found a way to reduce 1/6 total range.
python proto give a nice result for all known PK.
That is only passible when you know the private key, but WITHOUT the private key all those reductions aren't possible.
it's wrong.
for example 7-bit key can be found at 4 step (direct search from 2^6 to (2^7)-1 will find only at step 12, reduce 3 times).
1001001
11001010
21001011
31001100
4result: KwDiBf89QgGbjEhKnhXJuH7LrciVrZi3qYjgd9M7rF
U7hDgvu64y17-bit at 17318 (direct at 30287 step, reduce ~2 times)
...
10111011001001111
17318result: KwDiBf89QgGbjEhKnhXJuH7LrciVrZi3qYjgd9M7rF
iHkRsp99uC and so on... tested with known PK.
but still at researching.
8-bit key skiped by this method, therefore, unfortunately, there is a possibility of missing the desired value.
i was wrong about 1/6 (error with calculation was), anyway minimum 2 times reduce are possible, but with chance to skip needed value =(