed the order of words in a phrase in such a way that the only way to get a real phrase is to go through all the possible combinacies of the current 12 words.
1) How many combinations will it be necessary to go through an attacker in order to find a real seed? How long can it take?
2) Do you find such precaution really effective?
The answer is 12! = 479,001,600 possible mnemonics (less if there are repeated words). Out of these only ~29,937,600 (1 in 16) produce a valid seed. This is definitely brute forceable.
No, if you want to do this use a longer mnemonic (and make sure you permute them in a truly random way).
Permutation(12,12) = 12! = 479,001,600
Vanitygen on GTX 1080 can derive 50 million addresses in one second. 479/50 = ~9.5 thus it'll take 9.5 seconds to derive all addresses and find your private key. As this was a master password instead of a private key, it will take a little longer. Let's say at most 2 minutes.
Not effective at all, except that it might take up to week day for the thief to think of rearranging the words.
